Obstetrics and Gynaecology: Maternal Physiology Frames Everything
Obstetrics and gynaecology items test physiological adaptation to pregnancy, antenatal screening, hypertensive disease, labour and delivery, postpartum haemorrhage, contraception, and gynaecological malignancy. The single most useful habit is to interpret findings against pregnancy-specific normal ranges: blood pressure falls in mid-pregnancy, plasma volume rises by nearly half, and fibrinogen and D-dimer are physiologically elevated.
Core rules
- Hypertension in pregnancy. Pre-eclampsia is new hypertension after 20 weeks with proteinuria or maternal organ dysfunction. Severe features include blood pressure ≥160/110, platelets <100 × 10⁹/L, transaminases over twice normal, creatinine rise, pulmonary oedema, and neurological symptoms.
- Magnesium sulfate. Used for seizure prophylaxis and treatment of eclampsia, not as an antihypertensive; monitor reflexes, respiratory rate and urine output.
- Antihypertensives. Labetalol, hydralazine and nifedipine are standard; ACE inhibitors and angiotensin receptor blockers are contraindicated.
- Postpartum haemorrhage. Think tone, trauma, tissue and thrombin. Uterine atony is commonest; give oxytocin and tranexamic acid within 3 hours.
- Contraception. Combined hormonal methods are avoided with migraine with aura, uncontrolled hypertension, smoking over age 35, and prior venous thromboembolism.
Worked example
A woman at 33 weeks has blood pressure 162/110 mmHg, headache, protein-to-creatinine ratio 0.4, platelets 95 × 10⁹/L and AST 90 units/L. This is pre-eclampsia with severe features, with haematological and hepatic involvement. Admit, start magnesium sulfate with a 4–6 g loading dose followed by 1–2 g/h, give labetalol to keep blood pressure below 160/110, give antenatal corticosteroids, and plan delivery after 34 weeks or sooner if mother or fetus deteriorates. The reasoning: severity features determine timing, magnesium prevents seizures, and delivery is the only cure.
Traps that cost marks
- Using magnesium sulfate to lower blood pressure rather than to prevent seizures.
- Prescribing an ACE inhibitor for postpartum hypertension in a breastfeeding mother without checking suitability.
- Missing HELLP syndrome because the blood pressure is only moderately raised.
- Treating every postpartum haemorrhage with hysterectomy before trying uterotonics and uterine massage.