CMCA REPRO NOTES 2

REPRO NOTES 2

Pregnancy concepts

STAGES OF FETAL DEVELOPMENT

In just 38, a fertilized egg matures from a single cell carrying all the necessary genetic materials to a fully developed fetus ready to be born. Fetal growth and development is usually divided into three periods: Pre-embryonic (first 2 weeks beginning with fertilization): embryonic (from week 3 through 8): and fetal growth (from 8 week through birth). 

A. FERTILIZATION: BEGINNING OF PREGNANCY 

Fertilization is the union of the ovum and a spermatozoon. Another term used to describe this fact are conception, impregnation, or fecundation. Fertilization    

After ovulation, as the ovum is extruded from the graafian follicle, it is surrounded by a ring of mucopolysaccharide fluid (zona pellucida) and a circle of cells (corona radiata).  These structures increase the bulk of the ovum, facilitating its migration to the uterus. They also serve as a protection from injury. The ovum and surrounding cells are propelled into the near fallopian tube by currents initiated by the fimbriae, fine, and hair like structures that line the opening of the fallopian tube. Peristaltic action of the fallopian tube and movement of the cilia help propel the ovum along the length of the tube. Normally, only one ovum reaches maturity each month. Once released, fertilization must occur fairly quickly because an ovum is capable for fertilization for only 24 hours (48 hours at the most). 

An ejaculation of semen averages 2.5 ml of fluid containing 50 to 200 million spermatozoa per milliliter, or an average of 400 million per ejaculation. Sperm transport efficient close to ovulation that spermatozoa deposited in the vagina during intercourse generally reach the cervix within 90 seconds and the outer end of the fallopian tube within 5 minutes after deposition. 

Spermatozoa, moves from their flagella (tails) and uterine contraction through the cervix, the body of the uterus, and into the fallopian tube toward the waiting ovum. The mechanism whereby spermatozoa are drawn toward an ovum is most possibly a species-specific reaction, similar to an antibody-antigen reaction. Capacitation is a final process that sperm must undergo to be ready for fertilization, this process which happens as the sperm moves toward the ovum, composed of changes in the plasma membrane of the sperm head, which reveals the sperm binding receptor cites. Hyaluronidase (proteolytic enzyme) apparently released by the spermatozoa and acts to dissolve the layer of cells protecting the ovum. 

Directly after penetration of the ovum, the chromosomal material of the ovum and spermatozoon fuse, and the structure produce is what we call the zygote. Spermatozoon and ovum carried 23 chromosomes (22 autosomes and 1 sex chromosomes), a fertilized ovum has 46 chromosomes. When an X-carrying spermatozoon enters the ovum, the resulting child will have 2 X chromosomes and will be a female (XX), while if, Y-carrying spermatozoon fertilizes the ovum, the resulting child will have an X and Y chromosomes and that will be a male (XY).  Fertilization depends on three separate factors: (1) maturation of sperm and ovum, (2) ability of a sperm to reach the ovum, and (3) ability of the sperm to penetrate the zona pellucida and cell membrane to achieve fertilization. 

From fertilized ovum (zygote) the future child also the accessory structure needed for support during intrauterine life, such as placenta, fetal membrane, amniotic fluid, and the umbilical cord are formed.  

B. IMPLANTATION 

As the fertilization complete, the zygote migrates toward the body of the uterus, aided by the currents initiated by the muscular contraction of the fallopian tubes. It takes 3 to 4 days for the zygote to reach the body of the uterus. This time, mitotic cell division, or cleavage, begins. The first cleavage occurs at about 24 hours: cleavage division continue to occur at a rate of one at about every 22 hours. By the time the zygote reaches the body of the uterus, it composed of 16 to 50 cells. In this stage, because of its bumpy outward appearance it is termed a morula (from Latin word morus meaning “mulberry”) 

                TERM USED TO DENOTE FETAL GROWTH 

Morula continues to multiply as it floats free in the uterine cavity for 3 to 4 more days. In this stage, the structure is called a blastocyst, a structure that attaches the uterine endometrium. The cell in the outer ring is known as trophoblast, part of the structure that soon will form the placenta and the membranes. The inner cell mass (embryoblast cell) is the portion of the structure that soon will form the embryo. 

