Ovulation is a normal part of a woman’s monthly menstrual cycle. It signals the release of a mature egg from a follicle in one of her ovaries. If she does not have sex, or no sperm manage to reach the egg as it moves along the fallopian tube, it passes on through the uterus and then exits the body unnoticed. The lining of the uterus, which had been built up in case of pregnancy, is then lost as the monthly period. The cycle begins again on the first day of bleeding, with the next ovulation occurring 12–16 days later.
If a woman has sex and a sperm does meet the egg after ovulation, the egg can become fertilised. The DNA from the sperm enters the egg and fuses with the egg’s DNA to form a fertilised zygote. This is a single-celled embryo at this stage, but it then divides rapidly. By the time it reaches the uterus a few days later, it has usually become a blastocyst, a ball of about 120 cells.
Although the embryo has formed, pregnancy is not established until this ball of cells implants in the rich lining of the uterus. This is built up during the first half of the menstrual cycle so that it is ready to receive any egg that is fertilised after ovulation. The embryo embeds in the wall of the uterus in a process called implantation and pregnancy is fully underway.
This stage of very early pregnancy is a delicate one and many embryos that have formed from the fertilisation of an egg do not manage to implant, and are lost. Most women never notice this, or they may just think their period is a couple of days late. Pregnancies that do continue do so because the embryo implants successfully. A placenta is formed to nourish the growing baby, and it then starts to grow rapidly, expanding to become a recognisable fetus within a few short weeks.
If you have problems with infertility, one of the first tests you will have as a woman will check that you are ovulating regularly. If you are not, a number of fertility treatments can help you to achieve a pregnancy.