Fetal Development and Placental Function — Study Notes

Fetal Development: Key Concepts and Milestones

  • Overview

    • Fetal development = growth and maturation of the fetus from conception to birth.
    • Gestation duration: approximately 40 weeks40\text{ weeks}, measured from the first day of the last menstrual period (LMP).
    • Developmental periods/time frames:
    • Preembryonic: conception to day 14. Key events: fertilization, implantation, cell division.
    • Embryonic: weeks 3−83-8 (genesis). Critical period for organ development; highly sensitive to teratogens.
    • Fetal: weeks 9 to birth9\text{ to birth}. Growth and functional maturation of organs.
    • Major stages summary:
    • Preembryonic stage: zygote forms, blastocyst implants, placenta and amniotic sac begin forming (implantation around days 6–10).
    • Embryonic stage: major organ systems form (neural tube, heart, limb buds); placenta continues to develop; teratogens have a strong impact.
    • Fetal stage: organ systems refine and mature; rapid growth and functional readiness for life outside the womb.

  • Gastrulation and germ layers

    • During the 3rd week, gastrulation transforms the bilaminar disc (two layers) into a trilaminar disc with three primary germ layers:
    • Ectoderm: future skin, hair, nervous system.
    • Mesoderm: future muscle, bones, cardiovascular system.
    • Endoderm: future internal structures like gut, glands, lungs.
    • Gastrulation occurs around week 33; germ layers are established by the end of week 33.
    • NCLEX tip: germ-layer origin questions ask which structure originates from a given germ layer. Group organs by layer: surface or external + neural = ectoderm; muscle, skeleton, circulation = mesoderm; gut, glands, lungs = endoderm.

  • Stages of fetal development: trimester-by-trimester milestones

    • First trimester (weeks 1−121-12)
    • Weeks 1−21-2: fertilization, zygote formation, initial implantation begins; early placental development; amniotic sac forms.
    • Weeks 3−43-4: continued early placental development; amniotic sac forms.
    • Week 55: heart begins to beat.
    • Weeks 6−76-7: brain, lungs, liver, and GI tract begin developing.
    • Week 88: all major organs begin to form; limb buds visible.
      • Fetus size around 0.5−1 inch0.5-1\text{ inch} (approx. half to one inch); size often compared to a black bean.
    • Weeks 9−109-10: external genitalia begin forming; facial features become defined.
    • Weeks 10−1210-12: fetal heartbeat detectable by Doppler.
    • Weeks 12−1312-13: fingernails and toenails form; kidneys start functioning.
    • Week 1313: end of the first trimester.
    • Week 1414: genitals visible by ultrasound.
    • Second trimester (weeks 13−27/2813-27/28; often aligned with month references)
    • Week 1616: lanugo appears; limbs begin to move; vernix caseosa begins to develop.
    • Sex is identifiable by ultrasound; viability begins to approach around 24 weeks24\text{ weeks} with NICU support.
    • Fetus at roughly 5 inches5\text{ inches} long, weighing about 4 oz4\text{ oz}; comparable to the size of an avocado.
    • Weeks 18−2018-20: quickening (mother feels fetal movements); anatomy scan shows skin/hair development.
    • Week 2020: fetal length 9−10 inches9-10\text{ inches}; weight ~1 lb1\text{ lb}.
    • Week 2222: vernix caseosa covers the skin; lungs begin surfactant production; fetus considered viable with NICU care; length ~12 inches12\text{ inches}; weight ~2 lb2\text{ lb}.
    • Weeks 25−2825-28: continued rapid growth and maturation; more defined fetal features.
    • Weeks 28−3228-32: eyes open/close; rapid brain growth; rhythmic breathing movements begin.
    • Week 2929: mother may notice more kicks; Week 3030: increased fetal movement; potential sleep disturbance for mother.
    • Week 3232: pelvic pressure increases; frequent urination; fetus ~17−18 inches17-18\text{ inches} long and up to 5 lb5\text{ lb}.
    • Third trimester (weeks 32−4032-40; term = weeks 37−4037-40)
    • Week 3434: lungs nearing full maturity; nails fully formed.
    • Week 3636: fetus assumes vertex (head-down) position for birth; mothers may leak colostrum; nesting instincts increase; neonatal drop (lightening) reduces breathing effort; fetal length 17−19 inches17-19\text{ inches}; weight 6−7 lb6-7\text{ lb}.
    • Week 37−4037-40: full term; organ systems mature; final weight gain; lungs fully developed for extrauterine life.
    • Viability starts around 24 weeks24\text{ weeks}, but best outcomes occur after 37 weeks37\text{ weeks}.
    • At full term (weeks 37−4037-40): weight typically 6−9 lb6-9\text{ lb}; length 18−20 inches18-20\text{ inches}.
    • At week 40: fetal length ranges 18−20 inches18-20\text{ inches} and weight 7−9 pounds7-9\text{ pounds} on average.
    • Prenatal care reminders: warning signs (bleeding, absent fetal movement), importance of screenings and nutrition.

