Pregnancy and birth: how the body changes

Pregnancy begins with fertilisation in the ampulla of the fallopian tube, about a day after ovulation; implantation into the progesterone-primed secretory endometrium follows about a week later. The implanting embryo secretes human chorionic gonadotropin (hCG), which keeps the corpus luteum making progesterone so that the lining is not shed; it is the hormone pregnancy tests detect. Nausea and vomiting are common in the first trimester and tend to ease as hCG falls, although the cause is not fully understood. By about 10 to 12 weeks the placenta has taken over the production of progesterone and oestrogens. Pregnancy is dated from the first day of the last period, so birth is expected at about 40 weeks.

The placenta is both an exchange organ and an endocrine gland: maternal blood bathes the chorionic villi, so oxygen, glucose and antibodies cross to the fetus and waste passes back, without the circulations mixing. Blood volume rises by roughly 30 to 50%, plasma more than red cells, so the haemoglobin concentration falls (a physiological anaemia) as the need for iron rises, and cardiac output rises by about 30 to 50%. Progesterone relaxes vascular smooth muscle, so blood pressure falls in the first half of pregnancy and returns to its earlier level by term; a pressure that rises after 20 weeks, especially with protein in the urine, is the pattern of pre-eclampsia, so both are checked at every antenatal appointment.

Renal blood flow and filtration rise by about half, while the ureters dilate and drain more slowly, so urinary infection reaches the kidneys more easily. Placental hormones make maternal tissues less sensitive to insulin so that more glucose reaches the fetus; gestational diabetes develops when the pancreas cannot compensate, and a glucose tolerance test is offered to those with risk factors; treatment starts with changes to diet and physical activity, with metformin and then insulin added if blood glucose targets are not met. Relaxin softens ligaments, which lets the pelvis give a little in labour but produces pelvic girdle and back pain, and clotting factors rise while venous return slows, so venous thrombosis is more likely, above all after birth.

Labour is a positive feedback loop: the myometrium gains oxytocin receptors near term, prostaglandins efface the cervix, and pressure of the baby's head releases oxytocin from the posterior pituitary, which strengthens the contractions until birth. Labour has three stages: dilatation of the cervix, the birth itself, and delivery of the placenta, which ends with the uterus clamping its spiral arteries to limit bleeding. Prolactin then makes milk and oxytocin ejects it, the uterus involutes over about six weeks, and pelvic floor muscle training is advised. The same physiology applies to anyone with a uterus and ovaries, including trans men and non-binary people. Some changes are warnings: severe headache, visual disturbance, pain below the ribs, sudden swelling of the face or hands, a painful calf, vaginal bleeding or reduced fetal movements mean contacting a midwife or maternity unit the same day, and chest pain, severe breathlessness, fainting or heavy bleeding need emergency care.

Key facts