Pregnancy Physiology
Clinical Overview
Start with one idea and let everything hang off it: pregnancy is the maternal body running a second, growing organ — the fetoplacental unit — and every adaptation exists to feed that organ, defend it, then deliver it safely. Once you see each change as serving supply (deliver oxygen, glucose and nutrients to the fetus), protection (tolerate a half-foreign graft, store reserve against blood loss) or delivery (prime the uterus and clotting system for separation of the placenta), the long list of "pregnancy changes" stops being a list to memorise and becomes a logic you can rebuild from scratch in an exam.
From that single principle the whole physiology follows. To raise supply, blood volume expands, the heart pumps more, vessels relax so flow reaches the placenta, the kidney filters faster, and the mother becomes insulin-resistant so glucose floats toward the fetus. To protect, clotting is dialled up so placental separation does not exsanguinate her, and the immune system is re-tuned rather than switched off. To prepare for delivery, the uterus holds the pregnancy quietly for months and then, on cue, converts itself into a coordinated, powerful pump while the cervix softens out of the way. A normal pregnancy is therefore not "a non-pregnant body with a fetus added" — it is a whole-system reconfiguration, and that is exactly why disease in pregnancy reads differently.
The second idea, which the rest of this chapter keeps returning to, is the clinical consequence of the first: these adaptations both reduce reserve and disguise early illness, so non-pregnant reference values mislead you. A creatinine that looks normal outside pregnancy may signal renal impairment because the GFR should be higher. A "normal" PaCO2 may be ventilatory failure because the pregnant baseline is lower. Mild tachycardia can be physiological, yet persistent tachycardia in a bleeding, febrile or breathless woman is a warning sign. Oedema is often benign, but pulmonary oedema follows well-intentioned fluid loading in pre-eclampsia because oncotic pressure is low and capillaries leak. And labour is never "just oxytocin" — it is the final common pathway of fetal maturity, placental endocrine timing, inflammatory activation, cervical remodelling and coordinated myometrial coupling. Hold the supply–protect–deliver frame in mind and read on; each section names an adaptation, then shows where it can hurt you.
A handful of mechanistic chains carry most of the chapter. Learn the arrows, not just the endpoints:
Trophoblast implantation -> beta-hCG secretion -> corpus luteum rescue -> progesterone support -> decidual stability and uterine quiescence.
Progesterone -> increased respiratory drive -> lower PaCO2 -> renal bicarbonate loss -> mild compensated respiratory alkalosis -> a non-pregnant "normal" CO2 can be abnormal.
Placental and maternal vasodilatation -> lower systemic vascular resistance -> higher cardiac output -> mid-pregnancy BP fall -> cardiac disease may decompensate despite lower afterload.
Functional progesterone withdrawal plus prostaglandins plus oxytocin receptor upregulation -> coordinated myometrial contractions -> cervical stretch -> Ferguson reflex -> stronger contractions.
Core Knowledge
We build from the inside out. First the organ that drives everything (the maternal-fetal-placental unit), then the hormones it secretes, then — system by system — the maternal adaptations those hormones produce, and finally the endgame of parturition and the puerperium. Each section assumes only what came before, so read them in order on a first pass.
Maternal-Fetal-Placental Unit
The placenta is the new organ at the centre of the supply–protect–deliver story. It is simultaneously endocrine (it makes the hormones that drive maternal adaptation), transport (gases, nutrients and waste cross it), immune (it sits at the interface between fetal antigen and the maternal immune system) and vascular (it carves a low-resistance, high-flow bed out of the maternal circulation). It anchors the pregnancy through trophoblast invasion, remodels spiral arteries, exchanges substrates, secretes hormones and metabolises some drugs and hormones. Crucially, the fetus is not a passenger: fetal adrenal, hepatic and placental pathways together run steroid production, growth signalling and the timing of labour. The shorthand for this partnership is the fetoplacental unit — neither side has the full enzymatic toolkit, so they share the work.
| Component | Physiological role | O&G consequence |
|---|---|---|
| Syncytiotrophoblast | hCG secretion, nutrient/gas exchange surface, hormone production | Pregnancy tests, trophoblastic disease markers, placental endocrine function |
| Cytotrophoblast/extravillous trophoblast | Invasion and spiral artery remodelling | Abnormal placentation, pre-eclampsia and fetal growth restriction concepts |
| Decidua | Maternal endometrium transformed by progesterone | Implantation support and immune regulation |
| Fetal adrenal | DHEAS precursor for placental oestrogen synthesis | Fetal-placental steroid unit |
| Maternal circulation | Delivers oxygen/nutrients and removes waste | Maternal shock rapidly threatens fetal oxygenation |
Placental exchange depends on maternal perfusion, fetal perfusion, membrane thickness, surface area, concentration gradients, transporters and protein binding. Oxygen crosses down a gradient, aided by fetal haemoglobin's higher affinity. Glucose uses facilitated diffusion and follows maternal concentration; amino acids use active transport; IgG crosses via receptor-mediated mechanisms, especially later in pregnancy. This is why maternal hypoxia, severe anaemia, hypotension, sepsis and diabetes all have fetal implications through different mechanisms.
The "shared toolkit" idea explains why fetal wellbeing and placental hormone output are linked. Steroid synthesis is split: both placenta and fetus make progesterone from cholesterol, but only the fetus efficiently converts it onward to the androgen precursors (carried in an inactivated, sulphated form, which protects the fetus from premature exposure to active androgens). The placenta then uses sulfatase to unmask those precursors and aromatase to convert most of them into oestrogens, which it releases into the maternal circulation. The headline consequences are worth holding: the placenta becomes the dominant source of both progesterone and oestrogen after the first trimester, and a healthy fetal adrenal is required for normal placental oestrogen output. Anencephaly and placental sulfatase deficiency are the classic illustrations — low maternal oestriol — but the teaching point for the Primary is the principle, not a specific assay: maternal steroid levels are a joint product of fetal, placental and maternal pathways.
The placenta also surrounds the fetus in amniotic fluid, an actively regulated medium rather than stagnant water. In late pregnancy it is largely fetal urine, balanced against fetal swallowing and lung-fluid secretion, with exchange across the membranes. That turnover is why liquor volume is a window onto fetal urine output and swallowing: oligohydramnios points toward reduced fetal perfusion or renal/urinary-tract problems (or membrane rupture), while polyhydramnios points toward impaired swallowing or high fetal output. You do not need the numbers here — only the principle that liquor is a dynamic fetal signal.
Pregnancy Hormones and Beta-hCG
Early pregnancy is rescued by beta-hCG, a glycoprotein produced by syncytiotrophoblast after implantation. hCG acts like LH at the corpus luteum, maintaining progesterone until placental steroidogenesis is adequate. It becomes detectable in maternal blood before the missed period, rises rapidly in early pregnancy, peaks around the end of the first trimester, and then declines to a lower plateau.