  • Placenta: functions and hormonal support

    • The placenta is a vital temporary organ that forms during pregnancy and connects the fetus to the uterine wall via the umbilical cord.
    • Key roles:
    • Respiratory function: transfers O₂ from mother to fetus and removes CO₂ from fetal blood to maternal circulation.
    • Nutrient delivery: transfers glucose, amino acids, fatty acids, vitamins, and minerals by active and passive transport.
    • Waste removal: removes urea, uric acid, creatinine into maternal blood for elimination.
    • Immune support: semi-permeable barrier allows maternal antibodies to provide passive immunity to the fetus; protects against many infections, but some viruses (e.g., HIV, Zika) and bacteria can cross.
    • Endocrine function: temporary hormone production to maintain pregnancy, support fetal growth, and prepare for labor and lactation.

  • Placental hormones and their roles

    • Human chorionic gonadotropin (HCG)
    • Maintains corpus luteum in early pregnancy to sustain progesterone and estrogen secretion.
    • Basis for pregnancy tests; helps suppress maternal immune response against the embryo.
    • Human placental lactogen (hPL)
    • Alters maternal metabolism to increase nutrient availability for the fetus.
    • Increases insulin resistance to provide more glucose to the fetus; stimulates breast development for lactation.
    • Progesterone
    • Maintains the endometrial lining; prevents uterine contractions; promotes mammary gland development.
    • Estrogen
    • Stimulates uterine growth and blood flow; prepares breasts for lactation; increases uterine sensitivity to oxytocin near term.
    • Relaxin
    • Relaxes pelvic ligaments and softens the cervix for labor.
    • Corticotropin-releasing hormone (CRH)
    • Regulates fetal adrenal development; helps initiate labor by increasing prostaglandins and cortisol; rises sharply in late pregnancy.

  • The umbilical cord

    • Lifeline between fetus and placenta, essential for transporting blood, nutrients, and waste.
    • Blood transport roles:
    • Umbilical vein carries oxygenated blood from the placenta to the fetus.
    • Umbilical arteries (two) carry deoxygenated blood from the fetus to the placenta.
    • Wharton’s jelly cushions the vessels to prevent compression.
    • The cord links the fetal circulation to the placenta, which interfaces with maternal circulation.
    • Quick NCLEX facts:
    • Number of vessels: 3 (two arteries and one vein).
    • Protective tissue: Wharton’s jelly.
    • Arteries carry deoxygenated blood to the placenta; the vein carries oxygenated blood to the fetus.
    • Complications: cord prolapse and nuchal cord.
    • After birth, the vessels constrict and become ligaments.

