Development
Prenatal period
Begins with fertilization after secondary oocyte and sperm combine. Ends ~38 weeks later with birth.
Three periods are exhibited:
Pre-embryonic period: first 2 weeks of development post-fertilization when the now Zygote, becomes spherical multicellular blastocyst. Ends with blastocyst implanting in uterus.
Embryonic period: Extends from third through eight weeks of development. Rudimentary versions of major organ systems appear in the developing body as a embryo.
Fetal period: Thirty remaining weeks of development prior to birth, when organism is now called a fetus. Fetus grows and becomes more complex.
Pre-Embryonic Period
Zygote: Male sperm and female secondary oocyte unit to form single diploid cell.
Same size as secondary oocyte. Undergoes miotic cell divisions, and more cells appear forming the pre-embryo.
Fertilization
Fertilization: two haploid gametes fuse to form a new diploid cell containing info from both parents.
Corona Radiata Penetration: Sperm reaches secondary oocyte and burrows through the corona radiata, aided by enzymes that help break down the layers surrounding the oocyte.
Zona Pellucida Penetration: After the sperm successfully navigates through the corona radiata, it encounters the zona pellucida, a glycoprotein layer that protects the oocyte and facilitates sperm binding.
Acrosome reaction: Acrosome of sperm releases digestive enzymes which allow penetration into the zona pellucida, enabling the sperm to reach the plasma membrane of the oocyte for fertilization.
Fusion of Sperm and Oocyte Plasma Membranes and Fusion of Sperm and Ovum Pronuclei: Upon sperm and oocyte plasma membrane interaction, immediate fusing occurs.
Only the nucleus of the sperm enters the cytosol of the secondary oocyte. The midpiece and flagellum degenerate shortly afterwards.
Sperm nucleus in secondary oocyte causes second meiotic division and formation of an ovum.
Pronuclei: nucleus of sperm and nucleus of ovum; due to having 23 single chromosomes.
Zygote: when the two pronuclei fuse into a single nucleus, creating a fertilized egg with a complete set of 46 chromosomes, which will undergo multiple rounds of cell division as it begins the process of embryonic development.
Secondary Oocytes release chemotaxis signals which attract sperm to its location.
Capacitation: Physiologic conditioning of sperm before they can fertilize secondary oocyte.
Cleavage
Cleavage: Series of miotic divisions after zygote reaches 2-cell stage, where the cell size does not change but cell count increases.
Compaction: process when cells begin to adhere tightly to one another, forming a compact structure that is critical for successful implantation into the uterine lining.
Morula: 16-cell stage formed after several rounds of cleavage, characterized by a solid ball of cells that will eventually develop into a blastocyst.
After entering lumen of uterus, fluid leak through degenerating zona pellucida into morula. This forms a fluid-filled cavity called blastocyst cavity.
Blastocyst: After morula enters lumen.
Trophoblast: outer ring of cells surrounding fluid-filled cavity. These cells form chorion.
Embryoblast/inner cell mass: tightly packed group of cells located only within one side of blastocyst. It will form embryo proper.
Pluripotent: Able to differentiate into any cell or tissue type.
Implantation
Implantation: process by which blastocyst burrows into and embeds within endometrium.
First week of development: day seven is when implantation begins.
Trophoblast cells invade functional layer of endometrium and divides into two layers:
Cytotrophoblast: inner layer that forms the primary cell mass of the placenta
Syncytiotrophoblast: outer layer that facilitates nutrient exchange and hormone production.
Day 9: Blastocyst has burrowed into uterine wall and made contact with nutrients in uterine glands.
Synctiotrophoblast release Human Chorionic gonadotropin, which signal corpus luteum that fertilization have occurred, preventing degeneration and causing continued release of progesterone and estrogen which maintain uterine lining.
End of Second Week: Enough hCG produced to tell if a pregnancy is in progress.
Formation of Bilaminar Germinal Disc and Extraembryonic Membranes
During second week, embryoblast portion undergoes changes.
Day 8: Cells of embryoblast differentiate into two layers, which collectively form a flat disc called a bilaminar germinal disc:
Hypoblast: Small, cuboidal cells adjacent to blastocyst cavity.
Epiblast: layer of columnar cells adjacent to amniotic cavity.
