Spermatozoa are inserted into the vagina at ejaculation

Aprrox 5x10⁸ sperm in one ejaculate

Sperm are motile, moving at 1mm per minute

Contractions of the uterus assist in propelling the sperm along.

Approx 100 sperm reach to ovum (usually in the final third of the oviduct)

One of these penetrates the ovum using the enzymes in its acrosome. This sets off a series of mechanisms which prevents the entry of the other sperms. It also stimulates the second meiotic division in the ovum

The fertilised egg, or zygote, continues to the uterus multiplying by mitosis as it goes.

The zygote is at the morula stage (16 cells) by the time it enters the uterus. Fluid from the uterus enters the morula and the cells rearrange themselves to form a blastocyst which implants into the endometrium

The blastocyst has two parts: a trophoblast which is a layer of cells (which develops into various structures external to the embryo) encompassing the fluid-filled region and an inner cell mass which will develop into the embryo.

Full implantation occurs about 10 days after fertilisation.

The endometrial tissue breaks down to form large blood-filed spaces called lacunae. Projections from the trophoblast (called trophoblastic villi) penetrate these lacunae providing the zygote with nutrients. The trophoblast also forms three other membranes:

the chorion: which forms the placenta and the umbilical cord

the amnion: which surrounds the fluid-filled amniotic cavity giving protection to the foetus

the allantios: which also contributes to the placenta

The inner cell mass gives rise to the yolk sac which produces blood cells for the foetus until it develops its own liver.

The chorion produces projections called the chorionic villi. These invade the endometrium by enzymic digestion of the endometrial tissue and so become bathed in maternal blood. Blood vessels appear in the chorionic villi separated from the maternal blood only by the walls of the foetal blood vessels.

Note that these villi are literally inside the maternal blood and should be subject to an immune response because foetal tissue should be recognised as "non-self". However the villi are coated with molecules that prevent the imune response.

The placenta is the site of exchange between the foetal blood and the maternal blood. Oxygen, carbon dioxide, nutrients and wastes are all exchanged across the placenta.

The placenta is connected to the foetus by the umbilical cord. This develops from the allantios and the chorion. The umbilical cord contains two foetal arteries and a single foetal vein.

Alcohol, drugs (for example, nicotine and heroin) and infectious organisms (such as the viruses rubella, HIV and hepatitis B) may all be present in the maternal blood and thus are also exchanged into the foetal blood. For this reason babies may be born addicted to drugs, infected by viruses, or suffering the physical effects of exposure to nicotine and alcohol.

You do NOT need to know the details of foetal growth.

Birth (parturition) begins with the onset of labour and is divided into three stages:

stage 1: begins with the onset f uterine contractions and ends when the cervix is fully dilated

stage 2: lasts until the baby exits the vagina

stage 3: the placenta (afterbirth) is expelled through the vagina

The onset and maintenance of parturition are under hormonal control:

Milk is synthesised by secretory cells in the mammary glands under the stimulus of the hormone prolactin from the anterior pituitary.

Milk is ejected under the stimulus of suckling by the child. Suckling also stimulates the release of oxytocin by the maternal pituitary gland. The oxytocin stimulates contractions within the mammary glands which secrete the milk.

During the first three days following parturition milk is not produced. Instead colostrum is secreted. This contains less fat than the later mature milk but is very rich in proteins, particularly immunoglobulins i.e. the components of the immune response. Colostrum is therefore very important in conferring passive immunity to the newborn.