By Rut Gómez de Segura (Gynaecologist – Reg. No. 28/2908776). Article updated and medically reviewed on 27/10/2026 by María Calomarde Rees (Gynaecologist – Reg. No. 282863855).
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.
Estradiol plays a very important role in fertility because it is involved in follicular development and prepares the endometrium for a possible pregnancy. In assisted reproduction treatments, its levels can provide valuable information about the ovarian response, although they must be interpreted alongside other clinical and ultrasound findings, as they do not, on their own, determine the chances of achieving pregnancy.
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 release the eggs that can be fertilised in the laboratory. However, excessively high estradiol levels can be dangerous because they are associated with ovarian hyperstimulation syndrome. In certain circumstances, an excessive ovarian response can increase the risk of ovarian hyperstimulation syndrome (OHSS). In these cases, the medical team may recommend vitrifying the embryos and postponing the transfer to a later cycle to reduce treatment-related risks.
High estradiol levels can also impair implantation and make it necessary to defer 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.
Estradiol levels naturally fluctuate throughout the menstrual cycle, so a high or low result does not necessarily indicate a fertility problem. Correct interpretation requires consideration of factors such as when the test was performed, the woman’s age and the levels of other reproductive hormones.
When estradiol is measured at the beginning of the menstrual cycle, usually between days 2 and 4, it can help assess ovarian function alongside follicle-stimulating hormone (FSH). A high level at this stage may be associated with reduced ovarian reserve, although estradiol alone cannot diagnose low ovarian reserve or determine the chances of achieving pregnancy. For this reason, specialists also consider other indicators, such as anti-Müllerian hormone (AMH) and the antral follicle count, to build a more complete picture of reproductive function.
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Estradiol Signaling at the Heart of Folliculogenesis: Its Potential Deregulation in Human Ovarian Pathologies.
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