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The menstrual cycle: what is it?
The menstrual cycle lasts around 28 days and allows ovulation, i.e. the production of an ovum (or oocyte or egg) that can be fertilised and give rise to a pregnancy. Ovulation occurs in the middle of the menstrual cycle, around days 12-14 of the cycle. Ovulation is preceded by a preparatory phase or follicular phase of the menstrual cycle and followed by the luteal phase, which is essential for the implantation of the embryo.
When is the follicular phase? The phase leading up to ovulation or the follicular phase of the menstrual cycle is the first 10-14 days of the cycle.
What happens during the follicular phase? In its basal state - at the end of the previous menstrual cycle or after hormone treatment - the ovary has a certain number of follicles - measuring 2-9mm - on stand-by. These are the antral follicles. The number of antral follicles present at a given time - the antral follicle count (AFC) - reflects the ovarian reserve. In a woman aged between 20 and 40, the AFC is normally between 10 and 30 (cumulative on both sides). During the inter-cycle period, a signal from the brain - the hypothalamus and pituitary gland - initiates the start of the new menstrual cycle.
This signal, a slight rise in the FSH hormone produced by the pituitary gland, acts on the waiting follicles (the antral follicles) and initiates their growth. Under normal conditions, only one of the antral follicles will develop until the ovulation stage - this is known as the dominant follicle - while the other, smaller follicles degenerate - this is known as atresia - and are permanently lost. The process of selection and growth of the dominant follicle lasts 10-14 days, usually 11-12. This is the follicular phase.
The growth of the dominant follicle culminates in a hormonal event - the LH peak - which triggers ovulation. The LH peak is associated with hormone levels that are high enough to be detected in the morning urine using a test strip (blue transfer indicating the LH peak). As a general rule, - The ovum or oocyte (egg) leaves the ovary around 36 hours after the onset of the LH peak. Detecting the LH peak in the morning urine can therefore help optimise the timing of sexual intercourse with a view to procreation.
When is the ovulatory phase? The ovulatory phase of the menstrual cycle occurs around the 14th day of the 28-day cycle.
What happens during the ovulatory phase? At the end of the follicular phase, an egg is ready to be released. The mature follicle produces so much oestrogen that it causes the pituitary gland to release LH, which releases the egg. The egg then takes around 24 hours to reach the fallopian tubes. If spermatozoa are in the fallopian tube at this time, fertilisation can occur. In this case, the fertilised egg travels to the uterus to attempt to implant itself in the uterine wall. The egg survives for 24 to 48 hours, while the spermatozoa can live for up to 5 days.
When is the luteal phase? The luteal phase begins when the egg is released, around 14 days after the start of your period.
What happens during the luteal phase? After ovulation, the follicle that gave rise to the ovum or oocyte (egg) capable of being fertilised and the embryo capable of implantation undergoes transformations leading it to produce the pregnancy hormone progesterone. The follicle thus transformed is called the corpus luteum. The corpus luteum is a highly vascularised organ that can be identified by ultrasound.
Progesterone production lasts for 12-14 days in the absence of pregnancy. If the embryo implants and the patient is pregnant, the hormone produced by the embryo - hCG (used in pregnancy tests) - maintains the activity of the corpus luteum for around 6-8 weeks. The hCG produced by the implanted embryo induces an increase in progesterone production.
Around 8-10 weeks of pregnancy, progesterone is produced exclusively by the placenta and this remains the case until the end of the pregnancy. The function of the corpus luteum is no longer essential beyond the 1st trimester of pregnancy.
The result of the menstrual cycle: menstruation
In the menstrual cycle - with ovulation, but without pregnancy - the production of progesterone by the corpus luteum lasts around 12 days and then stops. The cessation of progesterone after prior exposure of the mucous membrane of the uterus - the endometrium - to progesterone results in desquamation of the mucous membrane and privative bleeding, during the menstrual period.
Menstruation is therefore the result of the menstrual cycle and ovulation. It is normal uterine bleeding occurring after ovulation and exposure of the uterine mucosa to progesterone.
Any other form of uterine bleeding that may occur is an abnormal phenomenon - known as dysfunctional bleeding - with the exception of withdrawal bleeding, which occurs with hormone treatments that mimic the menstrual cycle.
The cessation of progesterone also triggers uterine contractions, which help to control uterine bleeding. This normal control of bleeding by uterine contractions is responsible for the fact that, in principle, there is no activation of coagulation - no clots - during menstruation.
In addition to their regularity - hence their name - periods are also characterised by their bleeding pattern. Typically, periods increase to a maximum, then decrease and stop. Any other pattern - e.g. increase, decrease, and re-increase - is a sign of dysfunctional uterine bleeding.
When periods are accompanied by clots, the situation is abnormal. This may be due to anatomical problems - fibroids, for example - that interfere with the proper contraction of the uterus. Clots are also encountered in a particular condition, anovulatory bleeding. These are known as anovulatory cycles.
In the anovulatory cycle, the uterine mucosa continues to grow under the action of estrogens - estradiol or E2 - without progesterone - the absence of ovulation - transforming the mucosa. The lining then proliferates excessively, becomes fragile, and bleeds intermittently.
This may give the false impression of cycles and pseudo-regularity, hence the term anovulatory cycle. The treatment is hormone replacement.
Written by:
Prof. Dominique de Ziegler,
University consultant in the gynaecology department,
Foch Hospital
