Matthew Prior
Matthew Prior
MBBS, PhD, MRCOG, Newcastle Hospitals

The Menstrual Cycle

It starts with the egg

Girls are born with all the eggs they will ever have in their lifetime, about 2 million in total. Gradually over time, these eggs disappear until only a few remain, and the menopause happens. So, on average a woman will have about 500 ovulations in her life. Where do all the other eggs go? Most of a woman’s eggs will never leave the ovary, instead they disappear through a process called atresia.
For an egg to be useful, it needs to grow. This process takes several months and happens in follicles. A follicle is a fluid filled bubble on the surface of the ovary big enough to be seen on ultrasound scan. This process is happening throughout reproductive life and is independent to the menstrual cycle. Once tiny follicles, known as antral follicles, have developed, they stay at the edge of the ovary for a signal to mature.

Follicle Stimulating Hormone

The ovaries are controlled by hormones produced by the pituitary gland, a small pea sized structure at the base of the brain. At the beginning of the cycle, the pituitary gland produces a substance called follicle stimulating hormone, (FSH) which acts as a signal to the ovaries. FSH stimulates follicles to develop further. In a natural cycle, when follicles are exposed to FSH they develop and start to race each other until one becomes a larger dominant follicle. When follicles grow, they start producing oestrogen which acts as nature’s control switch. When the oestrogen levels rise to a certain level, it acts as a signal, telling the pituitary gland to stop making FSH.
The menstrual cycle

The dominant follicle now contains an egg which is nearly ready to be released, and the other follicles shrink away. Usually only one dominant follicle remains. This is why humans tend to only give birth to one baby at a time.

Oestrogen

The dominant follicle continues to produce oestrogen which has two effects. Firstly, oestrogen causes the lining of the womb to thicken which was shed with the last period. The womb lining is called the endometrium and regrows each cycle to be ready to accept a pregnancy if fertilisation occurs. Secondly, oestrogen continues to feedback on the pituitary gland. When its levels are high enough, another natural switch is flicked.

Luteinising Hormone (LH) Surge

The pituitary can sense when the ovary has an egg which is nearly ready by monitoring oestrogen levels. At this point it releases another hormone called luteinising hormone, in a surge, which triggers the ovary to release the egg about a day and a half later. Once the egg has been released, luteinising hormone keeps the follicle in place. The follicle is now called a corpus luteum. The egg then passes into the open end of Fallopian tube and travels slowly towards the uterus.

Progesterone

The corpus luteum produces the final hormone, progesterone. The clue is in the name. ‘Pro’- in favour of, ‘gestare’ – the period between conception and birth and ‘one’ – as in hormone. Progesterone maintains the lining of the womb to enable an embryo to implant.

Conception

If the woman has had unprotected vaginal sex during this cycle, sperm may be waiting in the fallopian tubes. Most sperm never make it this far, in fact only 1 in 100 make it past the cervix. However, usually around a few hundred swims as far as the fallopian tubes where they wait, sometimes for several days, to see if an egg comes along. Eggs are collected by the fallopian tubes and if an egg meets sperm fertilisation may occur. A fertilised egg is called an embryo.

Implantation

The embryo is transported down the tube where it begins to grow by the cells that it is made of dividing repeatedly division. By the third day there are about 8 cells and by the fifth day around 150 to 200 cells. By this point, the embryo should have arrived in the womb. The embryo is now ready to implant into the womb lining to form a pregnancy.

What next

If implantation occurs, the corpus luteum continues to produce progesterone to support the pregnancy until the placenta is big enough to make its own progesterone and takes over this role.

If there is no pregnancy, progesterone levels fall, and the woman will have a period to start a new menstrual cycle.