 Implantation, or contact between the growing structures and the uterine endometrium, occurs approximately 8 to 10 days after fertilization. After the 3^rd or 4^th day of free floating (about 8 days from ovulation), the last residue of the corona and zona pellucida are shared by the growing structure. The blastocycst brushes against the rich uterine endometirum (in the 2^nd phase of the menstrual cycle), a process termed apposition. It attaches to the surface of the endometrium (adhesion) and settles down into its soft folds (invasion) 

The blastocyst is able to invade the endometrium because, as the trophoblast cells on the outside of the structure touch the endometrium, they produce the proteolytic enzymes that dissolve the tissue they touch. If the point of implantation is low in the uterus, the growing placenta may occlude the cervix and make delivery of the child at term difficult (placenta privia). Implantation is an important step in pregnancy, as many as 50 % of zygotes never achieve it. Once implanted the zygote is called as embryo.  

DECIDUA. The decidua (Latin word “falling off”), it will be discarded after the birth of the child. Below is the three separate areas: 

1. Decidua basalis. Part of the endometrium lying directly under the embryo (portion where the trophoblast cell are establishing communication with maternal blood vessels)

2. Decidua capsularis. Portion of the endometrium that stretches or encapsulates the surface of the trophoblast

3. Decidua vera. Remaining portion of the uterine lining

CHORIONIC VILLI. Early as the 11^th or 12^th day, miniature villi, or probing”fingers” termed chorionic villi, reached out from the single layer of cell into the uterine endometrium. At term 200 villi will be formed. Chorionic villi have a central core of loose connective tissue surrounded by a double layer of trophoblast cells. The central core of the connective tissue contains fetal capillaries. The outer of the two covering layer is termed the syncytiotrophoblast, or the syncytial layer. This layer of cell is instrumental in the production of various placental hormone, like HCG, somatommamotropin (human placenta lactogen (HPL)), estrogen, and progesterone. The inner layer is called cytotrophoblast or Langhan’s layer, present as early as 12 gestations. It appears to function early in pregnancy protecting the growing embryo and fetus from certain infectious organism. Still this layer of cell disappears between the 20^th and 24^th week. This is why syphilis is considered to have a high potential for fetal damage late in pregnancy when cytotrophoblast cells are no longer present. 

C. PLACENTAL DEVELOPMENT. The placenta, Latin for “pancake” which is descriptive of its size and appearance of term, arise out of trophoblast tissue. It serves as the fetal lungs, kidney, and gastrointestinal tract and as separate endocrine organs throughout pregnancy. Its growth parallels that of the fetus, growing from a few identifiable cells at the beginning of pregnancy to an organ 15 to 20 cm in depth at term. It covers about half the surface area of the internal uterus. 

CIRCULATION. Early as the 12^th day of pregnancy, maternal blood begins to collect in the (intervillous spaces) of the uterine endometrium surrounding the chorionic villi. At the 3^rd week, oxygen and other nutrient diffuse from the maternal blood through the cell layer of the chorionic villi to the villi capillaries. From there, nutrients are transported back to the developing embryo. 

As the number of chorionic villi increases with pregnancy, the villi form an increasingly complex communication network with the maternal blood. Intervillous spaces grow larger and larger, becoming separated by a series of a partition or septa. In a mature placenta, there are as many 300 separate segments, called cotyledons; These compartments make the maternal side of the placenta at term look rough and uneven. About 100 uterine arteries supply the mature placenta. To provide enough blood for exchange, the rate of utero placental blood flow in pregnancy increases from about 50 mL/min at 10 weeks to 500-600 mL/min at term. There is no additional arteries appears after the first three months of pregnancy. At term, placental circulatory network is so extensive that a placenta weighs 40-600g (1lb) and is one sixth the weight of the baby. 

ENDOCRINE FUNCTION

HUMAN CHORIONIC GONADOTROPIN. The first hormone to be produced is HCG. This hormone can be found in maternal blood and urine as early as the time of the first missed menstrual period (shortly after implantation has occurred) through about the 100 days of pregnancy. HCG acts as a fail safe measure to ensure that the corpus luteum should fail and the level of the progesterone and estrogen. It also plays a role in suppressing the maternal immunologic response so placental tissue is not rejected. At about 8^th week of pregnancy in human the outer layer of cell of the developing placenta begin to produce progesterone. 

ESTROGEN. Referred to as the hormone of women, primarily estriol it is produced as a second product of the syncytial cells of the placenta. It contributes to the mother’s mammary glands development in preparation for lactation and stimulates uterine growth to accommodate the developing fetus. Assessing the amount of estriol in maternal serum was used in the past to test fetal well-being. 

PROGESTERONE. Referred to as the hormone of mother. It is necessary to maintain the endometrial lining of the uterus during pregnancy. It is present in serum as early as 4^th week of pregnancy as a result of continuation of the corpus luteum. It also appears to reduce the contractility of the uterine musculature during pregnancy, which prevents premature labor

HUMAN PLACENTAL LACTOGEN (HUMAN CHORIONIC SOMATOMAMMOTROPIN). Human Placenta lactogen (HPL) is a hormone with both growth-promoting and lactogenic (milk-producing) properties. It is produced by the placenta beginning as early as the 6^th week of pregnancy, increasing to a peak level at term. It can be assayed in both maternal serum and urine. It promotes mammary gland growth in preparation for lactation in the mother. It also serves the important role of regulating maternal glucose, protein, and fat levels so adequate amounts of these are always available to the fetus. 

D. UMBILICAL CORD 

The umbilical cord is  formed from the amnion and chorion and provides a circulatory pathway connecting the embryo to the chorionic villi. The function of the cord is to transport oxygen and nutrients to the fetus from the placenta and to return waste products from the fetus to the placenta. The umbilical cord is about 53 cm (21 in) in length at term. It is about 2 cm (3/4 in) in thickness. It contains one vein (carrying blood from the placental villi to the fetus) and two arteries (carrying blood from the fetus back to the placental villi). The bulk of the cord is a gelatinous mucopolysaccharide called Wharton’s Jelly, which gives the cord the body and prevents pressure on the vein and arteries. The outer surface is covered with amniotic membrane. 

Normally there are two umbilical arteries and one umbilical vein. About 1% of all infants are born with a cord that contains only a single vein and artery. About 15% of these infants are found to have accompanying congenital anomalies. Blood can be withdrawn from the umbilical vein or transfused into the vein during intrauterine life for fetal assessment or treatment (termed percutaneous umbilical blood sampling (PUBS)). The rate of the blood flow through an umbilical cord is rapid (350 ml/min at term). In about 20% of all births, a loose loop of cord is found at the fetal neck (nuchal cord). Once this loop is removed before the newborn’s shoulders are extruded, so there is no traction on it, the oxygen supply to the fetus remains unimpaired. 

E. MEMBRANES AND AMNIOTIC FLUID 

The chorionic villi on the medial surface of the trophoblast (those that are not involved in implantation because they do not touch the endometrium) gradually thin and leave the medial surface of the structure smooth (the chorion laeve, or smooth chorion.) the smooth chorion eventually becomes the chorionic membrane, the outermost fetal membrane. Once it becomes smooth, it gives support to the sac that contains the amniotic fluid. A second membrane lining the amniotic membrane or amnion forms beneath the chorion. Early in pregnancy, these membranes becomes so adherent that they seen as one at term. These membranes cover the fetal surface of the placenta and give that surface its typical shiny appearance. Similar to the umbilical cord, they have no nerve supply, thus when it rupture at term, neither mother nor the child experience any pain sensation. Contrast to chorionic membrane, the amniotic membrane not only offers support to amniotic fluid but actually produces the fluid. In addition, it produces phospholipids that initiate the formation of prostaglandins, which cause the uterine contractions and may be the “trigger” that initiates labor.

Amniotic fluid is constantly being newly formed and reabsorbed, so it is never stagnant within the membranes. Since the fetus constantly swallows the fluid, it is absorbed across the fetal intestine into the fetal blood stream. From there, the umbilical arteries exchange it across the placenta. Some fluid is probably absorbed by direct contact with the fetal surface of the placenta. At term, the amount of amniotic fluid ranges from 800 to 1200 ml. if, for any reason the fetus is unable to swallow (esophageal or anencephaly are the two most common reasons) excessive amniotic fluid or hydramnios   (more than 200 mL total or pockets of fluids larger than 8 cm on ultrasound) will result. Hydramnios also tends to occur in women with diabetes because hyperglycemia causes excessive fluid shift into the amniotic space. A disturbance of kidney function may cause oligohydramnios, a reduction in the amount of amniotic fluid (less than 300 mL total or n o pocket on ultrasound larger than 1 cm). amniotic fluid is an important defense mechanism for the fetus:

• it shield against pressure or a blow to the mother’s abdomen

• it protects the fetus from changes in temperature, because liquid changes temperature more slowly than air 

•  probably aids in muscular development, because it allows the fetus freedom to move 

• it protects the umbilical cord from pressure, protecting fetal oxygenation 

Amniotic fluid is slightly alkaline with a pH of about 7.2. Checking the pH of the fluid at the time of rupture helps to differentiate it from urine, which is acidic (pH 5.0-5.5). 

