Human pregnancy lasts ~40 weeks (280 days) from last menstrual period. Key developmental milestones: Month 1 (4 weeks): heart starts beating, neural tube forms, arm and leg buds appear. Month 2 (8 weeks): limbs and digits visible, all major organ systems forming, embryo becomes foetus. Month 3 (12 weeks): external genital organs differentiated, foetus looks human, kidneys begin urine production. Month 4 (16 weeks): skeletal ossification begins, meconium forming. Month 5 (20 weeks): quickening (first foetal movements felt), hair appears on head. Month 6 (24 weeks): lanugo covers body, eyelids separate, eyelashes form. Month 7 (28 weeks): eyes open, viability reached (can survive premature birth). Month 8-9: rapid weight gain, lung maturation, full term at 40 weeks.

The first trimester is the most critical period — all major organs form (organogenesis). Fertilisation occurs in the fallopian tube (ampullary region). Zygote undergoes cleavage → morula → blastocyst. Implantation in endometrium at 6-10 days. Inner cell mass → embryo. Trophoblast → placenta. Week 3: gastrulation → three germ layers form (ectoderm, mesoderm, endoderm). Week 4: heart beating, neural tube closing. Week 6-8: face forming, limbs with fingers and toes developing. Week 8: all major organ systems present in rudimentary form — embryo becomes foetus. Week 12: external genitalia differentiated. Most birth defects (teratogenesis) occur in this trimester when organs are forming. Teratogens (alcohol, drugs, infections like rubella) are most harmful in first trimester.

Second trimester: foetal growth and refinement of organs. Week 16: skeletal ossification starts (bones hardening), meconium (first faeces) begins forming. Week 18-20: quickening (mother feels movements for first time). Week 20: hair appears on head, vernix caseosa (white waxy coating) begins forming. Week 24: lanugo covers entire body, eyelids separate (previously fused), eyelashes start forming. Week 26: eyes fully open, lungs begin producing surfactant (essential for breathing). Mother feels strong movements regularly after 20 weeks. Foetal heartbeat audible with stethoscope (Doppler from 10-12 weeks). Quickening is earlier in subsequent pregnancies (16-18 weeks) than first (18-20 weeks).

Third trimester: rapid growth and final maturation. Week 28: viability — premature baby can survive with intensive neonatal care. Week 28-32: rapid weight gain (most weight gained here). Week 32-36: lanugo begins to shed, replaced by vellus hair. Week 36: lungs mature (sufficient surfactant for breathing). Week 37+: term pregnancy. Week 40: full term, average weight 3-3.5 kg. Foetus turns head-down (cephalic presentation) in preparation for birth. Week 38-40: engagement (foetal head descends into pelvis). Birth weight premature vs full term: premature (<28 weeks) 700-1000g. Full term 2500-4000g. Low birth weight (<2500g) = risk factor for neonatal complications.

Placenta develops from trophoblast cells + maternal endometrium. Structure: chorionic villi project into maternal blood sinuses. Functions: Nutrition (glucose, amino acids, fatty acids, vitamins) transferred to foetus. Gas exchange (O2 from maternal blood to foetal blood, CO2 in reverse direction). Waste removal (urea, CO2 from foetus to mother). Hormone production: hCG (maintains corpus luteum → maintains pregnancy), progesterone, oestrogen, hPL (human placental lactogen). Immune protection: maternal IgG antibodies cross to foetus (passive immunity). The placenta provides a selective barrier — most bacteria cannot cross, but viruses (HIV, rubella, CMV, herpes), drugs, alcohol, and some chemicals can cross and harm the foetus. The barrier is NOT perfect — this is why pregnant women must avoid teratogens, drugs, alcohol, and tobacco.

Parturition (childbirth) is triggered by complex hormonal changes. Mechanism: rising foetal cortisol → placenta increases oestrogen relative to progesterone → oestrogen increases oxytocin receptors on uterus → oxytocin from posterior pituitary triggers uterine contractions → positive feedback (Ferguson reflex): contractions stimulate more oxytocin → stronger contractions → cervix dilates → baby is born. Three stages: Stage 1 (Dilation): contractions begin, cervix dilates from 0 to 10 cm (8-12 hours in first labour, shorter in subsequent). Stage 2 (Expulsion): pushing, baby born (30 min - 2 hours). Stage 3 (Placental): placenta expelled within 30 minutes of delivery. Oxytocin (Pitocin): synthetic form used to induce or augment labour in clinical settings.

hCG (human Chorionic Gonadotropin): secreted by trophoblast from day 8-10 post-fertilisation. Maintains corpus luteum → continuous progesterone production. Detected in urine by home pregnancy tests. Peak at 8-10 weeks then declines. Used in multiple marker screening (Down syndrome risk). Progesterone: initially from corpus luteum (maintained by hCG), then from placenta after 10 weeks. Maintains uterine lining, prevents premature contractions, promotes breast development. Oestrogens (especially oestriol): promote uterine growth, breast duct development, loosen pelvic ligaments. Relaxin: from corpus luteum → relaxes symphysis pubis and cervix. Prolactin: suppressed during pregnancy by high progesterone → rises after delivery → milk production. Oxytocin: triggers labour contractions and milk ejection.

Prenatal tests detect foetal abnormalities. Ultrasound: non-invasive, detects structural defects, measures growth, confirms gestational age and foetal position. First trimester NT scan (nuchal translucency, 11-14 weeks): screens for Down syndrome risk. Anatomy scan (18-20 weeks): detailed structural survey. Amniocentesis: 15-20 weeks, samples amniotic fluid (contains foetal cells). Used for karyotyping (detects Down syndrome, Edwards, Patau), biochemical analysis, DNA testing. CVS (chorionic villus sampling): 10-13 weeks, earlier than amniocentesis. NIPT (Non-Invasive Prenatal Testing): analyses cell-free foetal DNA from maternal blood from 10 weeks. Screens for trisomies 21, 18, 13 and sex chromosome abnormalities. Non-invasive (no miscarriage risk). In India: sex determination by any prenatal technique is ILLEGAL under PCPNDT Act 1994 to prevent female foeticide and sex-selective abortion.