Every month, from puberty to menopause, women go through a three-stage menstrual cycle that takes around 26-32 days (typically 28 days).
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.
If you are trying to conceive, your most fertile time – or ‘fertility window’ – is when the egg is moving along the fallopian tube. Once released, the egg survives for 24 hours, so to maximise your chances of conceiving, you should aim to have unprotected sex in the two days prior to ovulation.
Day 1 of your cycle is your first day of your period, when the uterus sheds its lining from the previous cycle. During a menstrual cycle, the hormones progesterone and oestrogen nourish the lining of the uterus in preparation for pregnancy. If fertilisation and implantation does not take place, the ovaries stop producing these hormones and the lining of the uterus breaks down, which causes menstruation.
The drop in progesterone and oestrogen at the end of the previous cycle also sends a signal to the pituitary gland to increase production of follicle stimulating hormone (FSH). Between day 1 – day 5 of your cycle, a number of follicles (sacs of fluid containing eggs) develop within the ovaries.
The exact number of follicles varies, but you develop fewer follicles, and therefore eggs, as you get older. Although as many as 100 follicles might develop, between day 5 – day 12 the body will select one (sometimes two) to mature that month, to reduce your risk of multiple pregnancies. The dominant follicle secretes increasing oestrogen, increasing the thickness of the uterine lining (the endometrium) in preparation to support a pregnancy, and also changes consistency of the cervical mucus so sperm can pass through it more freely.
In an average 28 day cycle, ovulation occurs between day 12 – day 15. The pituitary releases a rapid surge of luteinising hormone (LH), which triggers ovulation – where the dominant follicle matures and releases the egg into the fallopian tube.
In the few days leading up to ovulation, the cervical mucus allows sperm to pass through the cervix and uterus into 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 – in reality, the average period of time is probably much less.
To maximise your chances of conceiving, you should aim to have unprotected sex a day or two prior to ovulation. 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 menstrual period starts.
Calculating your fertility window
Every woman’s cycle is different, and your own 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 (typically 14).
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 long, you can expect to ovulate on day 15. You may also notice other symptoms of ovulation around that time. Common physical symptoms include a change in the consistency of cervical mucus (clear and slippery, similar to raw egg white), or increased libido.