ORIGIN  AND DEVELOPMENT OF ORGAN SYSTEM

At the start of the fetal growth, progress proceed in a cephalocaudal (head to tail direction), that is head development occurs first and is followed by the development of the middle and finally, lower body parts. As a fetus grows, body organ systems develop from specific tissue layers called germ layer. 

PRIMARY GERM LAYER 

At the time of implantation, the blastocyst already has differentiated to a point at which two separate cavities appear in the inner structure: (1) a large one, the amniotic cavity, which is lined with a distinctive layer of cells, the ectoderm and (2) a smaller cavity, the yolk sac, which is lined with endoderm cell. Between the amniotic cavity and the yolk sac, a third layer of primary cells, the mesoderm, forms. The embryo now began to developed (from embryonic shield) at the point where the three layer cells (ectoderm, endoderm and mesoderm) meet. One reason rubella infection is always serious in pregnancy because this virus is capable of affecting of all the germ layer. 

All organ systems are complete, at least in a rudimentary form, at 8 weeks gestation (the end of embryonic period). During this early time of organogenesis (organ formation), the growing structure is most vulnerable to invasion be teratogens (any factor that affects the fertilized ovum, embryo, or fetus adversely). 

ORIGIN OF BODY TISSUE

CARDIOVASCULAR SYSTEM:

The septum that divides the heart tube forming as early as 16^th day of life, beating as early as the 24^th day. The septum that divides the heart into chambers develops during the 6^th or 7^th week. The heart beats may be heard with a Doppler as early as the 10^th to 12^th week of pregnancy. An electrocardiogram (ECG) can be recorded on a fetus as early as the 11^th week, although the accuracy of the ECG is in doubt until about 20^th week of pregnancy when conduction is more regulated. 

FETAL CIRCULATION. As early as the 3^rd week of intrauterine life, fetal blood has begun to exchange nutrients with the maternal circulation across the chorionic villi. Fetal circulation differs from extrauterine circulation in several respects. During the intrauterine life, the fetus drives its oxygen and excretes carbon dioxide not from oxygen exchange in the lungs but from the placenta. Blood does enter the lung while the child is in the utero, but this blood flow is to supply the cell of the lungs themselves not for oxygen exchange. Specialized structures present in the fetus shunt blood flow to supply the most important organs.

Blood arriving at the fetus from the placenta is highly oxygenated. This blood enters the fetus through the umbilical vein (vein even though it carries oxygenated blood, because the direction of the blood is toward the fetal heart). The umbilical vein carries the blood to the inferior vena cava through the accessory structure known as the ductus venosus, which receive most of the oxygenated blood from the umbilical vein to supply the fetal liver. It then empties the inferior vena cava. From the inferior vena cava, blood is carried to the right side of the heart. When the blood enters the right atrium, the bulk of it is shunted into the left atrium through an opening in the atrial septum, the foramen ovale. From the left atrium, it follows the course of normal blood circulation into the left ventricle and into the aorta. 

Blood from the arms and head is returned to the heart by the superior vena cava. The blood enters the right atrium and leaves it by the normal circulatory route, that is, through the tricuspid valve into the right ventricle, then through the pulmonary artery in the normal manner. A small part of this blood flow services the lung tissue. Still, the larger portion is shunted away from the lungs, through an additional structure called the arteries, even though they are now transporting deoxygenated blood, because they are carrying blood away from the fetal heart, back through the umbilical cord to the placenta villi, where new oxygen takes place. 

