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15.6 — Pregnancy
A pregnant woman's blood volume increases by 40 to 50 percent. Her cardiac output rises by the same. Her kidneys filter 50 percent more. Her oxygen consumption rises 20 percent, and her ligaments soften throughout her body.
None of this is a side effect. All of it is required, and every one of the discomforts of pregnancy is a direct consequence of a change that is doing something necessary.
Chapter 4.3 covered fertilisation and implantation. This chapter covers the 38 weeks that follow.
Dating
Pregnancy is dated from the first day of the last menstrual period, which is about two weeks before conception (Chapter 4.5).
So "40 weeks" contains a 38-week embryo and fetus, and the convention exists because the last period is a date a woman can report and ovulation usually is not.
Term is 37 to 42 weeks. Before 37 is preterm; after 42 is post-term.
And only about 4 percent of babies arrive on the estimated due date. It is the midpoint of a distribution, not an appointment, and describing it as such prevents a great deal of unnecessary anxiety in the final fortnight.
Dating by ultrasound in the first trimester is more accurate than by dates, because early embryos grow at a very consistent rate, and it is used to correct the estimate where they disagree.
The placenta
A fetal organ, made from the trophoblast (Chapter 4.3), reaching about 500 grams and 20 centimetres at term.
Its structure is a large surface presented to maternal blood. Fetal villi — branching projections containing fetal capillaries — sit in pools of maternal blood. Total villous surface area is around 12 to 14 square metres, in an organ the size of a dinner plate.
And the two circulations never mix. They are separated by a barrier a few micrometres thick — which is why blood group incompatibility only matters when cells cross at delivery or after trauma (Chapter 13.7).
The umbilical cord carries two arteries and one vein, and the naming is reversed relative to intuition: the arteries carry deoxygenated blood from the fetus to the placenta, and the vein carries oxygenated blood back — the same convention as the pulmonary vessels (Chapter 7.2).
A single umbilical artery, found in around 1 percent of pregnancies, is associated with a modest increase in other anomalies and prompts a careful scan.
What the placenta does:
Gas exchange — and fetal haemoglobin's higher affinity pulls oxygen across (Chapter 7.9).
Nutrient transfer — glucose by facilitated diffusion, amino acids by active transport against a gradient, and fatty acids more slowly.
Waste removal.
Hormone production, and it becomes one of the largest endocrine organs in the body (Chapter 12.7). hCG maintains the corpus luteum. Progesterone and oestrogen, taking over from the ovary at around 8 to 10 weeks. Human placental lactogen, which deliberately induces insulin resistance in the mother.
And that last one is worth understanding, because it explains gestational diabetes entirely.
The placenta makes the mother insulin resistant on purpose, so that her tissues take up less glucose and more remains in the blood for the fetus.
A healthy pancreas compensates by producing more insulin. A pancreas without adequate reserve cannot, and glucose rises — which is gestational diabetes.
So gestational diabetes is not caused by the pregnancy diet; it is the unmasking of a limited insulin reserve by a deliberate physiological challenge. Which is exactly why it identifies a woman at substantially raised lifetime risk of type 2 diabetes.
Immune protection — transferring IgG from around 20 weeks, most in the last trimester (Chapter 13.3).
And a barrier function that is partial. The old phrase "placental barrier" is misleading. Alcohol, nicotine, most drugs, many viruses and radiation all cross. The placenta filters some things and is not a wall.
Placental problems
Pre-eclampsia — and its cause traces back to implantation (Chapter 4.3).
In normal pregnancy, trophoblast invades the maternal spiral arteries and converts them into wide, low-resistance vessels that cannot constrict. In pre-eclampsia that conversion is incomplete, so the placenta is underperfused.
The stressed placenta releases factors into the maternal circulation that damage the lining of blood vessels throughout her body.
Which is why pre-eclampsia is a multi-system disease rather than a blood pressure problem. High blood pressure and protein in the urine are the markers; the disease affects liver, kidneys, brain, clotting and the placenta itself.
