Two ways to count age

• Gestational age (GA) — counted from the first day of the last menstrual period (LMP); used clinically
• Fertilization (conceptional) age — counted from conception, about 2 weeks less than GA

Embryology texts often use fertilization age; this topic gives GA unless stated.

From fertilization to implantation

• Fertilization usually occurs in the ampulla of the fallopian tube within about 12–24 hours of ovulation; sperm survive in the reproductive tract up to about 5 days
• The zygote divides as it travels down the tube: morula → blastocyst
• Implantation in the endometrium (now called the decidua) begins about 6–7 days after fertilization, usually in the upper posterior uterine wall; light "implantation bleeding" may occur
• The trophoblast becomes the placenta and chorion and secretes hCG, which keeps the corpus luteum producing progesterone until the placenta takes over (about 10 weeks)

Stages

Selected milestones (approximate, by gestational age)

Placenta and membranes

• The placenta exchanges oxygen, carbon dioxide, nutrients, and wastes by diffusion and active transport; maternal and fetal blood do not normally mix
• Endocrine functions: hCG; human placental lactogen (hPL) — raises maternal insulin resistance so more glucose reaches the fetus; estrogen; progesterone — maintains the endometrium and relaxes uterine muscle
• The placenta is not a true barrier: most drugs, alcohol, nicotine, some viruses, and IgG antibodies (including anti-D) cross
• Membranes: amnion (inner) and chorion (outer)

Amniotic fluid

• Functions: cushions the fetus, allows movement and lung development, maintains temperature, prevents cord compression, protects against infection
• Early pregnancy: comes mostly from fluid transfer across membranes and fetal skin; after about 16–20 weeks, fetal urine is the main source, and fetal swallowing is the main route of removal (plus lung fluid)
• Volume peaks near 34–36 weeks (roughly 800 mL–1 L)
• Oligohydramnios — amniotic fluid index (AFI) ≤ 5 cm or single deepest pocket < 2 cm: renal agenesis, urinary obstruction, placental insufficiency, ruptured membranes, postterm; risks: cord compression, pulmonary hypoplasia, limb contractures
• Polyhydramnios — AFI ≥ 24 cm or deepest pocket ≥ 8 cm: maternal diabetes, GI obstruction (esophageal or duodenal atresia), anencephaly and other neural tube defects (impaired swallowing), multiple gestation; risks: preterm labor, malpresentation, cord prolapse, postpartum hemorrhage

Fetal circulation — three shunts bypass the lungs and liver

Normal cord = 2 arteries + 1 vein (AVA). A single umbilical artery is associated with fetal anomalies (especially renal and cardiac) and growth restriction.

Changes at birth

• First breaths expand the lungs → pulmonary resistance falls; clamping the cord raises systemic resistance
• Foramen ovale closes functionally within minutes to hours as left atrial pressure exceeds right
• Ductus arteriosus constricts in response to rising oxygen and falling prostaglandins; functional closure usually within 1–3 days
• Ductus venosus and umbilical vessels close and become ligaments

Fetal hemoglobin (HbF) has a higher oxygen affinity than adult hemoglobin (HbA), so it pulls oxygen from maternal blood. Fetal hemoglobin and hematocrit are also higher. HbF is gradually replaced by HbA over the first months after birth (mostly by about 6 months).

• Fundal height (after 20 weeks, centimeters from symphysis to fundus ≈ weeks of gestation ± 2) — screens for growth problems; a gap of more than about 2–3 cm → ultrasound
• Fetal heart rate (FHR): normal baseline 110–160/min; tachycardia = baseline above 160/min for at least 10 minutes (maternal fever or infection, fetal anemia, hypoxia, drugs); bradycardia below 110/min
• Fetal movement: felt from quickening; decreased movement is a warning sign
• Ultrasound biometry: crown–rump length (first trimester dating), biparietal diameter, head and abdominal circumference (AC), femur length → estimated fetal weight (EFW)

• First-trimester ultrasound (crown–rump length) is the most accurate dating method
• Fetal growth restriction (FGR) — EFW or abdominal circumference below the 10th percentile for GA. AC reflects liver size and fat stores, so it falls early when nutrition is poor. Evaluation includes umbilical artery Doppler
• Anatomy ultrasound at about 18–22 weeks
• Fetal lung maturity: the L/S ratio ≥ 2:1 and presence of phosphatidylglycerol indicate mature lungs. These amniotic fluid tests are now rarely used — timing of delivery is based on gestational age and maternal–fetal indications, and antenatal corticosteroids are given to accelerate lung maturity when preterm birth is expected
• Maternal serum alpha-fetoprotein (MSAFP) — raised with open neural tube defects

Promoting healthy development

• Folic acid 0.4–0.8 mg (400–800 mcg) daily, starting at least 1 month before conception and continuing through the first 2–3 months, to reduce neural tube defects (anencephaly, spina bifida). A client who had a previous NTD-affected pregnancy needs a higher dose (commonly 4 mg daily) as prescribed
• Avoid known teratogens, especially during organogenesis:

Listed in priority order.

1. Identify fetal compromise — abnormal FHR, reduced fetal movement, fundal height lagging behind dates; report promptly
2. Screen for teratogen exposure — review all prescription and over-the-counter drugs, supplements, alcohol, tobacco, drugs, occupational hazards; coordinate safer alternatives before or early in pregnancy
3. Ensure preconception folic acid and accurate early dating
4. Support prenatal testing decisions with clear explanations of screening vs diagnostic tests

• Start folic acid before conception; take a prenatal vitamin
• No alcohol at any time in pregnancy; stop smoking and vaping
• Check with a clinician or pharmacist before taking any medicine or herbal product
• Food safety (toxoplasmosis, listeria): cook meat thoroughly, avoid unpasteurized dairy, wash produce; have someone else change cat litter
• Learn fetal development milestones and when to expect movement; report decreased movement later in pregnancy

• Embryonic period (weeks 3–8 after fertilization) = organogenesis = greatest teratogen risk
• Fertilization in the ampulla; implantation about 6–7 days later
• Heart beats by about 5–6 weeks GA; fingers and toes separate by the end of the embryonic period
• Quickening 16–20 weeks; fundus at umbilicus at 20 weeks
• Surfactant from about 24 weeks; L/S ratio 2:1 = mature lungs (test now rarely used)
• Cord = 2 arteries (deoxygenated) + 1 vein (oxygenated)
• Ductus arteriosus = pulmonary artery → aorta; foramen ovale = right → left atrium; ductus venosus bypasses the liver
• HbF has higher oxygen affinity; replaced by HbA after birth
• Fetal urine is the main source of amniotic fluid after mid-pregnancy
• FGR = EFW or AC below the 10th percentile
• Normal FHR 110–160/min; above 160 for 10 minutes = tachycardia
• Folic acid 0.4–0.8 mg daily from at least 1 month before conception prevents NTDs