Extraembryonic membranes formed from germinal disc and trophoblast, they preform multiple functions:
Yolk sac: First extraembryonic membrane to develop. Formed from hypoblast layer, it serves as a important site for blood cell and blood vessel formation.
Amnion: formed from epiblast layer of bilaminar germinal disc, it is a thin membrane which encloses entire embryo in a fluid-filled sac (Amniotic cavity) to protect embryo from drying out.
Chorion: Outermost extraembryonic membrane formed from cytotrophoblast cells and synctiotrophoblast cells (formed from trophoblast cells). These cells implant within function layer of endometrium and form the placenta.
Development of the Placenta
Blastocyst must obtain nutrients and respiratory gases from maternal blood supply further develop.
Placenta: Vascular “organ” which connects embryo/fetus to mother.
Exchange of nutrients, respiratory gases, and waste products between the maternal and fetal blood.
Transmission of maternal antibodies to developing embryo or fetuis.
Production of progesterone and estrogen to maintain and build uterine lining.
Formation of Placenta: Begins forming during second week of development. It develops from chorion, while the maternal portion forms from functional layer of uterus.
Connecting stalk: precursor to the umbilical cord, which facilitates nutrient and oxygen exchange between the mother and the fetus.
Chorionic villi: finger-like structures which form chorion, and contains several branches of umbilical cords.
Mature Placenta: Disc-shaped and mostly formed during fetal period, it adheres to wall of uterus and is expelled from uterus with child.
Embryonic Period
Gastrulation: three-layered structure that forms the three primary germ layers (ectoderm, mesoderm, and endoderm) which will differentiate into all body tissues.
Organogenesis: the process where the three germ layers develop into the internal organs of the body, typically occurring during the fourth to eighth week of gestation.
By week eight, main organ systems have been established.
Gastrulation and Formation of Primary Germ Layers
Gastrulation: Third week; most critical periods; Cells of epiblast migrate and form three primary germ layers: ectoderm, mesoderm, and endoderm, which will differentiate into various tissues and organs.
Primitive streak: Thin depression on surface of epiblast.
Primitive Node: Cephalic (head) end of streak, which is slightly elevated.
Primitive pit: Deeper depression on superior primitive node.
Invagination: inward movement of cells;
Epiblast cells detach and invaginate to form endoderm first, then a new layer of mesoderm. Remaining epiblast cells will form the ectoderm.
Folding of Embryonic Disc
Three-week developed embryo exhibits a flattened disc-shape.
Late third-fourth weeks, rapid cell division causes some regions to outgrow others. This causes a folding of embryonic disc, making a cylindrical shape.
Cephalocaudal folding: Embryonic disc and amnion grow rapidly, but yolk sac does not. This causes cephalic and caudal regions to fold on themselves.
Future head and buttocks.
Transverse folding: Lateral ends of embryo curve and migrate toward midline, pinching off yolk sac. Fusion occurs in midline, creating a cylindrical embryo.
Ectoderm solely on exterior now. Future trunk/torso.
Differentiation of Ectoderm
Neurulation (most important): Ectoderm after cephalocaudal and transverse folding, develops into nervous system tissue.
pituitary gland, lens of eyes.
Epidermal tissue and derivatives: After neurulation, ectoderm cells covering the embryo form the skin, and it’s derivatives.
Focus on contact of outside world: Ectoderm builds structures which facilitate interaction with the environment, including sensory organs and epithelial tissues.
Also forms: adrenal medulla and enamel of teeth.
Differentiation of Mesoderm
Notochord: tightly packed midline group of mesoderm cells. It forms the basis of central body axis and axial skeleton. Also inducing formation of neural tube.
Paraxial mesoderm: Lateral to neural tube (superior midline), it forms somites which give rise to axial skeleton, most muscle, most carilage, dermis, and connective tissue of the body.
Somites: blocklike masses
Intermediate mesoderm: Lateral to paraxial mesoderm, it forms most of the kidneys, ureters and internal reproductive organs.
Lateral plate mesoderm: Thin lateral-most layer which gives rise to the spleen, adrenal cortex, epithelial lining of blood and lymph vessels, heart, serous membranes of body cavities and all connective tissue components of limbs.
Head mesenchyme: Forms connective tissues and musculature of face.
Differentiation of Endoderm
Endoderm becomes most of the innermost tissue of the body when undergoing transverse folding.
Epithelial linings of respiratory, gastrointestinal, urinary and reproductive tracts.
