Pregnancy, parturition and lactation

The essentials

After implantation, finger-like projections called chorionic villi appear on the trophoblast and interlock with uterine tissue. Together they form the placenta, the structural and functional unit between the developing embryo (foetus) and the mother's body. The placenta supplies oxygen and nutrients, removes carbon dioxide and wastes, and is connected to the embryo by the umbilical cord. It also acts as an endocrine tissue and secretes hCG (human chorionic gonadotropin), hPL (human placental lactogen), oestrogens and progestogens. In later pregnancy, the ovary secretes relaxin. Levels of oestrogens, progestogens, cortisol, prolactin and thyroxine also rise in the mother.

The inner cell mass differentiates into ectoderm, mesoderm and endoderm, which give rise to all tissues. Key landmarks in human development: the heart forms after about one month; limbs and digits by the end of the second month; most major organ systems and external genitalia are well developed by the end of 12 weeks (first trimester); the first movements and hair on the head usually appear in the fifth month; eyelids separate and eyelashes form by the end of about 24 weeks (second trimester). Average human pregnancy lasts about nine months.

Parturition (childbirth) is induced by a complex neuroendocrine mechanism. Signals from the fully developed foetus and the placenta cause mild uterine contractions, the foetal ejection reflex. This triggers release of oxytocin from the maternal pituitary, which causes stronger contractions, which in turn cause more oxytocin release.

Key points

• Lactation: mammary glands produce milk towards the end of pregnancy.
• The first milk, colostrum, contains several antibodies (IgA) that protect the newborn.
• Placenta allows exchange but keeps maternal and foetal blood largely separate.

Exam tips

Remember which hormones are only made in pregnancy (hCG, hPL, relaxin).