The Menstrual Cycle: How hormones prepare for pregnancy or periods.

The menstrual cycle isn’t just about periods. On average the cycle lasts 28 days, whilst the period only accounts for five. However, these numbers can vary between women and can also change over a woman’s lifetime.

A women’s eggs, called oocytes, are all present from birth. The total number of eggs is approximately 2 million, which gradually deplete over time until the menopause. Throughout life, eggs undergo development independent of the menstrual cycle in small fluid filled sacs called follicles. As follicles grow, they develop receptors which respond to follicle-stimulating hormone (FSH). From this stage, the menstrual cycle influences further development of these follicles, allowing ovulation to occur.

So how does the cycle work?

The menstrual cycle is in fact is two cycles which interact and work together to adapt the uterus and ovaries to prepare the body for pregnancy. Each cycle influences the other using signalling molecules called hormones. 

The Follicular Phase

This phase accounts for days 1-14 of a typical menstrual cycle, which begins on the first day of a period, also known as menstruation, and ends at ovulation. It involves three main hormones: FSH, Luteinising hormone (LH) and oestrogen.

FSH is released from the pituitary gland, a pea sized structure located at the base of the brain. FSH levels rise and activates receptors on the follicles in the ovary allowing them to continue developing.

These follicles then begin to produce a type of oestrogen, called estradiol. The rise in oestrogen acts as a natural switch, telling the pituitary gland to stop producing FSH. Oestrogen makes vaginal discharge thinner, making it easier for sperm to enter the uterus. Oestrogen also influences the inner lining of the womb, the endometrium, which changes in thickness throughout the cycle in response to hormones. The rising oestrogen causes the womb lining to thicken from the last period to allow a fertilised egg to implant resulting in pregnancy.

As the follicles develop, only one, or occasionally two, grow big enough to develop mature eggs, this is known as the dominant follicle. Therefore, women tend to fall pregnant with one baby at a time. The rest of the follicles shrink away, hence explaining why women only have about 500 ovulations in their lifetime. Once the dominant follicle has an egg ready to go, the pituitary gland produces a surge of another hormone called LH which matures the egg and causes it to be released. This process is called ovulation. 

The Luteal Phase

This phase is always 14 days, whereas the follicular phase can vary between women. This occurs from ovulation to the first day of the next period, unless the woman becomes pregnant.

The dominant follicle which has released the egg, now becomes known as a structure called the corpus luteum. This structure releases a hormone called progesterone which maintains the thickness of the endometrium making the womb a favourable environment for pregnancy. The thick endometrium has a more enriched blood supply allowing it to support a developing embryo.

Conception

If a woman has had unprotected vaginal sex during this cycle, sperm may meet and fertilise the released egg in the Fallopian tubes resulting in pregnancy. A fertilised egg is called an embryo, a group of cells, which begin to divide and grow. By the fifth day there are approximately 150 to 200 cells, this is when it is ready to implant in the womb. If implantation does occur the corpus luteum continues to produce progesterone to support the pregnancy until the placenta takes over a few weeks later.

If fertilisation does not occur the corpus luteum degrades. As progesterone supports the lining of the womb, when progesterone falls the lining begins to shed causing a period. 

The whole cycle is then repeated.