The Implantation window: what is it and how does it affect pregnancy?

biopsia endometrio

Timing of the menstrual cycle is crucial to achieving pregnancy. Any irregularity or abnormality in the different stages of the cycle can hinder gestation.

We talk to Dr. Elena Santiago Romero about one of the main factors that prevents a couple from achieving pregnancy: the displacement of the implantation window.

What is the window of implantation?

The window of implantation is the period in which the endometrium is in optimal conditions to receive the embryo. 

  • In the natural cycle it corresponds to 5-7 days after ovulation.
  • In artificial cycles, where we use medication prior to an embryo transfer, the endometrium is usually receptive within a few days after the addition of progesterone.

How does the window of implantation affect fertility?

The window of implantation can shift, resulting in failure of embryo implantation despite good embryo quality, as the embryo is not transferred at the time when the endometrium is receptive.

To identify and adjust for this displacement, there are endometrial receptivity tests that assess whether the endometrium is ready for implantation. If not, they determine the ideal time to perform the embryo transfer. 

This allows the transfer to be personalised, thus increasing the chances of success in fertility treatments.

What is the ERA test?

The endometrial receptivity test, known as the ERA test, is a test that analyses the genetic expression of the endometrium to verify if it is in a receptive state.

This test is designed to synchronise the embryo transfer with the period of maximum receptivity of the endometrium, thus significantly increasing the chances of pregnancy in cases of implantation failure.

How is the endometrial receptivity test performed?

sistema witness

To perform the endometrial receptivity test, an endometrial biopsy needs to be performed on the woman in order to obtain a sample of endometrial tissue.

This sample will be taken on the day the implantation window period is due to begin. An endometrial preparation as close as possible to the actual endometrial preparation at the time of embryo transfer should be performed. This preparation can be done in natural cycle, modified natural cycle or substituted cycle.

ERA Test Results: How to know if the endometrium is receptive?

The ERA test, or Endometrial Receptivity Test, gives the following results:

  • Receptive endometrium: when the receptivity genes of the endometrial sample correspond to the day on which the biopsy was performed.
  • Non-receptive: when the endometrial receptivity genes do not correspond to the implantation window period.

In addition, to determine whether the endometrium is receptive, certain characteristics can be observed by ultrasound or by analysing the endometrial tissue by biopsy.

These characteristics are indicative of a receptive endometrium:

  • Endometrial thickening: A receptive endometrium has a thickness of between 7 and 10 mm, which can be measured by ultrasound.
  • Presence of pinopods: These are cytoplasmic projections that emerge from the endometrial cells and are a sign of receptivity.
  • Endometrial glands: The presence of glands in the endometrial tissue is typical of the receptive state.
  • Cell growth: A significant increase in the number of endometrial cells contributes to the thickening of the endometrium, a sign that it is in a receptive state.

These characteristics are essential for assessing endometrial receptivity and determining the optimal time for embryo transfer.

Benefits of the ERA test in fertility treatments with Vida Fertility

The ERA (Endometrial Receptivity Array) test offers numerous benefits during fertility treatment, particularly in IVF (In Vitro Fertilisation). Here are some of them:

  • Precise Identification of the Implantation Window:

The ERA Test allows you to determine precisely when the endometrium is at its most receptive state to receive the embryo. This significantly increases the chances of successful implantation and therefore a viable pregnancy.

  • Reduced Implantation Failure:

By knowing exactly the optimal time for embryo transfer, implantation failures are reduced. This is especially beneficial for women who have experienced repeated implantation failures in previous IVF cycles.

  • Optimisation of IVF Cycles:

Performing the ERA Test optimises IVF cycles by ensuring that embryo transfer is performed at the ideal time. This can reduce the need for multiple cycles, saving time, effort and financial resources.

The test allows customisation of each patient’s treatment, adapting to her unique cycle of endometrial receptivity. This increases the likelihood of success and offers a treatment experience more tailored to individual needs.

  • Greater Confidence and Peace of Mind for Patients:

Knowing that the timing of embryo transfer is scientifically determined gives patients greater confidence and peace of mind during treatment.

Experiences of women who have undergone the ERA test

Chloe's mom

It is incredible how from absolute resignation, these doctors lifted my spirits. They got me pregnant and gave me my Life back. They live up to their name.

Oliver's mom

I had to find a solution to my motherhood if I wanted to pursue my career and not settle for just any partner. I made the right choice.

Lucas' mom

I felt from the first moment I was listened to, understood and, above all, I felt that my treatment was finally going in the right direction.


Hello dear Mrs Ortega,
you and the whole team of Vida fertility make us so happy. Finally a relief that we can be helped! Many thanks to you and all the others for the great care and exceptional quality work. This high quality means a lot to us. We hope that in 2024 we can take a healthy baby full of love into our arms.❤️


Thanks to Dr. Kathi’s support and by following all her advice, the best moment of the treatment came: I got pregnant.


I didn’t know if I would ever be able to get pregnant and now I’m in my 5th month of pregnancy.


It hasn’t been easy an easy journey. But after two attempts, I’m happy to say that, in two months’ time, I’ll be meeting my little girl.

Lucy and Helen

Being able to experience this shared motherhood has been a gift for the two of us.
THANK YOU to this great team for everything!

Egg donation treatment

We highly recommend Vida Fertility to anyone who has already given up hope and wants to give it one last try.

Frequently asked questions about endometrial receptivity and the implantation window

What is the Implantation Window in IVF?

Dr. Elena Santiago answers

The window of implantation in IVF (In Vitro Fertilisation) is the specific period of the menstrual cycle during which the endometrium, the inner layer of the uterus, is in its most receptive state to allow the embryo to implant successfully.

In the natural cycle it corresponds to 5-7 days after ovulation. In artificial cycles, where we use medication prior to an embryo transfer, the endometrium is usually receptive a few days after the addition of progesterone.

Precise timing of the embryo transfer with this window of implantation is crucial to increase the chances of success of IVF treatment.

If the transfer is performed outside this window, even high-quality embryos may not implant properly, resulting in a failed cycle.

Implantation Window in a Substituted Cycle

Dr. Elena Santiago responds

In a substituted cycle, also known as an endometrial preparation cycle, the woman’s endometrium is prepared by administering exogenous hormones (oestrogen and progesterone) to mimic the phases of the natural menstrual cycle.

This type of cycle is commonly used in IVF treatments when the patient does not ovulate naturally or when frozen embryos are used.

The window of implantation in a substituted cycle is equally critical and must be perfectly timed with the embryo transfer.

By using the Endometrial Receptivity Test (ERA), the exact time when the endometrium is most receptive can be determined, allowing the embryo transfer to be timed with the utmost precision.

Implantation Window in Ovodonation

Dr. Elena Santiago answers

Ovodonation is an assisted reproduction technique in which donated eggs are used to achieve pregnancy.

The implantation window in oocyte donation treatments follows the same principles as in a conventional or substituted IVF cycle. The recipient’s endometrium must be prepared and receptive at the time of embryo transfer.

Synchronisation between the development of the donated embryo and endometrial receptivity is essential. In these cases, the ERA test can be especially useful in determining the optimal timing for embryo transfer, ensuring that the recipient’s endometrium is in the most receptive state possible, thus increasing treatment success rates in cases of previous treatment failures.

Understanding and optimising the window of implantation is fundamental to improve success rates in fertility treatments. If you have any questions or would like to take the ERA test, we invite you to contact Vida Fertility. Our team of experts is here to help you every step of the way to motherhood.

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