By Dr. Sara Fueyo Pestaña (Gynaecologist – Reg. No. 202008718).
The endometrium is the inner lining of the uterus, and its proper preparation is essential for successful embryo implantation. It is a highly dynamic and sensitive tissue that responds precisely to female hormones.
During the proliferative phase of the menstrual cycle, estrogens stimulate the thickening and regeneration of the endometrium after menstruation, while progesterone, in the secretory phase, transforms this tissue into a receptive and nourishing environment for the embryo. If fertilization does not occur, the endometrium is shed, leading to menstruation.
Beyond its biological function, the endometrium also reflects a woman’s hormonal and metabolic health. Factors such as stress, obesity, endocrine disorders, or prolonged use of certain medications can disrupt its development and compromise implantation capacity.

The menstrual cycle is a perfectly coordinated process involving several transformations in the endometrium. During the proliferative phase, the tissue regenerates and develops a denser network of blood vessels. Then, in the secretory phase, the endometrium acquires optimal conditions to receive the embryo, thanks to glandular activity releasing nutrients and the increase in blood flow.
Even minimal alterations in any of these phases can hinder embryo implantation, even when both the egg and the embryo are of good quality.
It is therefore essential to thoroughly assess the condition of the endometrium, whether a woman is trying to conceive naturally or through fertility treatments.
Several techniques are used to evaluate endometrial health. Transvaginal ultrasound allows measurement of endometrial thickness and analysis of its appearance. More precise methods, such as sonohysterography and hysteroscopy, enable medical professionals to examine the uterine cavity directly.

A more invasive option is an endometrial biopsy. Although it involves greater discomfort, it provides detailed information about hormonal response and tissue maturation.
Specialists also assess other parameters such as endometrial homogeneity, ultrasound texture, and the presence of possible structural abnormalities (polyps, fibroids, or adhesions) that may interfere with implantation. With this information, treatments can be personalized to improve the chances of success in assisted reproduction procedures.
Endometrial receptivity refers to the precise moment when the endometrium is ready to receive the embryo, making it the optimal time for implantation. This period, known as the “implantation window,” usually occurs between six and ten days after ovulation.
In recent years, endometrial receptivity tests have become increasingly important. These tests analyze the tissue’s genetic and hormonal expression to determine whether the endometrium is synchronized with embryo development. This enables more personalized embryo transfers, increasing the likelihood of achieving pregnancy.
As we have seen, a receptive and well-prepared endometrium is essential for implantation. When alterations occur in endometrial thickness, vascularization, or hormonal synchronization, even high-quality embryos may fail to implant. For this reason, endometrial evaluation is considered a crucial step in assisted reproduction treatments and an important diagnostic tool in recurrent infertility cases.
Studying the endometrium not only improves pregnancy rates but also helps anticipate potential complications such as endometrial-related miscarriages, allowing for safer and more personalized treatments.
Alongside medical monitoring, endometrial health is influenced by everyday lifestyle factors. Maintaining a healthy lifestyle, following a balanced diet, managing stress, treating endocrine disorders, and attending regular gynecological check-ups are all measures that can enhance endometrial receptivity and overall fertility.