The placenta, pregnancy, birth and twins

1Living inside another body

For about nine months, a fetus lives in fluid inside the uterus. It cannot breathe air, eat or get rid of its own wastes, yet it grows from a single cell into a baby of about 3 kg. Everything it needs passes through one temporary organ, the placenta, built partly from the embryo's tissue and partly from the mother's. This lesson explains how the placenta works, how the mother's body changes during pregnancy, how birth is controlled, how parents care for the newborn, and how twins arise.

2The structure of the placenta

The placenta is suited to exchange in the same ways as other exchange surfaces:

• the branched chorionic villi give a very large surface area;
• the barrier between the two bloods is thin, only a few cell layers, and becomes thinner as pregnancy goes on;
• maternal blood flows continuously through the spaces, and fetal blood through the villi, keeping steep concentration gradients;
• exchange uses diffusion (O₂, CO₂), facilitated diffusion (glucose), active transport (amino acids, some ions) and pinocytosis (antibodies).

Fetal haemoglobin has a higher affinity for oxygen than adult haemoglobin, so oxygen passes from the mother's haemoglobin to the fetus's even at the low partial pressures in the placenta.

What the barrier cannot stop. Alcohol, nicotine and carbon monoxide from smoking, many medicines and drugs, and some pathogens, such as the rubella virus and HIV, can cross the placenta. This is why pregnant women are advised to avoid alcohol and tobacco, to take medicines only on medical advice, and to be tested and treated for HIV so that it is not passed to the baby.

3Changes during pregnancy and antenatal care

Regular check-ups during pregnancy (antenatal care) detect problems such as high blood pressure, anaemia and infections early. The World Health Organization recommends a minimum of eight antenatal contacts with a health worker during each pregnancy.

4Birth and parental care

Near the end of pregnancy, the placenta's secretion of progesterone falls relative to oestrogen, so the muscle of the uterus becomes more sensitive to oxytocin. Contractions push the baby's head against the cervix; this sends impulses to the hypothalamus, so more oxytocin is released from the posterior pituitary, which causes stronger contractions: positive feedback (unit 7). Birth has three stages: the cervix widens (dilates); the baby is pushed out through the vagina; and the placenta and membranes, the "afterbirth", are delivered.

Human babies depend completely on their parents for many years, longer than the young of any other species. Parental care includes feeding, warmth, protection from danger, health care such as vaccination, and teaching and affection, which are needed for the child's emotional and intellectual development. Both parents share these responsibilities.

• Lactation: after birth, the fall in progesterone lets prolactin from the anterior pituitary stimulate milk production. A baby's suckling sends impulses to the hypothalamus, which causes oxytocin release and the ejection ("let-down") of milk: a neuroendocrine reflex.
• The first milk, colostrum, is rich in antibodies.
• The WHO recommends that infants be exclusively breastfed for the first 6 months of life, meaning no other foods or liquids, not even water, and then given safe complementary foods while breastfeeding continues.

5Twins and multiple births

Triplets and larger multiple births arise in the same ways, alone or combined. If an early embryo splits incompletely, the twins remain joined (conjoined twins), which is very rare. Fertility drugs that stimulate the ovaries, and IVF when more than one embryo is transferred (lesson 5), increase the chance of non-identical twins. Multiple pregnancies carry greater risks, such as premature birth, so they need closer antenatal care.