A woman’s fertility and her ability to conceive are related to the hormone changes that occur during her reproductive cycle. A typical menstrual cyclelasts about 28 days, but may vary between 21 and 40 days. Day one of a woman’s cycle is the day she starts her period. At this point, a number of eggs start to develop in the ovaries due to a hormone called the follicle stimulating hormone (FSH).
 
As her cycle progresses, the eggs continue to grow and one (or occasionally more) will reach a point where it’s mature enough to be released. Meanwhile, levels of the hormone oestrogen are rising and this causes the lining of the uterus (womb) to thicken in anticipation of a pregnancy.
 
Regardless of the length of a woman’s monthly cycle, ovulation – when the egg is released from the ovary – occurs 10 to 16 days before the start of her next period. What varies is the time between the woman’s last period and ovulation. This is why calculating the optimum time to conceive is not always straightforward.
 
Ovulation is triggered by a sudden rise in luteinising hormone (LH). During ovulation, the ovary releases an egg, which enters the fallopian tubeand travels towards the uterus. At this time, the hormone progesterone prepares the lining of the uterus so that it is spongy, thick and rich in nutrients. If the egg is fertilised by sperm, it is implanted in the wall of the uterus and pregnancy begins.
 
At this time, another hormone is produced – human chorionic gonadotropin (hCG). This is the hormone that is detected by pregnancy tests. If the egg is not fertilised, it passes through the uterus and out through the vagina. Hormone levels drop and the lining of the uterus is shed. The woman gets her period, and the cycle begins again.