Conception & Fetal Development.docx

Conception & Fetal Development

Lisa Smithers, MSN, APRN, CNM

1. Conception: the union of a single egg and sperm forming a baby
2. Gametogenesis:
   1. Spermatogenesis: the process of sperm formation, begins during puberty (age 13) in males. It occurs in the seminiferous tubules of the __________________. Mitosis is the replication of cells to keep the same number of chromosomes (46), then through the maturation process, the cells undergo __________ with division to decrease the chromosomal number from diploid ( 42 chromosomes) to haploid (21 chromosomes). The final product is 4 mature, motile sperm – either 22 ___ or 22 ___. During ejaculation, 40-250 million sperm are released and usually only survive 24 hours (although may survive up to 5 days).
   2. Oogenesis: the process of egg (ova) formation in the female begins during fetal life The female has all her ova by 30 weeks gestation as a fetus. The first meiotic divisions are complete starting at puberty with one primary oocyte maturing monthly This is when ovulation occurs, usually day 14 of a 28-day cycle. It cannot move by itself but is carried along by cilia (which line the uterine tubes) toward the uterus. The ovum may only survive 24 hours after its release at ovulation. The ruptured follicle (that contained the maturing ovum) is called the graafian follicle, and it secretes estrogen and progesterone to maintain the pregnancy until the placenta is established.

Graafian follicle 🡪 (becomes) corpus luteum

You only ovulate on day 14 ONLY IF you have a 28 day cycle. (32 🡪 ovulate on 18)

KEY!!!! You ovulate 14 days before your normal cycle (typically). First phase is very different for each woman. IN GENERAL it is AWLAY 14 days before ovulation. (cycle day – 14 = ovulation) you can get the past ovulation date NOT THE FUTURE (likely but unpredictable)

1. Fertilization: when the sperm enters the ovum.
   1. Fertilization usually takes place in the ampulla of the fallopian tube (closer to the ovary).
   2. When a sperm enters the ovum, a zona reaction occurs, which changes the zona pellucida surrounding the ovum and prevents other sperm from entering.
   3. The nucleus of the sperm (containing 23 chromosomes) merges with the nucleus of the ovum (also containing 23 chromosomes). This restores the diploid number to 46 chromosomes Through this process, the DNA of the chromosomes combine, forming the new person. Conception, the formation of a zygote, is now complete.
   4. The combination of the 22X egg with a 22Y sperm results in a female baby. The combination of the 22X egg a 22X sperm results in a male baby.
2. What happens after fertilization?
   1. The fertilized ovum (zygote) travels to the uterus in 3-5 days, while cell division is rapidly occurring, from 2 cells, to 4 cells, to 8 cells, to 16 cells (morula)
      1. Morula – a solid ball of 12-16 cells by about day 3 after conception
      2. Blastocyst – the cells continue to divide and then divides into two distinct layers: the embryo blast and the trophoblast
         1. The inner cell mass (embryoblast) becomes the embryo and then fetus
         2. The outer cell mass (trophoblast) develops the placenta and fetal membrane.
      3. Stem Cells – obtained from the embryoblast

Twinning

• Monozygotic Twins: identical twins - are always the same gender.

Always one ova, one sperm, and one fertilization.

They may divide very early, resulting in 2 separate implantations,

placentas, membranes.

They may divide later so that there is one implantation, one fused

placenta, and one chorion with separate amnions.

They may divide even later, so that both babies are inside the same

amniotic sac.

*no genetic component (random)

No genetic component

• Dizygotic Twins: fraternal twins – may be same or opposite genders.

Always two ova, two sperm, and two fertilizations.

There are two implantations, two placentas (which may fuse), two chorions (may fuse), two amnions.

Multiple ovulations (resulting in dizygotic twins) is an autosomal recessive trait, so it tends to repeat in families.

*Autosomal recessive trait

*Fraternal twins has to do with the females ovulation (not males) only matters about the mother’s history

• So are these twins monozygotic or dizygotic??
• Other multiple gestations

Triplets, Quadruplets etc: may be a combination of monozygotic and dizygotic.

