Endometrium Preparation for Embryo Transfer: How does it work?

The last step in the IVF process is the embryo transfer. In order to favor their implantation in the uterine cavity and start the pregnancy, it is necessary to prepare the endometrium, which is the mucous layer of the uterus where the embryos are implanted.

In this article we will talk about this treatment, which is indicated to increase the probability of implantation and pregnancy.

The different sections of this article have been assembled into the following table of contents.

The endometrium throughout the menstrual cycle

To understand how the endometrial preparation process takes place and why it is necessary, we must understand how the endometrium evolves throughout the ovarian cycle.

There are two main states of the endometrium:

• Proliferative phase, in which the endometrium grows due to the action of estrogens, reaching up to 10 mm. This period runs approximately from day 1 to day 14 of the cycle, that is, from the start of the cycle to ovulation (in regular cycles).
• The secretory phase, when the release of progesterone from the ovary begins. The action of progesterone on the endometrium makes it acquire the necessary aspect for implantation, thus favouring the nesting of the embryo and, therefore, the achievement of pregnancy. It occurs from day 15 to the end of the cycle.

Endometrial growth

When embryo implantation does not occur, progesterone levels drop and, as a result, the endometrium flakes. This produces menstruation, which marks the beginning of a new female cycle.

If, on the other hand, pregnancy occurs, progesterone levels do not drop and, therefore, the endometrium does not shed, which is why pregnant women do not have menstruation.

Favoring implantation

As we have seen, in a natural way, the endometrium acquires a certain thickness and aspect during the proliferative phase in order to favor the implantation of the embryos. In an in vitro fertilization (IVF) process we imitate this behavior so that the transferred embryos can implant and give rise to pregnancy. For this purpose, a woman's own hormone control is blocked and external hormones are administered.

The medication administered consists of several main steps:

• First phase: natural hormonal release from the ovary is blocked, usually through combined oral contraceptives (COCs) or GnRH analogues. The COC is administered for one month and, the day after finishing, the second phase begins directly. If we decide to opt for analogues, they are administered between the first and third day of the cycle for about 5 days.
• Second phase: it consists of the administration of estrogens to favour the growth of the endometrium, imitating the release by the ovary when it is maturing. It can be given with patches or pills. It is usual to start taking between the first and third day of the cycle.
• Third phase: the patient is prescribed progesterone, usually in the form of vaginal eggs, although it is also possible to give it orally. It is usually given about 3-5 days before the transfer and until your doctor tells you to, approximately between the 12th and 20th week of pregnancy. If pregnancy is not achieved, the medication will be discontinued.

Medication for preparing endometrium

During the second phase of the treatment, the patient goes once or twice to the gynecologist's office so that the gynecologist can assess the growth of the endometrium. Based on this, the appropriate time for embryo transfer is planned and it is determined when progesterone administration should be initiated.

The transfer should be performed when the endometrium has a trilaminar aspect (three parallel lines are observed) and an approximate thickness of between 7 and 10 mm. In this way, we increase the likelihood that embryos will implant.

This process is known as the substituted cycle, as we replace natural hormonal control with the administration of exogenous hormones.

In contrast, there is the so-called natural cycle, which consists of taking advantage of the natural release of female hormones. In this case, the woman only receives progesterone supplements 3-5 days prior to transfer, but no estrogens, COCs or analogues. We take advantage of the characteristic endometrial growth of the woman's cycle.

Prepare the endometrium of the gestational carrier

In surrogacy, erroneously known as surrogate motherhood, the endometrial preparation treatment falls on the gestational carrier, since she is the one who will carry the pregnancy to term.

In this reproductive process, the IVF process is divided into two parts, each corresponding to one person:

• On the one hand, ovarian stimulation and follicular puncture to obtain the eggs. This first stage is carried out by the future mother or the egg donor in the event that the mother is unable to provide the genetic endowment.
• On the other hand, the second part consists of the preparation of the endometrium and the transfer of the embryos. As mentioned above, it is the gestational carrier, also called surrogate mother who undergoes this part of IVF.

Although in cases of gestation by substitution the in vitro fertilization process is carried out between two women, the steps followed are practically the same. In any case, treatments should always be individualized according to the characteristics of each woman.

In the following link you can read more about in vitro fertilization:

FAQs from users

What's better during endometrium preparation - natural cylce or artificial cycle?

In cycles with frozen embryos, pregnancy rates per embryo transfer are the same whether the cycle is natural or artificial (with medication). No medication is needed for a natural cycle, which reduces the cost of treatment. However, setting a precise transfer date is more complicated than an artificial cycle.

What are the symptoms of the medication used in endometrium preparation?

The medication administered for the preparation of the endometrium is hormonal and, therefore, the symptoms or effects it may cause are related to abdominal swelling, fluid retention, tiredness, etc.

In any case, they are not serious or intense symptoms. In fact, many women do not notice changes during endometrial preparation treatment.

Could the surrogate mother refuse to undergo artificial cycle and demand natural cycle instead?

The demands that the surrogate mother, also called gestational carrier, may make must be stipulated in the surrogacy contract. In any case, it is recommended to follow the doctor's indications, because as a specialist, it is the doctor who has studied the characteristics of her menstrual cycle and her endometrium. Therefore, he will know which treatment offers the greatest probability of pregnancy.