Menstrual Cycle

Some background 

A newborn baby girl has up to 450,000 eggs stored in her ovaries. When she starts her periods between the ages of about 10 and 14, one of these eggs will ripen each month. The egg is released from the ovary and caught by the ferny ends of the fallopian tube, which transports it, using a gentle rippling motion, along to the uterus (womb). If the egg is fertilized by a sperm along its journey, it will bed down once it reaches the uterus and grow into a baby and a placenta. If fertilization doesn't take place, the egg will be flushed out, along with the lining of your uterus, when you have your period.

What's a normal menstrual cycle? 

An average menstrual cycle lasts 28 days - that's counting from the first day of one period to the day before the next. Some women have much shorter cycles, possibly lasting only 23 days, and some have much longer ones, lasting up to 35 days. Cycles which are shorter or longer than this are probably not normal, and you should see your obstetrician-gynecologist. You should also see your doctor if you bleed between periods or after sex. 

How your hormones work 

Your menstrual cycle is under the control of an array of hormones produced in various parts of the body: 

• Gonadotrophin-releasing hormone (produced in the hypothalamus, which is in the brain) 

• Follicle stimulating hormone (produced in the pituitary gland, which is also in the brain) 

• Luteinising hormone (produced in the pituitary gland) 

• Estrogen (produced in the ovaries)

• Progesterone (also produced in the ovaries) 

The whole process starts in the brain. The hypothalamus produces gonadotrophin-releasing hormone (GnRh), which travels to the pituitary gland and signals it to release follicle-stimulating hormone (FSH). FSH is carried round the body in the bloodstream and stimulates the ovaries to start ripening eggs. Between 15 and 20 egg-containing sacs, called follicles, then start to mature in the ovaries. One follicle (or very occasionally, two or more) grows faster than all the others. 

FSH also stimulates the ovaries to produce estrogen. 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 fertilization occur. 

Ovulation: the egg is released 

As estrogen levels rise, levels of FSH fall temporarily and then rise again, accompanied by a huge surge of luteinizing hormone (LH) from the pituitary gland. It is this 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 a thick, opaque mucus that sperm can't penetrate. Just before ovulation, though, estrogen changes the mucus so that it becomes thin, clear and stretchy. This allows the sperm to swim through the cervix into the uterus and up to the fallopian tubes where fertilization may take place. 

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. It 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 fertilized 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 fertilization occurs... 

If the egg is fertilized in the fallopian tube, it will continue to travel to the uterus, where it beds down in the lining. At this implantation stage the fertilized egg is made up of about 150 cells. The journey from ovary to uterus takes about five days. Your progesterone levels will stay high and you may start to feel the early signs of pregnancy. 

If fertilization doesn't occur ... 

If the egg isn't fertilized or doesn't successfully implant, it starts to disintegrate and the corpus luteum shrinks. Your estrogen 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 unfertilized egg, and your menstrual cycle starts again.