The first two weeks are critical for the success of the pregnancy, since, during those 14 days after ovulation, the embryo has to implant in the uterus, and any irregularity can cause implantation failure or the death of the embryo, even before the woman is aware of her early pregnancy. This is how the experts from the MARGen clinic in Granada explain it in an article published in the magazine Frontiers.

Thus, these health professionals point out that there are many women who become pregnant, but never know it, since their pregnancy is lost before it can be detected . In other cases, a pregnancy can be detected by blood tests, but is lost before the embryo can be seen in the uterus by ultrasound. These situations are defined as “biochemical pregnancy”.

The importance of progesterone

After ovulation, the follicle that has released the ovum is transformed into a structure called “corpus luteum”, through the process called “luteinization”, and assumes new functions relevant to pregnancy.

During the first two weeks of pregnancy, the corpus luteum is the main source of the hormone progesterone, which is essential for the changes in the uterus necessary to receive the embryos and support their subsequent evolution.

If there are , the implantation (nesting) of the embryo in the uterus fails or its further evolution is threatened. This situation is caused by insufficient secretion of progesterone, the hormone essential for a transformation of the cells of the inner lining of the uterus (endometrium), the glands of the endometrium, the contractility of the smooth muscle of the uterus (myometrium), the blood flow in the uterus, and the activity of the immune cells of the uterine cavity.

The hormone progesterone promotes the secretion of substances that nourish the newly implanted embryo by the endometrial glands, decreases the contractility of the myometrium thus reducing the risk of expulsion of the embryo, increases blood flow, necessary to bring nutrients to the embryo, and reprograms the immune cells from the uterus to promote implantation of the embryo instead of causing its rejection due to the presence of its foreign antigens from the father.

Threats to the embryo due to poor function of the corpus luteum

Insufficient secretion of progesterone by the corpus luteum can occur repeatedly in certain women after natural conception. However, according to Dr. Tesarik, this situation is much more frequent in assisted reproduction attempts , due to several factors related to ovarian stimulation.

Also, luteinization problems are more likely in assisted reproductive treatments compared to natural conception . “That is why -says Dr. Tesarik- it is essential to monitor the concentration of progesterone in the serum, even before knowing if the woman is pregnant or not. The risk of anomalies exists in all ovarian stimulation protocols, and not only in some specific ones, as was thought until now”.

According to the directors of the MARGen clinic in Granada, Dr. Jan Tesarik and Dr. Raquel Mendoza Tesarik, “in order to reduce failures in assisted reproduction attempts, it is important to determine the concentration of progesterone in the blood on the same day of the embryo transfer. and, where appropriate, adapt the replacement treatment appropriately. This step must be repeated every seven days after the embryo transfer. This is the only way to reduce avoidable pregnancy losses during the first two weeks.”