Epithelial lining of tympanic cavity and auditory tube.
Development of most of the liver, gallbladder, pancreas, portions of palatine tonsils, thyroid gland, parathyroid glands, and thymus.
Organogenesis
Organogenesis: Formation of organ development. Seen in week 4, with most rudimentary forms developed by week 8.
Teratogens (monster producing) substances: agents that may cause malformation or abnormal development of the embryo or fetus during pregnancy.
Peak Development period: Most cellular organization and construction of organ frame work occurs.
Although most rudimentary versions of organ systems are created by week eight, several of them still undergo finalization or peak development period.
Teratogens are especially harmful during this period.
Fetal Period
Fetal period extends from week nine to birth, it is characterized by maturation of tissues, organs and rapid growth of developing fetus.
Weeks nine-twelve: Primary ossification centers appear in most bones, external reproductive organs begin differentiation, coordination between nerves and muscles for movement begins to develop and brain enlargens.
Weeks thirteen to sixteen: Body grows rapidly, ossification continues, limbs become proportionate in length to body and brain and skull continue to enlarge.
Weeks seventeen to twenty: Muscular movements become stronger and more frequent (baby kicking), lanugo covers skin, vernix caseosa covers skin, limbs almost finalized, brain and skull enlarge more and external reproductive organs are able to be differentiated as male or female, indicating sexual development.
Weeks twenty one to thirty eight: Body gains major amount of weight, subcutaneous fat is deposited, eyebrows and eyelashes appear, eyelids open, testes descend into scrotum and blood cells formation centralized in red bone marrow.
Pregnancy of Mother
Timeline of Pregnancy
First Trimester: First three months of pregnancy, zygote develops into embryo then into early fetus.
Second Trimester: Months four to six; Marked by growth of fetus and expansion of maternal tissues.
Third Trimester: Months seven to nine; Fetus grows rapidly and mother’s body prepares for labor and delivery.
Important Note: Women’s experiences of pregnancy varies greatly. Some exhibit morning sickness, whereas some may be hospitalized due to extreme nausea and vomiting. Weight gain can be minor to extreme, and even length can be early or later.
Hormonal Changes
Human Chorionic Gonadotropin: Hormone produced by syncytiotrophoblast cells of blastocyst, which instructs the corpus luteum to continue production of progesterone and estrogen instead of degeneration (until placenta can take over)
Progesterone and Estrogen: During second and third trimesters, placenta produces high amounts of progesterone and estrogen. This suppresses FSH and LH release, halting ovarian cycle and follicular development.
Progesterone: Growth of functional layer of endometrium and prevention of uterine contractions to maintain pregnancy.
Estrogen & Progesterone: Uterine enlargement, mammary gland enlargement, and fetal growth. Increased growth of integumentary system (faster-growing and thicker hair, nails and skin).
Estrogen: Relaxation of ligamentous joints.
Relaxin: Released by corpus luteum and placenta, it promotes blood vessel growth in uterus.
Corticotropin-releasing hormone: Placenta produces this hormone to help regulate the onset of labor and support fetal development by influencing the maternal and fetal adrenal glands. As well, increase aldosterone to preserve fluid balance and regulate blood pressure, ensuring that both the mother and fetus receive adequate nourishment during gestation.
Human placental lactogen: Released increasingly linearly from fifth week to maximum by thirty-sixth week of development by placenta. It encourages fat oxidation, allowing for fetus to use glucose instead. Also inhibits effect of insulin, allowing greater blood glucose levels.
Named for lactating effect in animals. Different in humans though.
Prolactin: Produced by anterior pituitary, it works to increase milk production to ensure lactation.
Oxytocin: Produced by the posterior pituitary, it is essential for uterine contractions during childbirth and stimulates milk ejection during breastfeeding.
Uterine and Mammary Gland Changes
Implantation to Week four months: Uterine hypertrophy and hyperplasia begins, becoming noticeable in just four weeks after fertilization. As the uterus expands, impingement on bladder results in increased urinary frequency due to the pressure exerted on the bladder.
Melanocyte-stimulating hormone: Darkens areolae of nipples and stimulates melanin production in the skin, which helps prepare the body for breastfeeding by enhancing the visibility of the nipples.
Mammary glands become tender and sore and increase in acini occur, increasing breast size. Darkening of linea alba (ligamentous connection between lateral rectus sheaths) occurs.
