Fertility in women
Every month a fertile woman releases an egg and her body prepares for a fertilised egg to implant. Each egg is encased within a special set of cells called the follicle which helps the egg develop and grow. In a normal cycle, only one egg will develop to maturity and be released from its follicle. This is why most naturally occurring pregnancies result in a single baby.
During the time the egg is growing, the lining of the womb starts to thicken. This is a highly-specialised environment where the early embryo will attach and implant.
Approximately 14 days after the start of the last menstrual period, the ripe egg is released (ovulation). This is the best time to have intercourse and try for a baby.
The egg enters the fallopian tubes where, for about 24 hours, it may be fertilised. If the egg is fertilised it will begin to develop into an embryo as it passes down the tube into the womb where it may implant. If the egg is not fertilised, or the embryo does not implant, all the extra lining of the womb prepared for the embryo breaks down and results in a menstrual period.
Problems may occur because of a failure to ovulate, as a result of blocked fallopian tubes, or problems with the womb lining.