Every month, from puberty to menopause, women go through a three-stage cycle that takes between 26 and 32 days (typically 28). This cycle controls the maturation and release of an egg, and prepares the uterus to receive and nurture an embryo so a pregnancy can be created.
Your pregnancy window, or most fertile time, is when the egg is moving along the fallopian tube, waiting for fertilisation by sperm. Once released, the egg survives for 24 hours.
Day 1 is your first day of menstruation, when the uterus sheds its lining from the previous cycle. Your levels of progesterone and oestrogen, which are made by a gland called the corpus luteum, drop at the end of the previous cycle, sending a signal to the pituitary gland to increase production of follicle stimulating hormone (FSH).
A number of follicles (microscopic sacs of fluid containing eggs) develop within the ovaries in the days leading up to menstruation. The exact number of follicles varies, but you develop fewer follicles (and hence eggs), as you get older. This usually happens between Day 1 and Day 5 of your cycle, sometimes while you are still having your period.
Although as many as 100 follicles might develop, the body will select only one (sometimes two) to become the selected egg for maturity that month, to reduce your risk of multiple pregnancy. This occurs between Day 5 and Day 12 of the average 28 day cycle.
The dominant follicle secretes increasing oestrogen, in turn increasing the thickness of the lining of the uterus (the endometrium) in preparation for pregnancy, and also changing the cervical mucus so sperm can pass through it more freely.
In an average 28 day cycle, this happens between Day 12 and Day 15. The pituitary then releases a rapid surge of luteinising hormone (LH), which triggers ovulation, so the dominant follicle matures and releases the egg.
The far end of the fallopian tube moves across the ovary and sucks up the egg. For a few days before ovulation, the cervical mucus allows sperm to pass through the cervix and uterus to the fallopian tubes. The sperm can survive there for two or three days, awaiting arrival of the egg. The egg can only be fertilised by the sperm for up to 24 hours, and probably on average less.
If a sperm penetrates the egg, a membrane called the zona pellucida surrounds the egg and hardens like a shell so other sperm cannot enter. The sperm releases its contents and fertilisation occurs. The fertilised egg starts to divide into cells, the number of cells doubling with each division, and becomes an embryo.
The follicle that produced the egg now begins to make progesterone and oestrogen, providing nourishment for the endometrium. Once the embryo reaches the uterus, it hatches out of its shell (about five days after fertilisation), and implants in the lining of the uterus.
The embryo then starts to produce the pregnancy hormone, human chorionic gonadotrophin (hCG) – the hormone measured in pregnancy tests. The presence of this hormone drives the ovary to continue making oestrogen and progesterone to support the pregnancy.
If fertilisation and implantation does not take place, the ovary will stop making oestrogen and progesterone. Without these hormones, the lining of the uterus breaks down and the next period starts.
Your pregnancy window
Your pregnancy window is around 5-6 days before ovulation occurs and the egg is ready for fertilisation.
Once you know your pregnancy window, aim to have regular sexual intercourse so that sperm is already present in the fallopian tubes when ovulation occurs. Sperm can survive for 2 to 3 days, waiting for the egg to be released.
It is not necessary to have sex every day during this time. Most experts advise that sex every second day will give the same chance of conception.
When do I actually ovulate?
Every woman is different, and your cycle may also vary month to month. The time from the start of your period to ovulation could be as little as 8 days, or as many as 18 days. The time from ovulation to menstruation is more consistent though – typically 12 to 16 days.
To work out when you ovulate, subtract 14 days from the number of days in your cycle. So if your cycle is usually 29 days, you can expect to ovulate on day 15.
You may also notice other symptoms of ovulation around that time, such as fertile mucus or an increased libido.
If you are concerned about your menstruation cycle, we can carry out tests to see what is happening.
Ovulation induction can help women who produce low levels of the hormones that enable conception, or who are not ovulating at all. Treatments involve having FSH injections to increase the chance of conception.