Pregnancy

Gestational age is counted from the first day of the last menstrual period (LMP) — ≈2 weeks before conception — giving ≈40 weeks (280 days); the expected date of delivery is LMP + 9 months + 7 days (Naegele rule), refined by the crown–rump length on a first-trimester scan. Term is 37+0 to 41+6 weeks; birth before 37 weeks is preterm.

First trimester (to 13+6 weeks): implantation, the embryonic period (weeks 3–8 after fertilisation) of gastrulation and organogenesis — the period of maximal sensitivity to teratogens — and the start of the fetal period. A heartbeat is visible on transvaginal scan at ≈6 weeks. Nausea and vomiting peak at ≈9 weeks with hCG. Most miscarriages occur now. Combined screening (nuchal translucency, hCG, PAPP-A) is offered at 11–14 weeks. Second trimester (14–27+6 weeks): rapid growth; the anomaly scan at ≈18–21 weeks; fetal movements first felt at ≈18–20 weeks; the fundus reaches the umbilicus at ≈20 weeks; viability is reached around 22–24 weeks; screening for gestational diabetes at 24–28 weeks. Third trimester (28 weeks to birth): fat deposition and weight gain, maturation of the lungs (surfactant from ≈24 weeks, usually adequate by ≈34–36 weeks), descent of the head (engagement).

Maternal adaptations: plasma volume rises ≈40–50 % (more than red-cell mass → physiological dilutional anaemia); cardiac output rises 30–50 % with lower systemic vascular resistance and blood pressure in mid-pregnancy; the supine gravid uterus compresses the IVC (aortocaval compression); coagulation factors and fibrinogen rise (4–5-fold VTE risk, highest postpartum); tidal volume rises (progesterone) causing compensated respiratory alkalosis; GFR rises ≈50 % (lower creatinine) with physiological dilatation of the ureters; insulin resistance develops in the second half; TBG, CBG and prolactin rise; skin pigmentation increases (linea nigra, melasma).