Week 16 to week 28: Uterus expanded into abdominal cavity and fundus is midpoint level with pubic symphysis and umbilicus. Superior growth results in less pressure on urinary bladder.
Week 28 to week 40: Further expansion of the uterus occurs, with the fundus reaching the xiphoid process by the end of pregnancy, leading to significant changes in posture and compression of abdominopelvic organs. Resulting in ailments and trouble breathing.
Digestive, Nutrient and Metabolic Changes
Human placental lactogen: Released increasingly linearly from fifth week to maximum by thirty-sixth week of development by placenta. It encourages fat oxidation, allowing for fetus to use glucose instead. Also inhibits effect of insulin, allowing greater blood glucose levels.
Note: Can cause gestational diabetes if diet is not properly managed, necessitating monitoring of blood sugar levels and dietary adjustments throughout pregnancy.
Hyperemesis gravidarum: Severe form of morning sickness, which causes dehydration, imbalance, and weight loss.
Note: Unknown why Morning sickness occurs. It is suggested that it is a evolutionary response to prevent developing fetus from toxins. Expecting mothers will avoid meat and eggs (which are likely to have toxins or be spoiled) while preferring bland carbohydrates.
Intestinal Modifications: High levels of progesterone result in relaxation of smooth muscle, preventing peristalsis. Digested materials remain in GI tract longer. Heartburn and indigestion can be expected, as uterus expands and exerts pressure on the stomach.
Dietary Needs: Pregnant woman needs roughly only 300 extra calories daily to supply herself and her fetus. Excess become adipose tissue. Adequate levels of folic acid, calcium, protein, and iron are essential during pregnancy.
Cardiovascular and Respiratory Changes
Cardiovascular:
Plasma volume increases by 50%.
Cardiac output increases 30-50% from weak 6 to weeks 24-28 before dropping. Heart rate and stroke volume increases.
Increase in blood pressure can be exhibited in first semester. Typically reversed in second trimester due to a decrease in peripheral vascular resistance and decreased sensitivity of hormone angiotensin II.
Growth of uterus and fetus during third semester can result in venous return issues.
Respiratory:
Dyspnea: Shortness of breath in periods of exertion. In a expecting mother, this is due to the expanding uterus preventing full flattening of diaphragm, lowering total lung capacity.
Increased blood circulation and fluid retention in nasal cavity due to increased estrogen levels, causing nosebleeds and congestion.
Lower levels of blood CO2; Progesterone increases sensitivity of central chemoreceptors to blood carbon dioxide. Tidal volume increases by 30-40% and oxygen consumption increases about 20-30%.
Urinary System Changes
Glomerular filtration rate: increases about 30-50% to accommodate increased plasma volume and elimination of waste from fetus. This increases urine output slightly.
Progesterone: Smooth muscle relaxation in ureters, allowing expansion of ureters and renal pelvis of kidneys. This can cause urine stasis, potentially leading to urinary tract infections and other complications.
Labor and Delivery
Labor/Parturition: Physical expulsion of fetus and placenta from uterus.
Premature Labor: The onset of labor prior to 37 weeks of gestation, which may result in increased risks for both the mother and baby, including respiratory issues and developmental delays.
Factors resulting in labor:
Uterus enlarges and stretches, increasing myometrium strength.
Increasing levels of estrogen counteract inhibiting effects of progesterone on uterine myometrium by increasing general sensitivity and increasing its receptors to oxytocin.
Weak contractions will be present, increasing in strength.
False Labor/Braxton-Hicks contractions: These contractions mimic true labor but are typically irregular and do not signify the onset of labor, often serving as a practice mechanism for the body.
Irregularly spaced and lack increased frequency
Pain limited to lower abdomen and pelvic region instead of radiating through entire abdominal region and back.
Pain may change or stop in response to movement.
Relatively weak and do exhibit cervical changes.
True Labor
True Labor: Uterine contraction which increase in intensity and regularity and typically lead to progressive cervical dilation, resulting in the eventual onset of childbirth.
Regularly spaced and increase in frequency over time. Beginning with one every fifteen minutes to every five minutes. Intensity also increases.
Pain radiates from upper abdomen to lower back; pain does not go away in response to movement either.