  • Amniotic fluid and amniotic sac

    • Amniotic fluid is a clear, slightly yellowish liquid surrounding the fetus in the amniotic sac.
    • Functions:
    • Thermal regulation: maintains a constant temperature for the fetus.
    • Cushioning and protection from external pressure and trauma.
    • Musculoskeletal development: allows fetal movement, promoting muscle and bone development.
    • Lung development: fetus inhales and exhales fluid, aiding pulmonary maturity.
    • Hydration and nutrition: fluid balance and small amounts of nutrients.
    • Prevents adhesion: keeps membranes from sticking to fetus.
    • Facilitates practice for life after birth (breathing and swallowing).
    • Diagnostic value: amniocentesis can assess genetic conditions, infections, and lung maturity.
    • Quick mnemonic: CLEAN and WARM
    • CLEAN: C = Cushion; L = Lung development; E = Electrolyte balance; A = Adhesion prevention; N = Nutrition
    • WARM: W = Waste exchange; A = Antimicrobial; R = Regulates temperature; M = Movement support

  • Fetal circulation: key features

    • The fetus relies on the placenta for gas exchange; lungs are nonfunctional before birth.
    • Blood flow routing (to bypass nonfunctional lungs):
    • Umbilical vein carries oxygenated blood to the fetus and then through the ductus venosus to the inferior vena cava (IVC).
    • Foramen ovale: opening between right and left atria; shunts blood away from the lungs.
    • Ductus arteriosus: connects the pulmonary artery to the aorta; bypasses the lungs.
    • Umbilical arteries: carry deoxygenated blood from the fetus to the placenta.
    • Summary of pre-birth circulation: gas exchange occurs in the placenta, not the lungs; certain shunts direct blood to vital organs.
    • Postnatal changes: at birth, lungs expand with the first breath; the following structures close and eventually become ligaments:
    • Foramen ovale closes after birth.
    • Ductus arteriosus closes after birth.
    • Ductus venosus closes after birth.
    • Umbilical vessels close after birth and become ligaments.
    • Note: These closures are part of the transition to independent neonatal circulation.

  • Quick review and practical implications

    • Remember to discuss prenatal care, nutrition, and warning signs with patients:
    • Bleeding, severe abdominal pain, reduced fetal movement, decreased fetal heart tones, and other concerning symptoms warrant medical attention.
    • Amniocentesis and ultrasound-based assessments provide valuable information about fetal health, development, and maturity.
    • Understanding germ layers helps in predicting which organs may be affected by teratogens and congenital anomalies.
    • Knowledge of placental hormones aids in understanding pregnancy symptoms, metabolic changes, and labor onset.

  • Connections to broader principles

    • Development follows a tightly timed sequence from cell division to organogenesis, then growth and functional maturation.
    • Teratogens have the greatest impact during the embryonic period (weeks 3−83-8) when organ systems are forming.
    • Placental function integrates maternal physiology with fetal needs, supporting growth and protecting the fetus while enabling parturition readiness.
    • Ethical/practical implications include ensuring access to prenatal screening, informed consent for diagnostic procedures (e.g., amniocentesis), and counseling about potential risks of exposures (drugs, infections, environmental agents).

  • Mathematical and numeric references (LaTeX)

    • Gestation duration: 40 weeks40\,\text{weeks} from LMP.
    • Preembryonic stage: 0−14 days0-14\,\text{days}.
    • Embryonic stage: weeks 3−83-8.
    • Fetal stage: weeks 9−409-40 (birth).
    • Critical developmental milestones are described across weeks with approximate lengths and weights, e.g., 0.5−1 in0.5-1\text{ in} at week 88, 9−10 inches9-10\text{ inches} by week 2020, 12 inches12\text{ inches} by week 2222, and term ranges 18−20 inches18-20\text{ inches} and 6−9 lb6-9\text{ lb} at term.

  • Summary of key terms to remember

    • Zygote, blastocyst, implantation, gastrulation, ectoderm, mesoderm, endoderm, neurulation, neural tube, placenta, amniotic sac, amniotic fluid, Wharton’s jelly, umbilical cord, umbilical vein, umbilical arteries, foramen ovale, ductus arteriosus, ductus venosus, vernix caseosa, lanugo, quickening, viability, preterm vs term, NICU, colostrum, lightening (fetal dropping).