Fertility treatment increases chances of having higher order multiples

1. Implantation
   1. The endometrium (lining of the uterus) is now called the decidua and is in the secretory phase of the reproductive cycle. This provides the richest environment for implantation.
   2. The Blastocyst goes through a gradual process of implantation between days 6-10.
   3. Most implantation occurs in the fundus, which is the top of the uterus. The trophoblast cells secrete enzymes which lyse endometrial cells and erode maternal blood vessels to create lacunae (small blood lakes) which provide nutrition and oxygen for the blastocyst.
   4. “Implantation bleed may occur, resulting in some spotting.
   5. Fingerlike vascular projections called chorionic villi develop out of the trophoblast and extend into the endometrium (decidua). They form the fetal portion of the placenta, and fetal blood circulates through them, allowing for exchange of oxygen / carbon dioxide, nutrients, and wastes.
   6. Throughout this process, the embryo continues to develop.
   7. Implantation should occur in the uterus, resulting in an intrauterine pregnancy. If implantation occurs outside the uterus, such as in the fallopian tube or abdomen, it is called an extrauterine (or ectopic) pregnancy, and is non-viable long term.

• Fetal membranes: the “bag of waters”

Two Layers: Chorion and Amnion – partially fused

Chorion: outer membrane

One portion of the chorion covers the placenta on the fetal side and contains the

umbilical blood vessels that branch out over the surface of the placenta.

The other portion surrounds the amnion and is smooth with no blood vessels.

Amnion: inner membrane (closest to baby)

Thin, tough, translucent, avascular membrane which is filled with amniotic fluid.

The amnion covers the chorion on the fetal side of the placenta and covers the

umbilical cord.

• Amniotic fluid: Fluid surrounding the baby that protects the growing baby and promotes normal fetal development

Amount:

700-800 ml is normal at 40 wks gestation (due date/term)

Oligohydramnios: < 400 ml

Polyhydramnios (Hydramnios): > 2,000 ml

Color: clear or pale straw color is normal - Green when meconium present

pH: 7.0-7.25, slightly alkaline, turns nitrazine paper blue.

Composition: (98-99% water)

• • 1. Early in gestation, composition is the same as baby’s serum
    2. Fetus “breathes” fluid in and out of lungs, so amniotic fluid contains lecithin and sphingomyelin which are present in fetal lung fluid
    3. Fetus urinates in-utero, so amniotic fluid contains urea, creatinine, bilirubin, etc.
    4. Other things in amniotic fluid: skin cells, lanugo, vernix, albumin, leukocytes, glucose, electrolytes, and hormones.

Purpose of Amniotic Fluid:

Cushions the baby for protection

Maintains a stable temp

Allows freedom of movement for musculoskeletal development

Source of oral fluid for baby

Prevent the membranes from adhering to the developing fetal parts

Testing can be performed by obtaining a sample of amniotic fluid: Amniocentesis

genetic studies may be done on cells in the amniotic fluid

L/S ratio may be done to determine fetal lung maturity

• Umbilical Cord

Extends from the umbilicus to the placenta and is slightly longer than baby

Minimal cord length for vaginal delivery is 32 cm (slightly longer than a foot)

Normal cord length is approximately 2 ft

Less than 30 cm is considered abnormally short

Contains 3 vessels

2 arteries which carry de-oxygenated blood from baby to placenta

1 vein which carries oxygenated blood from placenta to baby

Contains no pain receptors.

Wharton’s jelly: soft substance that cushions the entire cord and prevents kinking or obstruction.

has high water content causing the cord to shrink (shrivel and dry) after birth

also contains prostaglandins which have a vasoconstrictive effect after cord is cut. Attempt to clamp within 60 seconds

Whartan’s jelly in the cord (as opposed to kinks or folds) are life-threatening to the fetus

• Placenta

Sustains pregnancy to permit normal fetal development

Shape and Size: large in comparison to fetus early in development

By full-term pregnancy, is >1b, approx. 6 inches in diameter, 1 inch thick

Maternal side - rough where it attaches to the uterus

Fetal side – smooth vessels covering the membrane covered surface

Central insertion of the umbilical to placenta are normal

Placental Functions:

Metabolic:

Placenta is an organ - produces some nutrients needed by the embryo and for placental function

Effective function depends on adequate maternal circulation (bp not too high or too low)

Adequate perfusion depends on:

1. Maternal BP avoid hypertension and hypotension
2. Avoiding strenuous exercise which diverts blood away from uterus to muscles

If she can’t exercise and carry a normal conversation she should lower exercise.

1. Maternal position: avoid supine position which compresses vena cava

(she’ll probably feel light headed 🡪 low bp & baby won’t be getting adequate blood flow/oxygen)

1. Uterine tone: excessive contractions curtail flow of maternal blood, decreasing oxygen and nutrients to baby
2. Healthy placental tissue: an aging placenta begins to calcify

Transfer:

Gas exchange – oxygen and carbon dioxide diffuse across the membrane. Respiration is one of the key functions of the placenta. Fetal blood gas levels are different than an adult. Oxygen levels are lower, but fetal hemoglobin is usually at a higher level than an adult and it binds oxygen more readily to aid with oxygen transfer. This allows the fetus to survive in the lower oxygen state.

