Hormonal changes
Progesterone, synthesized by the corpus luteum until 35 post-conception days and by the placenta mainly thereafter, it decreases smooth muscle excitability (uterus, gut, ureters) and raises body temperature. Oestrogens (90% oestriol) increase breast and nipple growth, water retention and protein synthesis. The maternal thyroid often enlarges due to increased colloid production. Thyroxine levels, see p 25. Pituitary secretion of prolactin rises throughout pregnancy. Maternal cortisol output is increased but unbound levels remain constant.
Genital changes
The 100g non-pregnant uterus weighs 1100g by term. Muscle hyperplasia occurs up to 20 weeks, with stretching after that. The cervix may develop ectropion (‘erosions’). Late in pregnancy cervical collagen reduces. Vaginal discharge increases due to cervical ectopy, cell desquamation, and ↑ mucus production from a vasocongested vagina.
Haemodynamic changes
Blood
From 10 weeks the plasma volume rises until 32 weeks when it is 3.8 litres (50% >non-pregnant). Red cell volume rises from 1.4 litres when non-pregnant to 1.64 litres at term if iron supplements not taken (↑18%), or 1.8 litres at term (↑30%) if supplements are taken—hence Hb falls due to dilution (physiological ‘anaemia’). WCC (mean 10.5 × 109/L), platelets, ESR (up 4-fold), cholesterol, β-globulin, and fibrinogen are raised. Albumin and gamma-globulin fall.
Cardiovascular
Cardiac output rises from 5 litres/min to 6.5–7 litres/min in the first 10 weeks by increasing stroke volume (10%) and pulse rate (by ~15 beats/min). Peripheral resistance falls (due to hormonal changes). BP, particularly diastolic, falls during the first and second trimesters by 10–20mmHg, then rises to non-pregnant levels by term. With increased venous distensibility, and raised venous pressure (as occurs with any pelvic mass), varicose veins may form. Vasodilatation and hypotension stimulates renin and angiotensin release—an important feature of BP regulation in pregnancy.
Other changes
Ventilation increases 40% (tidal volume rises from 500 to 700mL), the increased depth of breath being a progesterone effect. O2 consumption increases only 20%. Breathlessness is common as maternal PaCO2 is set lower to allow the fetus to offload CO2. Gut motility is reduced, resulting in constipation, delayed gastric emptying, and with a lax cardiac sphincter, heartburn. Renal size increases by ~1cm in length during pregnancy.
Frequency of micturition emerges early (glomerular filtration rate↑ by 60%), later from bladder pressure by the fetal head. The bladder muscle is lax but residual urine after micturition is not normally present. Skin pigmentation (eg in linea nigra, nipples, or as chloasma—brown patches of pigmentation seen especially on the face), palmar erythema, spider naevi, and striae are common. Hair shedding from the head is reduced in pregnancy but the extra hairs are shed in the puerperium.
Pregnancy tests
Positive eg from two weeks post-conception (or from the first day of the first missed period), until ~20 weeks of pregnancy, they remain positive for ~5 days after abortion or fetal death. Otherwise, the false +ve rate is low. They detect the Ã’-subunit of human chorionic gonadotrophin in early morning urine, so are positive in trophoblastic disease (p 264).
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