Warning signs that must be acted on: severe headache, visual disturbance — flashing lights or blurring — pain below the ribs on the right side, sudden swelling of the face and hands, and vomiting. These are taught to every pregnant woman for a reason: they can precede eclampsia — seizures — by hours.
Aspirin from 12 weeks reduces the risk substantially in women with risk factors, and it is one of the more effective preventive interventions in obstetrics.
The only definitive treatment is delivery, and management is a balance between maternal risk and fetal prematurity. Magnesium sulfate prevents and treats eclamptic seizures, and its adoption after the MAGPIE trial was a substantial global advance.
Placenta praevia — the placenta covering the cervix. Presents as painless bright red bleeding in the second half of pregnancy, and requires caesarean delivery.
Placental abruption — the placenta separating before delivery. Painful bleeding with a tense, tender uterus, and it is an emergency for both mother and baby. And the bleeding can be concealed, held behind the placenta, so the visible loss underestimates it.
Placenta accreta — the placenta invading too deeply, usually into a previous caesarean scar (Chapter 4.3). It will not separate at delivery and can cause catastrophic haemorrhage. Its incidence has risen with the caesarean rate, which is a genuine and cumulative consequence worth knowing.
The trimesters
First (weeks 1 to 12)
All organs form (Chapter 4.5), which is why this is the period of greatest vulnerability to teratogens (Chapter 4.6).
The heart beats from day 22. Limbs form. By 12 weeks the fetus is about 6 centimetres and recognisably human.
Maternal experience:
Nausea and vomiting affects 70 to 80 percent, typically from 6 to 12 weeks. "Morning sickness" is a poor name — it occurs at any time.
Its cause is not established; hCG is the leading suspect, supported by the fact that it is worse in twin and molar pregnancies where hCG is higher.
And there is a genuinely interesting observation: nausea in pregnancy is associated with a lower miscarriage rate. Whether it is protective or simply a marker of a robust placenta is unresolved.
Hyperemesis gravidarum is the severe form, affecting around 1 percent — vomiting causing weight loss, dehydration and electrolyte disturbance. It requires admission, fluids, antiemetics and thiamine (Chapter 1.6), and it is not "bad morning sickness" but a distinct and serious condition that has historically been dismissed.
Fatigue, often profound, from progesterone.
Breast tenderness and enlargement.
Urinary frequency, from the enlarging uterus and increased kidney filtration.
And miscarriage. Around 10 to 20 percent of recognised pregnancies, and probably 30 to 50 percent of all conceptions, most before the period is missed.
The overwhelming majority are caused by chromosomal abnormalities incompatible with development (Chapter 1.7), and nothing the woman did caused it or could have prevented it.
That sentence matters more than almost anything else in this chapter. Exercise, work, stress, sex and lifting do not cause miscarriage, and the guilt that follows is both universal and unfounded.
Recurrent miscarriage — three or more — affects around 1 percent and warrants investigation for chromosomal rearrangements in either partner (Chapter 2.7), antiphospholipid syndrome, uterine abnormalities and thyroid disease. Even after full investigation, around half have no identified cause — and the outlook for a subsequent successful pregnancy remains good.
Second (weeks 13 to 27)
Often the most comfortable period. Nausea usually settles; energy returns.
Fetal movements are felt from around 18 to 20 weeks in a first pregnancy, and 16 to 18 in later ones, because the woman recognises the sensation.
Growth is rapid. The anomaly scan at 18 to 22 weeks (Chapter 4.6).
Viability — around 22 to 24 weeks with intensive care, and survival rises steeply with each additional week.
Maternal changes:
Cardiovascular — blood volume up 40 to 50 percent, plasma rising more than red cells, which produces the "physiological anaemia of pregnancy": haemoglobin falls because the blood is diluted, not because iron is lacking. Distinguishing dilution from genuine iron deficiency matters, and it is done on the other red cell indices.
