Pregnancy

From puberty, the female body goes through a menstrual cycle of about 28 days, over and over, which prepares the uterus for a fertilised egg. Day 1 is the first day of a period.

Days	What happens
1 to about 5	The period: the lining of the uterus breaks down and leaves the body through the vagina, with some blood
About 6 to 13	The lining builds up again, thick and rich in blood vessels
About 14	Ovulation: an egg cell is released from an ovary into the oviduct
About 15 to 28	The lining stays ready. If the egg is not fertilised, the lining breaks down and the next period starts: back to day 1

28 days is an average. Cycles vary from person to person and from month to month. The cycle stops during pregnancy.

Pregnancy. If the egg cell is fertilised in the oviduct, it divides into a ball of cells and settles into the thick lining of the uterus: implantation. For about the first eight weeks after fertilisation it is called an embryo, while its organs form; after that it is a foetus. It floats in amniotic fluid inside the amniotic sac, which cushions it, and is joined by the umbilical cord to the placenta.

The placenta grows in the wall of the uterus. The mother’s blood and the baby’s blood come very close in it but do not mix. Oxygen and food pass across from the mother’s blood to the baby’s; carbon dioxide and waste pass back. Harmful substances can cross too: nicotine and carbon monoxide from smoking mean the baby gets less oxygen and may be born small or early; alcohol can damage the baby’s development, especially the brain; some drugs and some infections can harm it as well. Doctors advise no smoking, no alcohol, a healthy diet and regular check-ups during pregnancy.

Gestation is the time a baby develops in the uterus, from fertilisation to birth. In humans it is about 9 months (about 38 weeks from fertilisation; doctors count 40 weeks from the start of the mother’s last period); other mammals differ: a hamster about 16 days, a dog about 9 weeks, an elephant about 22 months.

Birth. After about 40 weeks (nine months), the muscles of the uterus begin to contract (labour), the cervix opens, the amniotic sac breaks and the baby is pushed out through the vagina. The placenta is delivered afterwards (the afterbirth). Identical twins come from one fertilised egg that splits; non-identical twins come from two eggs fertilised at the same time.