What is the implantation window, and why it’s that important?

The window of implantation is the period during which the endometrium is receptive and allows for the embryo to implant. Despite numerous studies attempting to pinpoint when it begins, how long it lasts, or when it ends, such as the endometrial receptivity array test (ERA), there is currently no conclusive evidence showing that conducting these tests can improve live birth rates for patients. Therefore, while we understand the concept, we lack a truly effective tool to determine when the window of implantation is fully open.

It is very important because it’s the window in which the embryo has to synchronize with the endometrium, so that the implantation can get place. It’s really important, it’s a lot of factors and hormones included in this window (in the opening and in the shutdown of this window). It’s really important to synchronize the moment between the embryo developing and the endometrium being prepared, so basically that’s why it’s so important.

The implantation window is the short period of time during a menstrual cycle when an embryo is able to implant in the endometrium. We know that it usually lasts 48 hours and it occurs between the 19th to 21st day of the menstrual cycle or between the 6th to the 7th day of the programmed IVF cycle with supplemented progesterone. We know that the menstrual cycle involves very important structural and functional arrangements, which are extremely important in obtaining the best option for further embryo development in the uterus. But interestingly, the embryo can implant in the fallopian tube and in the abdominal cavity any time during the menstrual cycle, but when the endometrium is absolutely resistant at the moment out of the window of implantation, the embryo starts to look for a place where it can survive. That is why the part of extra-uterine pregnancies can be explained by displacement of the implantation window. When an embryo comes to the uterus, and the endometrium doesn’t want to implant it, it can move forward to the fallopian tube and afterward implant there because the fallopian tube can implant the embryo independently and irrespective of what is happening in the uterine cavity.

The implantation window is a specific period when your endometrium is in an ideal state to accept an embryo. If you are struggling to conceive, and you have failed attempts in the past, it can be related to the condition of your endometrium on the transfer day. The preparation of the endometrium is a very crucial process, and the thickness of the lining is the key to the IVF cycle. Picking the right time for the embryo implantation is very important.

The hormones estrogen and progesterone can influence the growth and the development of the endometrium during 2 phases of the menstrual cycle: the endometrial lining change in thickness and the number of glands and vessels. All of this can impact the sensitivity of the inner lining of your uterus and its ability to accept an embryo on the embryo transfer day. Usually, the window of implantation is approximately on the 19th – 22nd day of a normal 28-day menstrual cycle. However, research shows that the implantation window can be displaced in 25% of women in the general population.

That’s why it is so important to understand this factor before the embryo transfer to avoid complications. The displaced window of implantation can be the reason why changes in the endometrium preparation protocol have no results or why multiple attempts fail to deliver a result, even if we transfer genetically good embryos. So make sure to ask your doctor about testing this.

Implantation represents a very delicate crosstalk between embryo and endometrium. Interestingly, the endometrium is characterized by the capability to accept and select the embryo. Thus, it plays the role of a natural biosensor of embryo competence. However, the period when the endometrium is receptive to the implantation and ready to accept an embryo is very short. That’s why it’s called the implantation window. Usually, it lies between days 19 and 21 of a menstrual cycle.

Unfortunately, about 30% of women have a displaced implantation window. Undoubtedly, the information about the optimal time for embryo transfer is one of the crucial points for IVF success. Nowadays, we have two technologies to identify the implantation window precisely: the ERA test and BeREADY test. In Ava-Peter clinic, we have implemented both of them, providing the personalized treatment in each case of infertility.

The implantation window is the period when the uterus is receptive to the implantation of an embryo. There is no full agreement, but we can say that the implantation window can last for about two or three days. When we are talking about fresh IVF cycles, there is no clear way of evaluating that window. So there is truly no way of knowing if the window is going to be open or closed.

However, we doctors know that when the progesterone is really high, the implantation window can be compromised. When it comes to a medicated FET (frozen embryo transfer), then the story is completely different. Of course, it is an artificial cycle, so there is a way of checking the implantation window during a biopsy. And after doing this biopsy, we can just do the same cycle and transfer the embryo at the proper time.

So the question is: why is the implantation window so important? Because if we are transferring a really good quality embryo and the implantation window is closed, the patient is not going to get pregnant. At Institut Marquès, it is always a part of our protocol to check for the implantation window with a biopsy. We do it just to be 100% sure that we are transferring the embryo at the right time.