FETAL HEMOGLOBIN. Fetal hemoglobin differs from adult hemoglobin in several ways:

• different composition (2 alpha and 2 gamma chains of adult hemoglobin)

• greater oxygen affinity (increases its efficiency)

• more concentrated ( at birth, newborns hemoglobin  level is about 17.1 g/100 mL compared with an adult normal level of 11 g/mL: a newborns hematocrit is about 53% compared with an adults normal level of 45&)

REPIRATORY SYSTEM. In the 3^rd week of uterine life the respiratory and digestive tract exist as a single tube. At the end of the 4^th week, a septum begins to divide the esophagus from the trachea, in that lungs bud appear on the trachea. Until the 7^th week of life, the diaphragm does not completely divide the thoracic cavity from the abdomen. During the 6^th week of life, lungs buds may extend down into the abdomen, reentering the chest only as the chest longitudinal dimension increases and the diaphragm becomes complete (at the end of the 7^th week). Below are the important respiratory development milestones:

• Alveoli and capillaries begin to form between the 24^th and 28^th weeks. Both capillary and alveoli development must be complete before gas exchange can occur in the fetal lung 

• Spontaneous respiratory movements begin as early as 3 months of pregnancy, continuing throughout pregnancy. 

• Specific lung fluid with a low surface tension and low viscosity forms in alveoli to aid in expansion of alveoli at birth: it is rapidly absorbed after birth

• Surfactant, phospholipid substance, is formed and excreted by the alveolar cells at about the 24^th week of pregnancy. This decrease alveolar surface tension on expiration, preventing alveolar collapse and improving the infant’s ability to maintain respirations in the outside environment

There are two components of a Surfactant: Lecithin and Sphingomyelin. In early formation of surfactant, sphigomyelin is the chief component. Lack of surfactant is a factor associated with the development of respiratory distress syndrome.

NERVOUS SYSTEM. Similar to circulatory system, nervous system start to develop extremely early in pregnancy. During the 3^rd and 4^th weeks of life, possibly before the woman even realizes that she is pregnant, active formation of the nervous system and sense organ has already begun. 

• A neural plate, (thickened portion of the ectoderm), is apparent by the 3^rd week of gestation, its top portion differentiates into the neural tube, which will form the central nervous system (brain and the spinal cord) and the neural crest, which will developed into the peripheral nervous system

• Brain waves can be detected on electroencephalogram (EEG) by the 8^th week 

• All parts of the brain (cerebrum, cerebellum, pons and medulla oblongata) form in the utero although they are not completely mature at birth. Growth continues to occur rapidly during the first year continues at high levels until 5 or 6 years of age 

• Eye and inner ear develop as projection of the original neural tube 

• By the 24^th weeks, the ear is capable of responding to sound, exhibits the papillary reaction, indicating light is present 

ENDOCRINE SYYSTEM. Early as endocrine organs mature in the intrauterine life, function begins, including the following:

• The fetal adrenal gland supply a precursor for estrogen synthesis by synthesis by the placenta

• The fetus pancreas produces the insulin needed by the fetus (insulin does not cross the placenta  from the mother to the fetus)

• The thyroid and parathyroid glands plat a vital roles in metabolic function and calcium balance 

DIGESTIVE SYSTEM. The digestive tract is separated from the respiratory tract at about the 4^th week. After this time, the intestinal tract grows and extremely rapidly. Initially solid the tubes canalize (hollow up) to become patent. Later, the endothelial cells of the gastrointestinal tract proliferate extensively, occluding the lumens once more. Atresia or stenosis can develop if either the first or second canalization does not occur. The proliferation of cells shed in the second recanalization forms the basis for meconium. 

Since the abdomen is too short to contain the intestine, apportion of the intestine, guided by the virtalline membrane (part of the yolk sac) enters the base of the umbilical cord during the 6^th week of the intrauterine life. Intestine remains in the base of the cord until about the 10^th week. By this time, fetal trunk has extended and enlarges the abdominal cavity so it is large enough to hold all the intestinal mass. As the intestine returns to the abdominal cavity, it must rotate 180 degrees. Failure to do so can result to inadequate mesentry attachment, possibly leading to volvulus of intestine. If any intestinal coils remain outside the abdomen, in the base of the cord, a congenital anomaly, omphalocele, develops. A similar defect, gastroschisis, occurs when the original midline fusion that occurred at the early cell stage is incomplete. Once the vitalline ducts does not atrophy after return of the intestines, a Meckel’s diverticulum (a pouch of intestinal tissue) or an opening between the intestine and umbilicus can result.

Meconium forms in the intestine as early as the 16^th week. It composed of cellular wastes, bile, fats, mucoprotien, mucopolysaccharides, and portion of the vernix, caseosa, the lubricating substance that forms on the fetal skin. Meconium is black or dark green (obtaining its color from the bile pigment) and sticky. 

