How your menstrual cycle works
Unlike men, who produce sperm every day throughout their lifetime, women are born with all the eggs they will ever have, stored in their ovaries. You will have been born with 1 to 2 million eggs but only a fraction of these will be released during your life. This reserve of eggs declines over time. When you start your periods, usually between the ages of about 10 and 14, only about 300,000 eggs are still viable. At least one of these eggs ripens each menstrual cycle, and is released from the ovary during ovulation. The egg is caught by the ferny ends of the fallopian tube, and moved along by a gentle rippling motion, to the uterus.
If the egg is fertilised by a sperm along its journey, it will bed down once it reaches the uterus. Here it will grow into a baby and a placenta. If fertilisation doesn’t take place, the egg will be flushed out, along with the lining of your uterus, when you have your period. Read more about how babies are made.
What’s a normal menstrual cycle?
An average menstrual cycle lasts 28 to 29 days. That’s counting from the first day of your period to the day before the next. Some women have much shorter cycles, lasting only 22 days, and some have much longer ones, lasting up to 36 days. It’s also common for the length of your cycle to vary from month to month. In between your periods, it’s normal to have a milky white vaginal discharge, which will change in consistency as your hormone levels rise and fall throughout your menstrual cycle.
As you get older, and nearer menopause, you’re likely to find that length of your menstrual cycles change. Your cycle length may shorten to two to three weeks, or you may find that your cycle lengthens to many months. If you’re under 40, and experiencing very long gaps between your periods, or your periods seem to have stopped completely, it’s worth seeing your GP to get some blood tests. You should also see your GP if you bleed between periods or after sex. Some methods of contraception, such as the hormone injection and intrauterine device, can cause irregular bleeding. If this doesn’t apply to you, it’s best to get yourself checked out.
How do my hormones work?
Your menstrual cycle is under the control of a range of hormones produced in various parts of your body:
- Gonadotrophin-releasing hormone (GnRh) is produced in the hypothalamus, which is in your brain. It stimulates your body to make and release follicle stimulating hormone and luteinizing hormone.
- Follicle-stimulating hormone (FSH) is produced in the pituitary gland, which is also in your brain. It makes the eggs on your ovaries ripen ready for release.
- Luteinising hormone (LH) is also produced in your pituitary gland. It stimulates your ovaries to release eggs.
- Oestrogen is produced in your ovaries. It has many roles in your body, including the changes in your body during puberty that take you from girl to womanhood. It plays a very important part in your reproductive cycle.
- Progesterone is also produced in your ovaries. It works with oestrogen to keep your reproductive cycle regular and to maintain pregnancy.
The whole process of the menstrual cycle is started in the brain, when the hypothalamus produces GnRh. This hormone travels to the pituitary gland and tells it to release FSH. FSH then travels round the body in the bloodstream and stimulates the ovaries to start ripening some eggs. Between 15 and 20 egg-containing sacs, called follicles, start to mature in the ovaries. One follicle, very occasionally two or more, grows faster than all the others. FSH also stimulates the ovaries to produce oestrogen. This encourages the eggs to mature and starts to thicken the lining of the uterus so that it’s ready to support a pregnancy, should fertilisation occur.
What happens during ovulation?
As oestrogen levels rise, levels of FSH fall temporarily and then rise again accompanied by a huge surge of luteinising hormone from the pituitary gland. It is luteinising hormone that triggers ovulation, the moment that the most mature egg bursts out of its sac and away from the ovary. The egg is immediately caught up by the ends of the fallopian tube. Normally your cervix, the neck of your uterus, produces thick, opaque mucus that sperm can’t penetrate. You’ll notice this mucus in the gusset of your pants or when you wipe after you’ve urinated. Just before you ovulate oestrogen changes the mucus so that it becomes thin, clear and stretchy. You may have noticed this change in your mucus for yourself. This fertile mucus allows the sperm to swim through the cervix, into the uterus, and up to the fallopian tubes, where fertilisation may take place.
What happens after ovulation?
In the ovary, the now empty follicle collapses and becomes a corpus luteum. This small yellow mass of cells starts to produce the hormone progesterone. Progesterone changes the mucus in the cervix so that, once again, it becomes impenetrable to sperm. You may notice that your vaginal discharge becomes thicker and stickier during this stage of your cycle.Progesterone also acts on the lining of the uterus, which becomes thick and spongy as a result of an increased blood supply, ready to receive a fertilised egg. As levels of progesterone rise, your breasts may feel stretched and tingly. The pituitary gland stops producing FSH so that no more eggs mature in your ovaries.
If the egg is fertilised in the fallopian tube, it will travel on to bed down in the lining of the uterus. At this implantation stage the fertilised egg is made up of about 150 cells. The journey from ovary to implanting into the uterus takes six to 12 days. Your progesterone levels will stay high and you may start to feel the early signs of pregnancy.
What if fertilisation doesn’t take place?
If the egg isn’t fertilised or doesn’t successfully implant, it starts to disintegrate and the corpus luteum shrinks. Your oestrogen and progesterone levels drop and the lining of your uterus starts to produce prostaglandins. These chemicals cause changes in the blood supply to your uterus, breaking up the lining, and stimulating the uterus to contract. Your period starts and the lining of your uterus is shed along with the unfertilised egg. Your menstrual cycle then starts all over again.