Cardiac output up 30 to 50 percent, and blood pressure actually falls in the middle trimester because the placenta is a low-resistance circuit.
Respiratory — tidal volume rises by around 40 percent, driven by progesterone. Which produces a mild respiratory alkalosis (Chapter 10.4), and a subjective breathlessness in around 70 percent of women that is entirely normal.
Gastrointestinal — progesterone relaxes smooth muscle, so reflux and constipation are near-universal, and gallstone risk rises because the gallbladder empties less efficiently (Chapter 9.5).
Musculoskeletal — relaxin softens ligaments, and the shifted centre of gravity increases the lumbar curve. Back pain and pelvic girdle pain are common and treatable with physiotherapy rather than being simply endured.
Skin — increased pigmentation of the nipples, the midline of the abdomen, and sometimes the face.
Third (weeks 28 to 40)
Weight gain accelerates; the fetus gains around 200 grams a week.
Lung maturation with surfactant production (Chapter 8.2), which is why steroids are given if preterm delivery is anticipated.
IgG transfer peaks.
Maternal changes: breathlessness from the uterus pressing on the diaphragm, worsening reflux, difficulty sleeping, swelling of the ankles, and Braxton Hicks contractions — irregular, painless tightenings that are practice rather than labour.
And there is a specific piece of advice about sleeping position that is worth knowing. From 28 weeks, sleeping on the back is associated with an increased risk of stillbirth, because the uterus compresses the inferior vena cava and reduces venous return.
Going to sleep on either side is the advice, and it has been the subject of national campaigns based on consistent observational evidence.
What antenatal care is looking for
And knowing what each check is for makes the appointments make sense.
Blood pressure and urine protein — pre-eclampsia.
Fundal height — the distance from the pubic bone to the top of the uterus, which in centimetres roughly equals the weeks of gestation from about 24 weeks. A discrepancy prompts a growth scan.
Fetal heart rate.
Blood tests at booking — blood group and antibodies, full blood count, hepatitis B, HIV, syphilis, rubella immunity.
And two of those deserve emphasis. Treating syphilis in pregnancy prevents congenital syphilis entirely (Chapter 4.6), and treating HIV reduces mother-to-child transmission from around 25 percent to under 1 percent — one of the most effective interventions in all of medicine.
Glucose tolerance testing at 24 to 28 weeks for those at risk.
Anti-D for Rh-negative women (Chapter 13.7).
Vaccination — pertussis, influenza and RSV, to transfer antibody to the fetus (Chapter 13.5).
Group B streptococcus — carried by 20 to 30 percent of women harmlessly, and a cause of severe neonatal infection. Managed by intrapartum antibiotics, with screening policies differing between countries.
Common questions, answered accurately
Alcohol — no safe level has been established, and the advice in most countries is none (Chapter 4.6).
Caffeine — limit to around 200 mg a day, roughly two cups of coffee.
Exercise — recommended, and it reduces gestational diabetes, excessive weight gain and back pain. Avoid contact sports, scuba diving and anything with a fall risk. The old advice to keep the heart rate below 140 has been abandoned; women who exercised before pregnancy can continue at similar intensity.
Food — avoid unpasteurised dairy and soft cheeses (listeria), undercooked meat (toxoplasma), high-mercury fish, and liver in large quantity (excess vitamin A).
Flying — safe until around 36 weeks, with increased clot risk on long flights.
Sex — safe in uncomplicated pregnancy.
Hair dye, nail varnish, and most cosmetics — no evidence of harm from normal use.
And medication. The reflex to stop everything is frequently the wrong one. Uncontrolled epilepsy, asthma, depression or thyroid disease is more dangerous to a pregnancy than most of the drugs treating them. The decision needs a specialist rather than a rule, and it is best made before conception rather than after (Chapter 4.6).
What the next page fixes
Chapter 15.7 covers labour and birth — the three stages, what actually happens mechanically, pain relief, caesarean section, and the emergencies that make obstetrics what it is.