pregnancy

The development of a human embryo. Division of the fertilized egg, or ovum, begins within hours of conception. Within a week a ball of cells – a blastocyst – has developed. After the third week, the embryo has changed from a mass of cells into a recognizable shape. At four weeks, the embryo is 3 mm/0.1 in long, with a large bulge for the heart and small pits for the ears. At six weeks, the embryo is 1.5 cm/0.6 in, with a pulsating heart and ear flaps. At the eighth week, the embryo is 2.5 cm/1 in long and recognizably human, with eyelids, small fingers, and toes. From the end of the second month, the embryo is almost fully formed and further development is mainly by growth. After this stage, the embryo is termed a fetus.

Ultrasound scan of a 12 week-old fetus showing (from right) the head, an arm, the body, and a foot. Such scans are often taken at around the twelfth week of pregnancy to determine whether any obvious developmental abnormalities have occurred.

Structure of the human placenta. The fetal blood vessels attach to the placenta through the umbilical cord. The mother's blood supply runs through the placenta, which supplies the fetus with nutrients and removes waste products from the fetal blood.

In humans, the process during which a developing embryo grows within the woman's womb. It begins at conception and ends at birth, and the normal length is 40 weeks, or around nine months.

Menstruation usually stops on conception. About one in five pregnancies fails, but most of these failures occur very early on, so the woman may notice only that her period is late. After the second month, the breasts become tender, and the areas round the nipples become darker. Enlargement of the uterus can be felt at about the end of the third month, and after this the abdomen enlarges progressively. Fetal movement can be felt at about 18 weeks; a heartbeat may be heard during the sixth month. Pregnancy in animals is called gestation.

Occasionally the fertilized egg implants not in the womb but in the Fallopian tube (the tube between the ovary and the uterus), leading to an ectopic (‘out of place’) pregnancy. This will cause the woman severe abdominal pain and vaginal bleeding. If the growing fetus ruptures the tube, life-threatening shock may ensue. Toxaemia is characterized by rising blood pressure and, if left untreated, can result in convulsions leading to coma.

According to 2005 World Health Organization (WHO) figures, 530,000 women die annually from preventable pregnancy-related causes; 99% of these deaths occur in developing countries.