Fertilisation usually occurs in the fallopian tube about twelve days after the start of menstruation, and implantation into the lining of the womb a few days later. At fertilisation the chromosomes contained in the sperm and the egg fuse and produce one single cell. This divides to produce two, the two divide into four and so on, until the single cell becomes millions of very different cells with different functions, which eventually becomes a being capable of producing, in turn, progeny of its own. At the time of implantation there are two cell types in the embryo, the outer of which form the placenta and the amniotic sac that will surround the baby, and the inner of which form the true embryo.

In the first few weeks the developing embryo produces a hormone - called human chorionic gonadotrophin - whose function is to support the corpus luteum, the yellow body left in the ovary from which the egg has emerged. The corpus luteum produces progesterone which supports the developing embryo until the placenta is large enough to produce enough progesterone, at round about 20 weeks. Human chorionic gonadotrophin can be detected in the blood by very sensitive tests in the early stages of gestation, and in the urine at about the time of first missed period. This is the basis of most pregnancy tests.

Most of the rudimentary organ systems are in place by about day 40 of the pregnancy - the primitive heartbeat can be detected by sensitive ultrasound around about this stage - though the embryo is hardly more than one on a half centimetres in length. By the end of the 10th week, when the embryo becomes a foetus, it has clear human features with eyes, external genitalia and rudimentary limbs. Between now and birth, which occurs 40 weeks after the date of the last menstrual period, there is rapid maturation and growth of all body systems. By 12 weeks the foetal heartbeat can be heard through the abdominal wall using a heartbeat monitor. Movements, which have been present from the start of the foetal period, are felt by the mother at 18 to 20 weeks (it's called quickening). In the later stages of pregnancy foetal movements are a reassuring indicator of foetal wellbeing.

During pregnancy the uterus enlarges to accommodate the developing foetus. The normal nonpregnant uterus lies deep in the pelvis and weighs about 50 grams. It grows along with the developing infant so that by 12 weeks it can be felt just above the pubic bone at the bottom of the abdomen; by 20 weeks it's at the level of the umbilicus, while by 36 weeks it’s up against the ribs. By the end of pregnancy the uterus will be weighing about a kilogram.

The placenta performs a number of functions. It is the organ of respiration, supplying oxygen and removing carbon dioxide, it provides nutrition to the foetus and it excretes the waste products produced during development. It also plays a part in preventing the mother from rejecting the baby - which is, after all, foreign to her, being of different genetic material. At birth it weighs about one-seventh of the weight of the baby.

There are a number of changes which occur in the mother throughout the pregnancy. There is increased fluid retention and an increase in the circulating blood volume, which produces a relative anaemia, called the physiological anaemia of pregnancy. The heart beats more strongly and more quickly, and the blood pressure drops as a result of the blood vessels becoming dilated. There is an increase in the amount of oxygen transferred during respiration, with an increase in the maternal effort required, which gives a subjective sensation of shortness of breath. The increase in size of the foetus, the placenta and the uterus, together with the retained fluid and an increase in fat stores are responsible for weight gain throughout the nine months. It is recommended that women who are of normal weight at the start of pregnancy should aim to gain no more than 12-15 kilograms. For those who start off overweight it might be better to aim for the lower end, since being too big during your pregnancy can put you at risk of complications, and make it more difficult to shed the weight afterwards.

Hormonal changes affect different organs, causing for example the breasts to enlarge and the nipples to darken. Developing aversions to or cravings for certain foods are commonplace.

The progress of a pregnancy is monitored during antenatal care, which in the uncomplicated case is provided mainly by midwives, with medical attention or intervention not necessarily being required. The tests required in the early stages are measurement of haemoglobin and glucose, the determination of the blood group, and the detection of any antibodies either to blood group constituents (A, B or O) or the so-called Rhesus factor, which we either do or do not possess - so we’re either Rhesus positive or negative - and urine examination to exclude infection. At each antenatal visit blood pressure is measured, the urine tested for the presence of glucose and protein, and foetal development assessed by examination of the height of the uterus after the 12th week. There are additional screening tests available: the so-called triple test measures the levels of three hormones - Alpha foeto-protein, human chorionic gonadotrophin and oestriol, which can give an indication of the likelihood that a foetus is affected by either Down's syndrome or a neural tube defect. Testing for hepatitis B and H I V is also offered. Appropriate discussion of the implications of these tests is necessary, since they are not without problems of their own. Ultrasound examination is usually performed at an appropriate stage of development (about the 18th week of pregnancy) to exclude foetal abnormality. In many centres there is the assessment of the ability to metabolise glucose, called a glucose tolerance test, for all women in order to exclude incipient Diabetes Mellitus which can affect the development of the foetus; in others it is only performed if there is a history that puts the woman at risk. Further blood tests are performed at roundabout 28 weeks and 36 weeks to check on the level of haemoglobin and to ensure that there has been no development of antibodies.

