What is the implantation window, and what does it depend on?

It’s the window in which the embryo has to synchronize with the endometrium, so that the implantation can get place. Mainly it depends on hormones: the estrogen in the first part of the embryo development which makes the endometrium getting thicker and we the correct characteristic with the triple line and then, the most important one, which is the progesterone which is the one that get ready though that those cells and that endometrium er to be receptive for the embryo. So basically the most important is the progesterone but it’s also a sort of a complete hormonal regulation that it’s involved in the receptivity of the endometrium.

I don’t believe in the implantation window. If you want to put it that way because you know they have spoken about it. The implantation window it’s something that I don’t think can be measured and I have seen pregnancies happen at very strange conditions, for example, when I started doing IVF, I was separating the embryos that were blastocysts on the sixth day and on the fifth day. I said that when I don’t know something, I look at what nature is doing. So okay, I thought this embryo became good on the sixth day so, I would put it back on the sixth day of the cycle after replacing for example. I was trying to be very precise now i don’t check anymore if the embryos are of the fifth or the sixth day – why? because the body is waiting for the embryo and when the embryo touches the womb and then starts the implantation process through the immune system then the reception is ready for it. The reception is initiated also by the contact of the embryo to the uterus. That’s why I don’t believe that there is any reproductive window or implantation window. A correct embryo will implant at the right time.

We discussed this before; it depends mainly on the progesterone which initiates the window of implantation. But recently, it has been shown that there is also one important player in the receptivity of endometrium; the uterine natural killer cells. It has been shown by recent studies of Nicolas Macklin- an outstanding person in reproductive medicine. He showed us that the number of uterine natural killer cells correlates with the receptivity of the endometrium. It was shown in this study that in pre-receptive and post-receptive endometrium, the number of natural killer cells is lower and the irreceptive endometrium in the face of receptivity; their number is higher. It’s still unknown whether by influencing the activity of natural killers, we can somehow improve the window of implantation, it’s still unknown, and whether we can replace it, it doesn’t look so yet. But anyway, new studies are coming and it is becoming more and more interesting.

The implantation window is the final period of the optimal synchronization between the endometrium and the embryo – it is when the uterus is receptive for the embryo to implant. In a physiological menstrual cycle, that is usually something that corresponds to the twenty-first to twenty-fourth day of the period of twenty-eight-day cycles. This means that between four and seven days before the expected cycle, implantation can occur. The fact that the womb is ready for implantation is determined by a very sensitive balance between estrogen and progesterone.

Some patients can have their implantation windows displaced – it can be a bit earlier than usual, or it can be a little later. So, in patients who have already undergone several IVF cycles with their own eggs and good quality embryos that did not implant, or in patients who have undergone an egg donation cycle with several good quality embryos that did not implant, whenever we have a morphologically normal endometrium, it would be indicated to do a study which is called an endometrial receptivity test. This would check if the window of implantation is displaced, and that way, we can personalize when is the best moment to do the embryo transfer and do that accordingly to increase the chances of the success of the cycle.

The implantation window is the time that the endometrium is receptive for an embryo for implantation. In general, the majority of women have implantation on day five after the progesterone levels start to be high, so after ovulation. However, there are small fractions of women who are not ready for implantation after five days of progesterone. They can be pre-receptive or post-receptive, which means that we should confirm when there is an appropriate day to implant the embryo.

So, this is called an implantation window because the endometrium opens the window and is ready to implant, but after 12-24 hours, the window closes, so if the embryo is trying to implant after the implantation window closes, then the endometrium is no longer receptive, and the embryo cannot implant. That’s why it is very important to check this before the embryo transfer, to check that the implantation window is suitable for that embryo transfer, and it really depends on hormone profiles and endometrium morphology.

The window of implantation defines the period when the uterus becomes receptive for the implantation of an embryo. This period is usually short and depends on the programmed sequence of actions of the two hormones estrogen and progesterone on the inner layer of the uterus, the endometrium. In humans, the window of implantation opens around 6 days after ovulation and remains receptive for around 4 days; this means around day 20-24 of a theoretical 28-day cycle.

In assisted reproductive technique cycles, several markers have been explored to determine the best moment to put back an embryo to increase the chances of pregnancy. However, the morphological, hormonal, and even genetic markers are still poor predictors of pregnancy. This is why this subject is under intense research nowadays.