Fertility in women is determined by whether the egg that she releases from one of her ovaries at the time of ovulation meets with sperm that swim up through her cervix and uterus to reach the fallopian tube. As you can imagine, timing is crucial as sex must occur when the woman is at her most fertile.
Ovulation occurs midway through the menstrual cycle, on around day 12–16. The egg is released from the ovary and it starts to make its way towards the uterus, but it only lives for about 24 hours. It needs to meet the sperm just at the end of the fallopian tube or at the top of the uterus, so that it has a few days after being fertilised to get into the uterus to implant in the lining.
Sex must therefore take place about 40 hours after ovulation so that the egg and sperm meet at exactly the right place. The 2–3 days after the egg has been released are therefore when a woman is most fertile, and when couples should aim to have regular sex if they want to conceive.
If the egg and the sperm meet too high in the fallopian tube, the embryo can implant in the wall of the fallopian tube, and this leads to an ectopic pregnancy. There is no room for the embryo to grow, and the tube eventually ruptures, causing bleeding and a medical emergency.
If the sperm doesn’t meet the egg until it is well into the uterus, it may be already starting to die, and even if it is fertilised, it may not be able to implant to establish a pregnancy.
Finding out when a woman is most fertile involves her checking for signs that ovulation is just about to occur. This is possible by checking for: