By Dr. Rut Gómez de Segura (Gynaecologist – Reg. No. 28/2908776).
Estradiol levels are crucial for the success of a pregnancy, whether natural or resulting from assisted reproductive treatment. It is a sex hormone from the estrogen group responsible, among other functions, for promoting the maturation of ovarian follicles and, to some extent, the maturation of sperm in men.
For this reason, estradiol is closely monitored during assisted reproduction cycles, since correct estradiol levels can mean the difference between success and failure of a treatment.
In women, estradiol is mainly produced by the ovaries and is directly involved in the maturation of ovarian follicles, where the eggs develop. Therefore, it is essential for the start of the menstrual cycle, ovulation, and preparing the uterus for possible embryo implantation.
In men, estradiol is produced in smaller amounts than in women, primarily by conversion from testosterone. Its main function is sperm maturation, though it also plays a role in maintaining libido and hormonal balance.

In in vitro fertilization (IVF), estradiol is one of the most closely monitored hormones because its level is an indicator of how the ovaries are responding to ovarian stimulation.
The more estradiol the body produces, the greater the number and size of the follicles, which will later be responsible for releasing the eggs that can be fertilized in the laboratory. However, excessively high estradiol levels can be dangerous due to their association with ovarian hyperstimulation syndrome. Additionally, high estradiol can be harmful for implantation and may require postponing embryo transfer.
Before in vitro fertilization, it is necessary to have oocytes and sperm, whether from the future parents or from donors. In any case, one part of the process involves egg retrieval. This is where estradiol plays a fundamental role, as its levels indicate the ideal time to administer the trigger injection, which matures the eggs prior to ovarian puncture.
Moreover, this hormone allows medical professionals to predict the quality of the ovarian response, as well as determine the best time for embryo transfer or whether it is necessary to freeze the embryos due to very high hormone levels.
On the other hand, progesterone is another hormone that is closely monitored. After ovulation or ovarian puncture, progesterone helps prepare the endometrium to receive the embryo. An adequate balance between these two hormones is essential for successful implantation and maintaining the pregnancy.

Estradiol levels during ovarian stimulation are measured through several blood tests, enabling the medical team to adjust hormone doses, prevent complications such as ovarian hyperstimulation syndrome, and safely plan the procedure.
Estradiol levels should progressively increase during the IVF process, reaching approximately 200-300 pg/mL per mature follicle. Higher levels may signify an overly strong ovarian response, increasing the risk of ovarian hyperstimulation syndrome (OHSS) and may be related to polycystic ovary syndrome (PCOS).
Conversely, lower levels may indicate a poor ovarian response, possibly due to premature menopause, hypothalamic or pituitary problems, or ovarian failure. In some cases, if hormone levels are inadequate, medication dosages may need to be adjusted or the cycle may even be canceled.