Following intercourse, sperm must swim into the cervical mucus through to the uterus and into the fallopian tube, where fertilisation with the egg may occur. From millions of sperm only a couple of hundred will reach the egg. Although many sperm may try to penetrate the egg, only one will succeed. Couples trying to conceive should have sexual intercourse every two to three days around the time of ovulation. Sperm can survive for 48 to 72 hours within the woman’s reproductive tract, allowing an egg to be fertilised at any time during that period. The egg, however, is only viable for fertilisation no longer than 24 hours after ovulation.

Once fertilised the egg, now an embryo, travels down the fallopian tube to the uterus. Approximately 5 days after fertilisation the embryo will implant into the endometrium (uterine lining). It begins to secrete human chorionic gonadotrophin (hCG), a hormone that tells the ovary to continue progesterone production, which supports the embryo as it grows into a fetus. Blood tests detecting hCG can confirm a pregnancy at the time when a woman’s period is due.