The frequency of antenatal visits is a matter of some controversy; the recommendation that they should be monthly until 28 weeks, fortnightly until 36 weeks and then weekly thereafter is thought by some to be too many. Most are agreed however that there should be close monitoring towards the end of the pregnancy, and that it is helpful for the mother to monitor the movements of the baby by counting the number of times it kicks: reduced foetal movements may indicate a foetus at risk. All are agreed that too little antenatal care is more dangerous than too much.

Nutritional requirements during pregnancy increase both as a result of the demands of the foetus and as a result of the increased metabolic rate, so that the mother requires an extra 300 kilo calories per day, of which 60 grams should be protein. Mineral and vitamin supplements, particularly Folic acid, are recommended by some, but care should be taken with regard to the intake of vitamin A, since an excess of this has been associated with the development of foetal abnormalities (so some authorities advise that pregnant women should avoid eating liver, which is a potent source of vitamin A). In view of the fact that unpasteurised milk products can contain the infectious agent listeria, which can cause miscarriage, they might best be avoided.

Labour is the process by which the uterus develops co-ordinated and regular contractions and the baby is delivered. Vaginal delivery consists of three stages of labour: during the first stage the cervix, the entrance to the womb which protrudes into the vagina, dilates from one centimetre to 10 centimetres, a process taking about 10 to 12 hours for a first delivery (rather less for a second or subsequent child). The second stage of labour consists of the delivery of the child, and lasts an hour for the first child, and less in subsequent pregnancies. The third stage of labour consists of the delivery of the placenta, and lasts about 30 minutes or less. Uterine contractions occur under the influence of a hormone called oxytocin; the changes in the cervix are influenced by another hormone called prostaglandin. During normal delivery, where the presenting part of the baby is its head, the passage down the birth canal is aided by the process of moulding of the foetal skull. The bones of the skull consist of the frontal bone at the front, the occipital bone at the back, and two parietal bones at either side. There are places where the bones are deficient, and replaced by a membrane which is flexible, called the fontanelles, one towards front of the skull and one towards the back which is smaller. The flexibility provided by these allows the bones of the skull to move relative to each other as the head passes through the pelvis.

The delivery of the head is controlled by the midwife because fast - or precipitate - delivery, involving rapid movement of the skull bones with respect to each other, can cause bleeding in the brain. Once the head has been delivered the shoulders and the rest of the body usually follow rapidly, though it is wise to ensure that the umbilical cord is not tightly wrapped around the baby's neck - if it is it may need to be clamped and divided. The baby usually takes its first breath in a scream of protest at its sudden exposure to the harsh light and cold of reality, and in response to the stimulus provided by the use of a suction catheter to clear the nostrils and mouth of amniotic fluid and secretions in which it has been bathing for the past months. (The scream is, of course, the most welcome sound in the world to all concerned, not least those responsible for the delivery).

Lactation is the production of milk by the breasts. Towards the end of pregnancy, and for the first two or three days after birth the breasts secrete a milky fluid called colostrum, which is rich in antibodies. During pregnancy the placenta has secreted a hormone called lactogen, which inhibits the production of prolactin by the pituitary gland. Without the placenta, and stimulated by the infant’s suckling, there is an increase in the levels of both prolactin and oxytocin, and the breasts begin to produce and secrete milk, becoming engorged by about the third day. During establish breast-feeding a mother will produce between 600 and 900 millilitres of milk a day, a process requiring 300 kilo calories of energy and adequate fluid and calcium intake.

After birth the baby loses up to 10 per cent of its birth weight, a loss that is normally made up during the first week or so, after which there is usually rapid weight gain. Babies that fail to regain their birth weight, or that do not gain weight satisfactorily are said to be failing to thrive. There may be many causes (described in the section on paediatrics) but commonly there is inadequate production of breast milk. There is no doubt that for babies breast is best, but not if it is insufficient, and a mother should feel no guilt at turning to the bottle.