The menstrual cycle and fertilisation

1A cycle of about 28 days

From puberty, the female reproductive system goes through the menstrual cycle, which gets the uterus ready for a possible pregnancy. It usually takes about 28 days, but it varies from person to person.

menstrual cycle

2A period

On days 1 to 5, the thick lining of the uterus breaks down. It leaves the body through the vagina, with a little blood. This is a period (menstruation).

periodmenstruation

3The lining builds up

After the period, the lining of the uterus grows thick again, full of blood vessels. Meanwhile, an egg ripens inside one of the ovaries.

lining

4Ovulation

At about day 14, an ovary releases the ripe egg into the oviduct: ovulation. If the egg isn't fertilised, the lining breaks down at the end of the cycle and the next period begins.

ovulation

5Fertilisation

During sexual intercourse, sperm are released into the vagina. They swim through the cervix and uterus into the oviducts. If one meets an egg, their nuclei join: fertilisation.

sexual intercoursefertilisation

6Implantation

The fertilised egg divides into a ball of cells, the embryo, as it moves down the oviduct. It sinks into the thick lining of the uterus: implantation. Pregnancy has begun.

embryoimplantation

7Twins

Usually one egg is released and one baby grows. Identical twins come from one fertilised egg that splits in two. Non-identical twins come from two eggs, each fertilised by a different sperm.

identical twinsnon-identical twins