The gastrointestinal tract is sterile before birth. Because Vitamin K is synthesized by the action of the bacteria in the intestines, this can cause vitamin level to be low in the newborn infant. In addition, sucking and swallowing reflexes are not mature until the fetus is about 32 weeks or the fetus weighs 1500 g. 

Ability of the gastrointestinal tract to secrete enzymes essential to carbohydrate and protein digestion is mature at 36 weeks. Though, an amylase, as enzyme found in saliva and necessary for digestion of complex starches, is not mature until three months after birth. Many newborn are not developed lipase, an enzymes needed for fat digestion. 

The liver is active throughout gestation, its function is to filter between the incoming blood and the fetal circulation and the deposit for fetal store such as iron, and glycogen. Still, it is immature at birth, possibly leading to hypoglycemia and hyperbilirubinemia, two serious problems in the first 24 hours after birth. 

MUSCULOSKELETAL SYSTEM. The fetus can be seen to move on ultrasound as early as the 11^th week, although the mother does not always feel this movement (quickening) until nearly 20 weeks. In the first 2 weeks of fetal life, cartilage prototypes provide position and support. Ossification process continues all through fetal life and actually until adulthood. Carpals, tarsal, and sternal bones generally does not ossify until birth coming up. 

REPRODUCTIVE SYSTEM. At the 6^th week of life, the gonads (ovaries or testes) form. 

• When testes form, testosterone is secreted, apparently influencing the sexually neutral genital duct to form other male organs (maturity of the wolffian, mesonephric, duct)

• In the absence of testosterone secretion, female organs will form (maturation of the mullerian, or paramesonephric, duct) 

Eventually, testes descend from the pelvic cavity, where they first form into the scrotal sac late in the intrauterine life, at the 34^th to 38^th week. 

URINARY SYSTEM. Urine is formed by the 12^th week and is excreted into the amniotic fluid by the 16^th week of gestation. At term, fetal urine is being excreted at the rate of 500 mL/day. An amount of amniotic fluid that is less than normal (oligohydramnios) suggest that fetal kidneys are not secreting adequate urine. 

Complex structure of the kidneys is gradually developed during pregnancy and for months afterward. Glomerular filtration and concentration of urine in the newborn are not efficient because the kidneys are not fully mature even by birth. 

In the early stage of the urinary system development, the bladder extends to the umbilical region. On unusual occasion, an open lumen between the urinary bladder and umbilicus fails to close. Termed a patent urachus, this is discovered at birth by the persistent drainage of a clear, acid-pH fluid (urine) from the umbilicus. 

INTEGUMENTARY SYSTEM. The skin of the fetus appears thin and most translucent until subcutaneous fat begins to be deposited about 36 weeks. Skin is covered by soft downy hairs (lanugo) and a cream cheese like substance, vernix, caseosa, important for lubrication and keeping skin from macerating. 

IMMUNE SYSTEM. IgG maternal anti bodies cross the placenta into the fetus primarily during the 3^rd trimester of pregnancy, giving a fetus temporary passive immunity against disease for which the mother has antibodies. Frequently include poliomyelitis, rubella, (German measles), rubeola (regular measles), diphtheria, tetanus, infectious parotitis (mumps) and pertussis (whooping cough). The level of the passive IgG immunoglobulins peaks at birth and then decreases over the next 9 months while infants begin to build up their own stores of IgG as well as IgA and IgM. 

MILESTONES OF FETAL GROWTH AND DEVELOPMENT

END OF 4 GESTATION WEEKS:

• Length is 0.75 to 1 cm

• Weight is 400 mg 

• Spinal cord is formed and fused at the midpoint 

• Lateral wings that will form the body are folded forward to fuse at the midline 

• Head folds forward, becoming prominent, comprising about ¹/₃ of the entire structure

• Back is bent so the head almost touches the tip of the tail 

• Rudimentary heart appear as a prominent bulge on the anterior surface 

• Arms and legs are budlike structures

• Rudimentary eyes, ears and nose are discernible 

END OF 8 GESTATION WEEKS:

•  Length is 2.5 cm (1in) 

• weight is 20 g 

• organogenesis is complete 

• heart with a septum and valves is beating rhythmically 

• facial features are definitely discernible 

• extremities have developed 

• external genetalia are present, but sex is not distinguishable by simple observation 

