For this to happen, the woman needs to produce an oocyte in one of her ovaries, which must be able to move freely to the fallopian tube.

The man needs to produce a high number of good quality sperm, capable of progressing through the cervix to the fallopian tube, where fertilization takes place.

The timing of sexual intercourse is crucial: an egg survives for only 24 hours and sperm for up to 5 days.

Once fertilization has occurred, a normal uterus is necessary for the embryo to implant and develop.