Pregnancy in humans lasts an average of 280 days counting from the first day of the last menstrual period. Hormones play a crucial role in the course of the pregnancy and in the physical and psychological changes in the mother and child.

Pregnancy is not only a hormonally controlled biological event. It is also a time of psychological and social change. The life of the mother-to-be undergoes fundamental changes that affect her relationship and her career. She has to define her role as a mother. Her emotional response to the pregnancy will also depend to a significant extent on whether or not the expected child was planned.

The following sections outline briefly what happens during pregnancy, birth and breast-feeding.

Fertilization and implantation

After ovulation the egg is capable of being fertilized for 12 hours at the most. For fertilization to take place, the sperm must be present in the fallopian tube during this time. About six days elapse between fertilization and implantation of the egg in the uterine lining. During this time the egg undergoes repeated cell divisions, forming a ball of cells called the morula. The morula then develops an inner cavity, thereby becoming a blastocyst.

The blastocyst implants in the lining of the uterus. The action of progesterone leads to development of the so-called decidua, the endometrium of the pregnant uterus. The decidua is particularly well supplied with blood and provides a favorable environment for the further development of the embryo. It is not shed and therefore there is no menstruation. A missed period is thus one of the first signs of a pregnancy.

Physical changes in mother and child

The blastocyst develops into the embryo which is protected by the amniotic sac. This also contains the amniotic fluid (popularly called the 'waters') which provides a drinking and elimination reservoir for the fetus and also an environment in which it can move about freely. The amniotic sac is made up of two membranes, the amnion on the inside and the chorion on the outside.
The chorion develops finger-like projections (chorionic villi) which fuse with the uterine lining to form the placenta.

The placenta is the most important organ for the exchange of oxygen and nutrients between mother and child. The blood vessels contained in the chorionic villi join to create an increasingly large vascular system and finally form the umbilical cord. This carries oxygen- and nutrient rich maternal blood to the fetus and transports the waste products of the fetal organism to the maternal metabolic organs (lung, liver, kidney) for elimination.

The placenta also produces important hormones. In the first weeks it produces hCG (human chorionic gonadotropin). This hormone helps the corpus luteum of pregnancy to produce sufficient progesterone. Chorionic gonadotropin can already be detected in a woman's blood or urine 2-8 days after a missed period as a reliable pregnancy sign (hormonal pregnancy test).

But the placenta also produces progesterone, the predominant hormone of pregnancy. Progesterone causes growth of the muscular layer of the uterus and also reduces its contractility (prevention of premature labor). It also prepares the breast glands for their milk-producing function.

However, the effects of the sex hormones are not limited to the reproductive organs. They increase the blood supply to all parts of the body, which is the reason why pregnant women often have such a healthy complexion. On the other hand the hormonal changes also cause problems, particularly at the beginning of a pregnancy (e.g. constipation, accumulation of water in the legs).

Birth

The onset of regular painful contractions marks the beginning of the birth process. Labor pains are caused by the contraction of the strong uterine muscle. The sex hormones do not play a significant role in the initiation of labor. This task is performed by other hormones – particularly oxytocin and prostaglandins.

In the first stage of labor the force of the contractions pushes the baby's head into the pelvic inlet, the cervix dilates and the amniotic sac ruptures. In the second stage of labor the contractions become stronger, pushing the baby further downwards until it is finally delivered.

In the third stage of labor the placenta detaches and is expelled.

Breast-feeding

During pregnancy the sex hormones have already prepared the milk glands in the breasts for their milk-producing function. Milk production is stimulated by the hormone prolactin.
The colostrum, the first milk produced by the mother, is very important for the newborn baby as it is particularly rich in protein, fat-soluble vitamins and minerals. Colostrum and breast milk are the best sources of nutrition for the child. The main stimulus for increased milk production is suckling which triggers production not only of prolactin but also of the hormone oxytocin which is responsible for 'let-down', the flow of milk into the breasts. Early and frequent putting the child to the breast helps to increase milk production.

Return to the menstrual cycle

The occurrence of a pregnancy and the subsequent nursing period temporarily interrupt the menstrual cycle. Periods resume between 6 and 24 weeks after delivery. Before this, ovulation is rare but not impossible. It is therefore also possible to become pregnant during this phase.

After delivery the uterus has to return to its pre-pregnancy size and the wound left by the placenta must heal. The wound secretions cause a vaginal discharge known as lochia which lasts about 4 to 6 weeks. As long as this postpartum discharge continues scrupulous hygiene is important to avoid infections. The hospital will normally provide you with pads to absorb the lochia.
Menstruation usually starts again a few weeks after weaning. Often the intensity of bleeding changes after a pregnancy. Ask your doctor when you can start using tampons again.