• primitive tail is regressing 

• abdomen appears large as the fetal intestine is growing rapidly 

• sonogram shows a gestational sac, diagnostic of pregnancy

END OF 12 GESTATION WEEKS (FIRST TRIMESTER): 

• length is 7 to 9 cm 

• weight is 45 g 

• nail beds are forming in fingers and toes 

• spontaneous movements are possible, although usually too faint to be felt by the mother 

• some reflexes, such as Babinski reflex, are present 

• bone ossification are forming 

• tooth buds are present 

• sex is distinguishable by outward appearance 

• kidney secretion has begun, although urine may not be evident in amniotic fluid

• heart beat is audible by a Doppler 

END OF 16 GESTATION WEEKS  

• length is 10 to 17 cm 

• weight is 55 to 120 g

• fetal heart sound are audible with an ordinary stethoscope 

• Lanugo (fine, downy hair on the back and arm of newborn, apparently serving as a source of insulation for body heat) is well formed

• Liver and pancreas are functioning 

• Fetus actively swallows amniotic fluid, demonstrating an intact but uncoordinated swallowing reflex

END OF 20 GESTATION WEEKS 

•  Length is 25 cm 

• weight is 223 g 

• spontaneous fetal movements can be sensed by the mother 

• antibody production is possible 

• hair forms, extending to include eyebrows and hair on the head 

• meconium is present in the upper intestine 

• brown fat, a special fat that will aid in temperature regulation at birth, begins to be formed behind the kidneys, sternum, and posterior neck

• fetal heart beat is strong enough to be audible through the abdomen with an ordinary stethoscope 

• vernix caseosa, a cream-cheese-like substance produced by he subcutaneous glands that serves as a protective skin covering intrauterine life, begins to form

• definite sleeping and activity pattern are distinguishable 

END OF 24 GESTATION WEEKS (SECOND TRIMESTER): 

• length is 28 to 36 cm 

• weight is 550 g 

• Passive antibody transfer from mother to fetus probably begins as early as 20^th week of gestation, certainly by the 24^th week of gestation . 

• meconium is present as far as the rectum 

• active production of lungs surfactant begins 

• eyebrows and eyelashes are well defined 

• eyelids, previously fused since the 12tg week, now open 

• pupils are capable for reacting to light 

• when fetuses reach 24 weeks, or 601 g, they have achieve a practical low-end age of viability if the are cared for after birth in a modern intensive care facility

END OF 28 GESTATION WEEKS 

• length is 35 to 38 cm 

• weight is 1200 g 

• lung alveoli begin to descend into the scrotal sac from the lower abdominal cavity in males 

• blood vessels of the retina are extremely susceptible to damage from high oxygen concentration 

END OF 32 GESTATION WEEKS 

• length is 38 to 43  cm 

• weight is 1600 g

• subcutaneous fats begins to be deposited (former stringy, “little old man” appearance is lost)

• Fetus is aware of sounds outside the mothers body 

• Active Moro reflex is present 

• Delivery position (vertex or breech) may be assumed 

• Iron stores that provide iron for time during which the neonate will ingest only milk after birth are beginning to be developed 

• Fingernails grow to reach the end of fingertips 

END OF 36 GESTATION WEEKS 

• Length is 42 t0 49 cm 

• Weight is 1900 to 2700 g (5 to 6 lb) 

• Body stores of glycogen, iron, carbohydrate, and calcium are augmented 

• Additional amounts of subcutaneous fat are deposited 

• Sole of the foot has only one or two crisscross creases compared with the full crisscross pattern that will  be evident at term 

• Amount of lanugo present begins to diminish 

• Most babies turn into a vertex or head-down presentation during this month 

END OF 40 GESTATION WEEKS (THIRD OF TRIMESTER)

• Length is 48 to 52 cm; crown to rump is 35 to 37 cm 

• Weight is 3000 g (7 to 7 ½ lb.) 

• Fetus kick actively –hard enough to cause the mother considerable discomfort 

• Fetal hemoglobin begins it conversion to adult hemoglobin, the conversion is so rapid that, at birth at about 20 % of hemoglobin will be adult in character 

• Vernix caseosa is fully formed 

• Fingernails is extend over the fingertips 

• Creases extend over the feet cover at least two thirds of the surface 

In primiparas (women having the first babies), the fetus often sinks into the birth canal during the last two weeks, giving the mother feeling that her load is being lightened. This occurrence is called lightening, a fetal announcement that the third trimester of the pregnancy has ended and