Essential in the maintenance and good continuation of a pregnancy, the corpus luteum, or corpus luteum, is formed each month under the influence of female hormones. What is its origin? How does it contribute to the good progress of gestation? What happens if the egg has not been fertilized? Explanations.

The corpus luteum is intimately linked to the menstrual cycle. Each month, the ovarian cycle is divided into several phases:

• The follicular phase : During these first 12 to 14 days of the cycle, several ovarian follicles, kinds of small bags allowing the oocyte they contain to evolve, grow in size under the effect of hormonal impregnation (FSH and estrogen). At the end of this phase, one of the follicles matures. This is called De Graff’s follicle, according to the doctor who first identified it in the 17th century.
• L’ovulation : when mature, De Graff’s follicle eventually ruptures under the influence of the hormone LH (luteinizing). The oocyte is expelled and travels to the fallopian tube where it awaits fertilization.
• The luteal phase: it is during this last period of the female cycle that the corpus luteum is formed.

The corpus luteum, named after a yellow pigment, lutein, is the name given to the ovarian follicle after ovulation. Also described by the term corpus luteum, the old follicle is adorned, under the effect of the hormone LH again, with capillaries which will allow blood to circulate. The cell then becomes a gland whose main mission is to secrete estrogen and especially progesterone.

Thanks to the secretion of this hormone, the corpus luteum plays a key role in the success of pregnancy. Indeed, progesterone helps prepare the endometrium to receive the egg after fertilization. Under its influence, the uterine lining, very thin at the start of the ovarian cycle, thickens considerably in order to provide a favorable environment for the future embryo thanks to its numerous folds, glands and blood vessels. On the 20th day of the cycle, the uterus is ready … It remains to be seen whether the oocyte is fertilized.

If there has been fertilization, the embryo implants in the uterus after 8 days. Here again, a clever hormonal balance is taking place. Thus, the outer layer of the egg which will soon become the placenta (the trophoblast) secretes the hormone HCG (chorionic gonadotropin). Its role: to allow the corpus luteum to continue to function by preventing the uterus from contracting as it would normally during menstruation. For 3 months, the corpus luteum then secretes estrogen and especially progesterone more and more intensively. The embryo can continue to develop and the pregnancy is maintained.

It is also under the influence of this double hormonal action of the trophoblast and the gestative corpus luteum that the first “symptoms” of pregnancy appear. If the absence of menstruation at the 3 rd week of pregnancy (5 weeks) is the most obvious sign of the gestation which begins, other manifestations can be observed from the very first days of the hormonal impregnation: swelling of the breasts , first nausea, fatigue, hyperemotivity or irritability …

Around the 4th month of pregnancy, the placenta is ready for mother-child exchanges. Having no longer any utility, the corpus luteum regresses.

In 15% of cases, the pregnancy unfortunately ends with a miscarriage, usually before the end of the 2nd month (70% of miscarriages). Generally involved: a defect that appeared during embryogenesis. The embryo is then not viable and the gestation cannot be prolonged.

Much more rarely, a hormonal insufficiency is the cause of the miscarriage. In this case, the termination of the pregnancy is due to a deficiency of the gestating corpus luteum. Note: An episode of miscarriage is generally not considered to be symptomatic of a gynecological health problem. It is only in the event of repeated miscarriages or a late spontaneous termination of pregnancy (2nd trimester) that additional examinations are recommended.

When the oocyte is not fertilized, the corpus luteum is said to be cyclical. At this point in the luteal phase, the corpus luteum stops producing progesterone before it degenerates. With the end of the hormonal impregnation of the corpus luteum, the uterus and more particularly the small blood vessels of the uterine lining contract. The surface layer of the endometrium then detaches in a succession of small hemorrhages. The blood is evacuated by the body for a few days during the period. The follicular phase of the menstrual cycle then begins again.