Nutrition – Water, proteins, carbohydrates, fats, vitamins, minerals, electrolytes pass from maternal to fetal blood. Glucose is the major energy source for fetal growth and metabolic activities.

Waste removal – besides the removal of carbon dioxide, other metabolic waste products leave the fetal blood and enter the maternal blood for disposal.

Antibody transfer – immunoglobulin G (IgG) provides passive (temporary) protection against some diseases. Transfer occurs primarily in the last term of pregnancy. Preterm and small for gestational age babies are at higher risk.

Endocrine:

Human Chorionic Gonadotropin (hCG) – preserves function of corpus luteum which serves to produce estrogen and progesterone for the first 6-8 weeks until the placenta is fully developed and functioning

Human Placental lactogen – promotes growth and nutrition of fetus and development of maternal breasts for lactation

Estrogen – most plentiful is Estriol – stimulates uterine growth and uteroplacental blood flow, enlargement of breast and external genitalia

Progesterone – essential for continuation of pregnancy. Maintains the endometrium and prevents uterine contractions. Also stimulates development of breast alveoli.

Protective barrier:

Semipermeable membrane keeps some undesirable particles out

Many drugs cross the placenta including alcohol, nicotine, cocaine, OTC’s

Viruses (including HIV), bacteria, other toxic substances cross placenta

Fetal Development:

Developmental Timeline

• Gestation = time in weeks since the first day of LMP (last menstrual period)
• Conceptional age (Embryonic age) = time in weeks since fertilization at conception (38 weeks)
• EDD (estimate due date) = 40 weeks gestation, but baby 38 weeks conceptional or embryonic age
• Term gestation – 37-42 wks gestation

Early Term – 37-38wks 6days gestation

Full Term – 39-40wks 6days gestation

Late Term – 41-41wks 6 days gestation

Post Term – 42 wks +

Intrauterine Periods (Stages)

Germinal first 2 weeks after conception - from fertilization to formation of embryonic disc with 3 germ layers (weeks 1-4 gestation)

Embryonic day 15-8 weeks after conception - formation of major organ systems is occurring - The embryo stage is when the baby is most susceptible to teratogens (harmful things; less likely to make a dangerous harm to baby before week 15 because baby’s organs are not yet all functional/formed, now (week 15<) it is very dangerous; increased risk)

Fetal beginning of week 9 to birth - growth and development of organ systems is occuring

Embryonic Stage

Germ layers which differentiate into all tissues and organs during the embryo stage:

ectoderm upper or outer layer - forms skin, nails, nervous system, tooth enamel

mesoderm middle layer - forms connective tissue, teeth, muscles, blood, vascular system

endoderm lower or inner layer - forms epithelial linings of GI and respiratory tracts, endocrine glands, auditory canal

Principles of development:

Development is cephalocaudal (head to toe)

Development is peripheral to peripheral

Development is general to specific

Landmarks in fetal development:

Conceptional/Embryonic/Fertilization Age: (approximately 2 wks less than gestational age)

Week 3 – germ layers form (ectoderm, mesoderm, endoderm)

Week 4 – neural tube closes (if it does not-neural tube defect-spina bifida, anencephaly)

Week 5 – heart is beating (conceptual age, wk 7 is gestational age [heart beating then])

Week 8 - all organs have formed

Week 9 – embryo becomes a fetus (fetal period begins)

Gestational Age:

Week 10 – fetal heart tones heard with Doppler

* MEMORIZE for exam!!!!!

Week 12 – external genitalia differentiate

(we can do genetic testing, this wk gender can be seen though by ultrasound)

Week 16 – all structures have formed

Week 16-20 – fetal heart tones can be heard with fetoscope – quickening occurs

*fetoscope: like a stethoscope, uses bone conduction for sound

Week 20 – baby is considered legally viable (although babies don’t typically have chance of survival if born until approx. 22 weeks gestation)

Week 24 – alveoli form in lungs

Week 27 – surfactant forming in lungs; lean body

Week 30-31 – subcutaneous fat forming

Week 32 – surfactant near mature levels

Week 34 – lecithin levels in lungs start to increase indicating lung maturity

Week 36 – rounded body, skin pink

Fetal Growth:

Terms:

Gametogenesis

Sperm & Ovum (Ova)

Spermatogenesis

Miosis

Cilia