Dear women: Know your cycle

Anna Grynnerup
7 min read

Most of us become aware of our fertility once a month when we get out period - or if it doesn’t come. Understand the female cycle.

If you understand your cycle, you'll also better understand what can go wrong. We've asked Anna Grynnerup to give us in-depth details about the female hormonal and menstrual cycle. Anna is an MD and a member of Doctors Intermediate. She has long worked at the Gynaecological-Obstetrics department at both Hvidovre and Rigshospitalet, where her research included taking AMH measurements to measure egg reserve in women.

If we take two fundamental principles that characterise female fertility and where it differs from male fertility: 1) female fertility is cyclical, which is why you can only get pregnant for a short period each month, and 2) fertility is time-limited. We'll go into more detail about the female hormonal and menstrual cycle. I have taught many primary school pupils and later medical students this subject, and I know that many (perhaps especially boys and men) think these are two boring topics - but stick around. You are not fertile for the rest of your life - menopause, which you may have heard of, marks the end of your fertility. It's exciting! If you understand what happens during the cycle, you also better understand what can go wrong.

Menstrual cycle and female sex hormones. Let's first recap the basics of a cycle so we understand exactly what the hormones are doing and what happens each day of a woman's cycle.

What is a hormone? To start with, we need to establish what a hormone is. Hormones are small signalling substances released from one organ (a gland), carried around the body in the blood and act on another organ. Different hormones control different mechanisms in the body. For example, insulin is a hormone secreted by the pancreas that regulates our blood sugar. Some hormones work very quickly. For instance, adrenaline makes your heart pound when you are about to crash into traffic, while other hormones work more slowly. For example, testosterone makes a young man's beard grow slowly. The menstrual cycle, and the hormones that regulate it, aim each month to 1) mature an egg and release it from the ovary and 2) prepare the lining of the womb to receive and nourish the fertilised egg so that a pregnancy can attach and grow. The hormones that start this process come from the brain (pituitary gland) and are called follicle-stimulating hormone (FSH) and luteinising hormone (LH), respectively, and they relay the message to our ovaries. The ovaries on either side of the uterus send signals back to the pituitary gland through the hormones oestrogen and progesterone.

So what happens during a menstrual cycle? A cycle starts on the first day of your period and ends the day before your next period. In this description, I have listed days from the first day of your period. Remember that the times below are examples within a typical range. There are big differences between individual women's menstrual cycles.

Days 1-5:

Your period starts, and you bleed because you have not become pregnant, so oestrogen production decreases. The brain senses that the amount of oestrogen is low and signals back to the ovaries, with FSH, that new follicles need to mature for a new cycle. Ovarian follicles are small fluid-filled pockets that contain an egg cell. When they mature, they produce oestrogen. Therefore, the concentration of oestrogen starts to increase at the beginning of the cycle

Days 5-13:

During menstruation, the inner lining of the uterus is bled out. Therefore, just after menstruation, the lining is completely thin, and you will not be able to get pregnant. Oestrogen from the maturing follicles makes the lining inside the uterus grow thicker to accommodate a potentially fertilised egg. At the same time as the uterus is getting ready, the maturing follicles in the ovaries will be fighting to be the one that finishes maturing for ovulation (sometimes two eggs mature, and then the woman can get pregnant with fraternal twins). We call this period of the ovarian cycle the "follicular phase" because this is where the follicles grow. This part of the cycle is called the "proliferative phase" for the uterine lining.

Day 14:

When about 14 days have passed from the start of the menstrual cycle, the leading follicle has matured and is producing a lot of oestrogen. When you take an ovulation test, you measure the hormone LH. The brain takes this as a signal that the ovarian follicle is ready to ovulate. Therefore, the pituitary gland briefly releases a lot of the hormone LH. The egg, loose inside the ovary, is then "released" into the abdominal cavity. This causes the mature ovarian follicle to burst - better known as ovulation.

Day 15-20:

What happens to the rest of the yolk after it bursts? The shell itself, which is used to encapsulate the egg, now becomes the so-called yellow body, which is a small gland (also called the corpus luteum). The corpus luteum starts producing the hormone progesterone quite quickly. Progesterone aims to make the lining of the uterus ready for pregnancy and produce nutrient-rich secretions needed for the pregnancy to stay in the uterus.

The yellow body hormone signals the pituitary gland not to mature new egg follicles at this time. This is convenient because the uterus is just getting ready for pregnancy - and therefore doesn't need any more eggs. When fertilisation occurs: The released egg will be captured by the oviduct's tentacles, which will carry the egg to its destination - the uterus. This stage of the ovarian cycle is called the "luteal phase"; for the uterine lining, this part is called the "secretory phase".

If the egg meets sperm through the fallopian tube and is fertilised, the cell divisions (development of the early pregnancy) start on the way to the uterus.

Days 21-28:

Hormones help the fertilised egg and the uterine lining grow ready. By the time the fertilised egg reaches the uterus, it has become a clump of about a hundred cells that can attach to the lining. In the following days, the fertilised egg will grow and start producing the hormone hCG (popularly called pregnancy hormone). hCG signals the yellow body to continue producing oestrogen and progesterone - thus maintaining pregnancy and avoiding menstruation. hCG can be measured in blood and urine and is the hormone that can give you two lines on a pregnancy test. If a pregnancy does occur, it will typically not provide a positive test until about 14 days after ovulation (and about 28 days after the last menstrual period), when the level of hCG in the urine is sufficiently high. A normal menstrual cycle is about 28 days long but can be anywhere from 21 to 35 days and still be completely normal. The luteal phase is relatively constant, about 13-14 days, while the follicular phase is of varying length. Therefore, it can be assumed that ovulation takes place about 14 days before the first day of menstruation - regardless of whether you have a short or a long cycle.

But what happens if the egg is NOT fertilised? Then the egg will not attach to the mucosa, and no hCG will be produced. Without hCG, the yellow body gets no signal to continue its hormone production. The corpus luteum will then simply "burn out" and stop producing hormones about 14 days after ovulation (28 days after the last menstrual period). This causes two things to happen: 1) the thick lining of the uterus breaks down and bleeds out as menstruation, and 2) the pituitary gland detects that hormone production is decreasing and takes this as a signal that new follicles are now maturing. And THEN we're back to square one, and a new cycle can begin.