fetal period

**Timing (high-yield):** - Embryologic (developmental) age: fetal period = week 9 → birth. Embryonic period = weeks 3–8 (organogenesis). Weeks 1–2 = pre-embryonic ("all-or-none" period). - Note: Gestational age (from LMP) is ~2 weeks ahead of embryologic age (from fertilization), so the fetal period begins at ~week 11 gestational age.

**Key features:** - Organogenesis is essentially complete; the fetal period involves growth, differentiation, and functional maturation of tissues and organs. - Rapid growth in length occurs in the 3rd–5th months; greatest weight gain occurs in the last 2 months (3rd trimester). - The head is proportionally large early on, and lower limbs are relatively short; proportions normalize over time.

**Selected milestones:** - Weeks 9–12: Intestines return into the abdomen from the physiologic umbilical herniation (which occurs ~weeks 6–10, with return by week 12); external genitalia differentiate (sex distinguishable by ~week 12); bone marrow begins hematopoiesis (spleen/liver predominate earlier); urine production begins. - Weeks 13–16: Skeletal ossification is evident on radiographs; ovaries are differentiated. - Weeks 17–20: Quickening (mother feels fetal movement); lanugo and vernix caseosa appear; brown fat forms. - Weeks 20–25: Fetal movements coordinated; type II pneumocytes begin to appear (~week 24–28) and produce surfactant. - Weeks 26–29: Lungs capable of breathing air with adequate surfactant (viability limit ~22–24 weeks with intensive care); central nervous system can direct rhythmic breathing and control temperature; eyes open; erythropoiesis shifts to bone marrow. - Weeks 30–38: Surfactant sufficient (L/S ratio ≥2 at ~35 weeks); fat deposition; testes descend (~weeks 28–32 into the inguinal canal, scrotum by ~week 33–35); myelination of the CNS continues after birth.

**Clinical significance:** - Teratogens: In the fetal period, susceptibility to major structural malformations is lower than during the embryonic period, but teratogens can still cause growth restriction, functional/CNS deficits, and specific defects. Examples: ACE inhibitors (renal dysgenesis/oligohydramnios, pulmonary hypoplasia, skull hypoplasia), tetracyclines (teeth discoloration, bone growth inhibition), fetal alcohol exposure (CNS effects, growth restriction), maternal diabetes (macrosomia), and androgen exposure (virilization of female external genitalia). - Fetal hemoglobin (HbF, α2γ2) predominates in the fetal period and has a higher O2 affinity than HbA (it binds 2,3-BPG poorly); switch to HbA occurs around birth. - Neonatal respiratory distress syndrome: surfactant deficiency in premature infants; antenatal corticosteroids (betamethasone) accelerate lung maturity. - Congenital diaphragmatic hernia leads to pulmonary hypoplasia (fetal period lung development is impaired). - Intrauterine growth restriction (IUGR) can be symmetric (early insult; e.g., infection, chromosomal abnormalities) or asymmetric (late insult; e.g., uteroplacental insufficiency, with head-sparing). - Prenatal screening/diagnosis is done during this window (e.g., quad screen at 15–22 weeks, anatomy ultrasound at 18–22 weeks).

**Mnemonic:** "Embryo = Establish organs; Fetus = Fatten and Function."