Hypothalamic changes in fetus and mother: Near beginning of true labor, oxytocin is released by mother and fetus. This causes: (a) contraction of smooth muscle of uterus and (b) stimulation of placenta to secrete prostaglandins (which are local hormones that stimulate smooth muscle contraction) to soften and dilate cervix.
Positive feedback mechanism: More intense uterine contractions result in fetus’s head being pushed against cervix, which stimulate stretching and dilation of cervix. This stretching initiates nerve signals to hypothalamus which triggers the release of oxytocin, further enhancing uterine contractions and progressing labor.
Placenta also secretes more prostaglandins in response to uterine contractions.
Stages of True Labor: dilation stage, expulsion stage, and placental stage.
Dilation Stage: Longest phase which exhibits an onset of regularly spaced uterine contractions that increased in intensity and frequency. The baby’s head pushes against the cervix, and ends with cervix thinned and and dilated.
Nulliparous woman experience longer dilation stage": 8-24 hours instead of parous women which range from 4 to 12 hours.
Rupture of amniotic sac/”water breaking”
Expulsion Stage: Begins with full dilation of cervix and ends with expulsion of fetus from mother’s body. This stage typically lasts for 30 minutes, however can take up to several hours. Uterine contractions push fetus through vagina (can be helped if woman uses valsalva maneuver).
Crowning: Baby’s calvarium distends the vagina, and the head is slightly present.
Episiotomy: A surgical incision made to widen the vaginal opening during childbirth, often performed to prevent tearing and facilitate delivery.
Umbilical cord clamped and tied off after expulsion.
Placental stage: Uterus continues to contract, to expel the remaining placenta from the uterus, typically occurring within 5 to 30 minutes after the birth of the baby.
Afterbirth: Placenta and remaining fetal membranes.
Postnatal Changes for Newborn
Neonate: term for newborn.
First breath usually within first ten seconds of breath to initiate the respiratory process, causing significant changes in circulation and oxygenation.
Fetal circulation changes to postnatyal circulation as the newborn's lungs expand and begin to facilitate gas exchange, leading to the closure of shunts present during fetal development.
Changes in Mother after Delivery
Post-partum: first six weeks after birth. Mother’s body undergoes changes to feed the neonate and return to a normal body.
Hormonal Changes
Estrogen & Progesterone: Levels plummet after delivery. Integumentary system regresses back to its normal cyclical growth patterns. Respiratory system tidal volume decreases due to decreased sensitivity to CO2 levels as progesterone levels drop.
Baby Blues: A transient period of mood swings, irritability, and sadness that many new mothers experience in the days following childbirth, often linked to hormonal fluctuations and the adjustment to motherhood.
Corticotropin-Releasing Hormone: Levels plummet as well. This change leads to a decrease in cortisol production, which can affect the mother's stress response and overall energy levels.
Post-Partum depression: Many new mothers experience post-partum depression due to these hormonal fluctuations, which can impact emotional well-being and familial relationships.
Prolactin & Oxytocin: Levels drop after birth as well. Surges are present however due to their role in stimulating milk production and enhancing maternal bonding, highlighting the complex interplay between hormones during this critical time.
Blood and Fluid changes
Lochia: Similar to a menstrual period, lochia is the vaginal discharge that occurs after childbirth, consisting of blood, mucus, and uterine tissue as the body gradually returns to its pre-pregnancy state.
Progressive lessening of intense bleeding, with the worse being the first five days.
Expulsion of retained fluids due to hormonal change: Increased urination as a byproduct of a decline in CRH, which causes aldosterone release. Within 24 hours after birth, perfuse bouts of urination can be present. Sweating is also likely.
Lactation
Lactation: Production and release of breast milk from mammary glands.
Prolactin is responsible, oxytocin must be present for secrection. Inhibited by dopamine.
Estrogen causes prolactin release even though both cause an increase in mammary gland acini. Paradoxically, high levels of estrogen and progesterone prevent milk production.
Colostrum: Watery, yellowish, milk-like substance which has lower concentrations of fat but is rich in immunoglobulins (mainly A); this helps infant acquire immunity and causes laxative effects.
True Milk: Higher fat content, several growth factors, essential fatty acids, enzymes which aid digestion of milk and large array of immunoglobins.
Direct breastfeeding is always better than formula. An exception is vitamin D shockingly.
