Conception (fertilisation) is the beginning of pregnancy, when an egg is fertilised by sperm. As part of the normal menstrual cycle, one egg (ovum) is released from one of the ovaries about 14 days before the next menstrual period. Release of the egg is called ovulation. The egg is swept into the funnel-like end of one of the fallopian tubes where fertilisation may occur and is transported to the uterus. If fertilisation does not occur, the egg degenerates and passes through the uterus with the next menstrual period. If the egg is penetrated by a sperm, it is fertilised and begins to grow into an embryo through a series of cell divisions. If more than one egg is released, a multiple pregnancy occurs.

At ovulation, the layer of mucus in the cervix becomes more fluid, allowing sperm to enter the uterus rapidly. Sperm may move from the vagina to the funnel end of the fallopian tube – the usual site of conception – within 5 minutes. The cells lining the fallopian tube facilitate fertilisation and the subsequent development of the fertilised egg.

The fertilised egg divides repeatedly as it moves down the fallopian tube; it enters the uterus in 3-5 days. In the uterus it becomes a blastocyst (a hollow ball of cells). Inside the uterus the blastocyst implants into the uterine wall where it develops into an embryo and a placenta.