What exactly is the window of implantation and what factors affect it?
Please find 3 answers recorded with 3 IVF experts below.
Answer from Dr. García de Miguel
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 is a small fraction of women who are not ready for implantation after five days of progesterone, so they can be pre-receptive or post-receptive, which means that we should confirm when is the 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.
Answer from Dr. Arque
The implantation window is the final period of the optimal synchronisation 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 to seven days before the expected cycle, implantation can occur. The fact that the womb is ready for the implantation is determined by a very sensitive balance between estrogen and progesterone. There are some patients who can have their implantation windows displaced – it can be a little bit earlier than usual or it can be a little bit 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 personalise when is the best moment to do the embryo transfer and do that accordingly, to increase the chances of the success of the cycle.
Answer from Dr. Castillo
The window of implantation defines the period when the uterus becomes receptive for the implantation of an embryo. This period of time 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 in order to determine the best moment to put back an embryo in order 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.