Positive feedback loop: Milk letdown is an hormonal reflex. Sucking of the breast causes stimulation of mechanoreceptors in nipple and areola, which alert the hypothalamus. The hypothalamus releases oxytocin in response, which targets myoepithelial cells mammary acini (which cause release of breast milk) to squeeze. This release of milk encourages more sucking from the infant, perpetuating the cycle and ensuring adequate milk supply.
Mothers who regularly breastfeed have their ovulation cycles arrested as a result of increased levels of prolactin, which suppresses fertility during the lactation period.
Uterine Changes
Uterus is enlarged due to hypertrophy and hyperplasia during pregnancy; Oxytocin facilities shrinkage by uterine contractions, promoting the return of the uterus to its pre-pregnancy size postpartum. The contractions are referred to as afterpains.
Regular breastfeeding causes these contractions more frequently, which helps get rid of after-pains quicker.
Heredity
Heredity: transmission of genetic characteristics from parent to child.
Genes: Field of biology which studies heredity and transmission patterns.
Human Genetics
Human Somatic Cell: Contains 23 pairs of chromosomes.
Karyotype: A photographic or digital image of an individual's set of chromosomes, typically arranged in pairs and organized by size and shape for analysis.
Twenty two pairs are autosomes, which play no role in the sex of the individual. The last two chromosomes are the sex chromosomes which play a role in sex of the individual.
Genes: Discrete units of DNA that provide instructions for production of specific proteins.
Locus: Region where a variant of a gene is found.
Dominant alleles exhibit their effects while a Recessive allele’s effects are hidden.
Punnett Square: Method which allows for the visualization of genetic variation and predictions of the probability of inheritance of traits in offspring.
Genotype: Genetic makeup of a individual;
Phenotype: Physical expression of the genotype.
If someone does have a widows peak or not.
Homozygous: An organism that carries two identical alleles for a particular gene, which can be either dominant or recessive.
Heterozygous: An organism that carries two different alleles for a specific gene, resulting in a phenotype that is typically determined by the dominant allele.
Patterns of Inheritance
Strict dominant-recessive inheritance: Dominant allele is ALWAYS expressed in the phenotype, regardless if an individual is homozygous or heterozygous for that allele.
Dominant: Widow’s peak (WW or Ww); Recessive: Straight hairline (ww)
Incomplete dominance: Phenotype of two heterozygous alleles is intermediate between dominant or recessive alleles.
Sickle Cell: Most folks carry AA, however some carry As, which produces an abnormal form of hemoglobin. This form of hemoglobin crystalizes when oxygen levels are low, leading to brittleness and a formation of a sickle shape.
Codominant Inheritance: Rare and an exception, both alleles are expressed in the phenotype. Such people with ABO blood.
Three forms of phenotypes: (a) the phenotype produced by homozygous dominant alleles, (b) the phenotype produced by homozygous recessive alleles, and ( c) phenotype produced by heterozygous alleles.
Polygenic inheritance/multiple gene inheritance: Multiple genes interact to produce a phenotypic trait. Most human traits are a result of this.
Sex Linked Traits
Sex Linked Traits: Expressed by genes on X and or Y chromosomes; ~900-1400 genes on the X chromosome, whereas Y contains ~70-200 genes. Y chromosome only present in males.
X-linked recessive trait always expressed in a genetic male because they only have one X chromosome. While females only exhibit x-linked recessive phenotype if there is two recessive alleles for that trait, which is an exception.
Carrier (female specific): Individual has one X-linked recessive allele only and does not exhibit any phenotypic effects. However, it still can be passed unto the offspring in cross over.
Color-blindness: A common example of an X-linked recessive trait, which predominantly affects males due to their single X chromosome, while females can be carriers without showing symptoms.
X-linked dominant traits: Rare but possible, traits are seen in both sexes, however males are severely affected. Most males will be aborted because of high rate of lethality associated with X-linked conditions, leading to a higher survival rate for females who can often carry these traits without being affected.
Environment & Genes
Penetrance: Percentage of population with a specific genotype which exhibits expected phenotype.
Hereditary Pancreatitis: Penetrance level of 80%; 20% of individuals will experience no symptoms.
Teratogens: Agents that can cause malformation or abnormal development of an embryo or fetus, leading to potential birth defects.
Fetal alcohol syndrome: characterized by mental retardation and distinctive facial features.
Poor nutrition: Twin study shows that different environment differences impact their growth and developmental outcomes, emphasizing the importance of a healthy diet during pregnancy.