Corpus Luteum and Pregnancy
Once ovulation has occurred, the follicle transforms into the corpus luteum or yellow body. The formation of the corpus luteum indeed depends on the delicate balance of hormonal values that arise: the preovulatory peak of LH plays a central role.
After the rupture of the follicle and the expulsion of the oocyte, the corpus luteum is formed: the granulosa cells begin to accumulate proteins, lipids, and lutein, a yellowish pigment, and the rich vascularization allows the arrival of cholesterol necessary for the synthesis of progesterone. The corpus luteum is responsible for the production of progesterone, a true marker of the ovulation process, in quantities of 8-10 ng/L, peaking on the 7-8th day.
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This hormone remains high with constant production up to 12-14 days post-ovulation and has a blocking action on the hypothalamus and reduces the responsiveness of the granular cells to LH. This latter mechanism leads in about 2 weeks to the degradation and disappearance of the corpus luteum. This is the course of events in a regular menstrual cycle phase, without pregnancy.
If pregnancy occurs, a structure is promptly formed that secretes human chorionic gonadotropin or beta hCG (measured in pregnancy tests), with LH-like action and maintenance of the corpus luteum, responsible for the endocrine control of pregnancy in the first trimester. From the 11th-12th week, control passes to the placenta.
Understanding the physiology and biological role of the corpus luteum in a woman’s fertile life is crucial, not only in light of possible attempts at natural pregnancy but also, and above all, in the event of resorting to medically assisted reproduction methods.
During embryo transfer techniques, for example, one must begin luteal supplementation of progesterone.
Pregnancy Corpus Luteum
Usually, progesterone is produced by the pregnancy corpus luteum, which doesn’t undergo death because it is supported by the HCG synthesized by the embryo. It is considered the quintessential hormone related to reproduction, as it determines:
- the secretive transformation of the glandular elements of the endometrium;
- the decidualization of the stroma: that is, the preparation of the endometrium to accommodate the arriving embryo;
- vascular proliferation and the implantation of the blastocyst;
- hyper-polarization of muscle fibers, reducing the possibility of contraction: this prevents sudden and excessive contractions that could lead to a failure in the implantation phase and, therefore, to a miscarriage;
- the fundamental hormonal support for maintaining pregnancy in its initial stages.
Consequently, any defect in the function of the corpus luteum can negatively affect endometrial maturity and its ability to support pregnancy in the early stages.
It might seem counterintuitive to carry out luteal supplementation since many follicles have undergone ovulation (and indeed miscarriages due to luteal insufficiency are only 8%), and therefore, a good amount of progesterone should be produced. However, the supplementation is based on very specific rationales:
- One cannot be certain a priori of the amount of progesterone produced by the corpus luteum; therefore, it is preferred to have a greater degree of certainty with supplementation.
- The high levels of estradiol in the luteal phase due to ovarian stimulation cycles have a negative effect on endometrial receptivity.
- The use of GnRH analogs results in prolonged inhibition of pituitary function, depriving the corpus luteum of its physiological support.
The administration can be done orally, intramuscularly, trans-dermally, or vaginally (the most commonly used method because it’s more similar to the natural form and has no side effects; vaginal suppositories, gels, and creams are available). It is taken in the evening before bedtime, as progesterone can induce fatigue and sleepiness.
Regarding timing, progesterone supplementation should begin on the day of the embryo transfer and continue for at least two weeks (until the possible menstruation). If the pregnancy test is positive, progesterone supplementation will have to be continued, but there is no consensus on the duration: according to some, for thirty days after the embryo transfer; according to others, until the visualization of fetal cardiac activity; and yet others, up to the twelfth week of gestation.
The rationale behind this last term is that from the twelfth week, the endocrine control of the pregnancy is ensured by progesterone produced not by the corpus luteum but by the placenta; the latter synthesizes it from cholesterol. Therefore, an increase in cholesterol in the second and third trimesters of pregnancy is physiological.
Overall, as can be seen, the corpus luteum is a fundamental and indispensable element for managing the delicate hormonal balance of a woman, whether during a regular menstrual cycle or in the case of pregnancy. Its importance is so crucial that it must be strongly considered even when approaching medically assisted forms.
On the one hand, with its limited duration in non-fertile cycles, it maintains menstrual activity, and on the other hand, with its greater persistence during the pregnancy phase, it induces the production of hormones that allow the initiation and maintenance of the pregnancy itself.