Why is the endometrium important in assisted reproduction?

By (gynecologist), (embryologist), (gynecologist), (embryologist) and (psychologist).
Last Update: 11/29/2021

The endometrium is the inner lining of the uterus. It is a layer that varies throughout the menstrual cycle, with the purpose of allowing the embryo to implant in it in order to give rise to gestation.

Taking this premise into account, in assisted reproduction treatments (ART) it is of vital importance to control that the endometrium has the appropriate characteristics to favor embryo implantation.

For this reason, there are several tests that can be performed to assess the patient's endometrium prior to embryo transfer. The aim is to ensure that the endometrium is in the best condition to receive the embryo.

How does the endometrium change?

The endometrium is composed of a basal layer and a functional layer. The basal layer is responsible for producing a new functional layer each cycle, as this functional layer is regenerated each month in the event that no gestation has occurred.

To this end, the endometrium undergoes changes throughout the menstrual cycle. These variations are driven by estrogens and progestogens produced by the ovary. In this way, the endometrium goes through different phases:

  • Proliferative phase: the stromal cells and endometrial glands increase due to estrogenic influence. This phase takes place until ovulation, i.e., day 14 of the menstrual cycle.
  • Secretory phase: glandular secretion of substances necessary for embryo implantation. This change occurs in response to progesterone released by the ovarian corpus luteum after ovulation.

If pregnancy has not occurred, the degeneration of the corpus luteum leads to the shedding of the endometrium, which will be expelled with menstruation.

You can learn more about endometrial changes in this link: What is the endometrium - Thickening, types and pathologies

Tests for endometrial assessment

The embryo should implant in the endometrium approximately 6 days after fertilization. For this, not only is the quality of the embryo important, but the condition of the endometrium plays an equally decisive role. In addition, embryo and endometrium must be synchronized.

Currently, there are several tests that can be performed to assess the status of the endometrium.

Ultrasound monitoring of the endometrium

As a non-invasive technique, vaginal ultrasound allows us to study the endometrium throughout the menstrual cycle. In assisted reproduction, endometrial thickness and pattern are evaluated in a standard way to assess and estimate its receptivity.

Ideally, in order to perform the embryo transfer, the endometrium should be of adequate thickness. It seems that an ideal endometrium is one that reaches 7-9 mm in thickness and with a trilaminar pattern. A thin endometrium appears to be associated with lower pregnancy rates.

In egg donation programs or frozen embryo transfers, vaginal ultrasound plays a fundamental role in confirming that the endometrium is ready to receive the embryo. In these cases, the endometrium must be prepared in advance by administering hormones (estrogens and progesterone), or by taking advantage of the natural cycle if the woman still has regular menses.

Assisted procreation, as any other medical treatment, requires that you rely on the professionalism of the doctors and staff of the clinic you choose. Obviously, each clinic is different. Get now your Fertility Report, which will select several clinics for you out of the pool of clinics that meet our strict quality criteria. Moreover, it will offer you a comparison between the fees and conditions each clinic offers in order for you to make a well informed choice.

Endometrial receptivity test

In assisted reproduction cycles, a great effort is made to obtain optimal embryo development. Subsequently, a selection is made among the available embryos in order to transfer the one that will have the best chances of success. However, this would be of no use if the embryo is transferred into an endometrium that is not exactly at the right time to receive it.

This phase is known as the window of implantation and, in women with regular cycles, usually corresponds to days 19-21 of the menstrual cycle. However, this window of implantation may be displaced in time in some women, which may result in failure to achieve pregnancy after several attempts with good quality blastocyst transfer.

Nevertheless, there are tests available for the analysis of endometrial receptivity. These tests are performed on the basis of an endometrial biopsy and indicate whether the endometrium is receptive or not. If the endometrium is receptive, the embryo transfer will be performed under the same conditions as the biopsy. In the event that the endometrium is not receptive, the results would indicate when the embryo transfer should be performed.

Endometrial receptivity tests allow for a more personalized embryo transfer, increasing ART success rates.

If you are interested in this type of endometrial receptivity test, we recommend you read this article: What is the ERA endometrial receptivity test, and what is it for?

Study of the endometrial microbiome

This type of test analyzes the endometrial flora or microbiome, that is, the microorganisms found in the endometrium. Thus, based on an endometrial biopsy, the test provides a result as to whether the endometrial microbiome is suitable for embryo implantation to occur. This is because an imbalance in the endometrial flora could hinder embryo implantation.

If the endometrial flora is not dominated by Lactobacillus, it will not be the most favorable to host the embryo. In this case, the test report will recommend a personalized treatment with probiotics or antibiotics, depending on the result.

However, there is also a test that detects the main bacteria causing chronic endometritis (inflammation of the endometrium). This test is also very important to improve the results of ART, especially when there is recurrent implantation failure since chronic endometritis is present in a high percentage of these patients.

FAQs from users

Is endometriosis the same as endometritis?

By Alicia Francos Pérez M.D., M.Sc. (gynecologist).

Endometrititis and endometriosis are different concepts.

Endometritis is the inflammation of the endometrium caused by different reasons such as an operation or uterine manipulation after childbirth, abortion, IUD insertion, etc.

Endometriosis, on the other hand, is a proliferative disease of a benign nature, but highly damaging to the reproductive system.
Read more

Can endometrial thickness be improved wit natural remedies?

By Rut Gómez de Segura M.D. (gynecologist).

Endometrial thickness grows naturally in the middle of the cycle in women who have regular cycles . Drugs and supplements with anti-inflammatory effects should be avoided to avoid interfering with normal endometrial growth.

Abundant hydration and daily physical exercise, along with a varied diet and a healthy lifestyle, will promote the natural growth of the endometrium. In patients without periods (menopause) or patients with very irregular menstrual cycles, it will be necessary to administer drugs to help the endometrium grow.

How much does the ERA test cost?

By Victoria Moliner BSc, MSc (embryologist).

The cost of this diagnostic technique varies from clinic to clinic. The approximate costs are $800 in which costs for fertility medication and endometrial biopsy are not included.

What endometrial thickness is needed to perform IVF?

By Silvia Azaña Gutiérrez B.Sc., M.Sc. (embryologist).

Ideally, in order to perform an embryo transfer after an IVF cycle, the endometrium should reach a thickness of about 8 mm and present a trilaminar aspect by ultrasound.

However, an endometrial thickness of 8 mm does not guarantee that the embryo will implant. Similarly, gestation is also possible with thinner endometria, although these endometria result in lower pregnancy rates.

If you would like to learn more about how the endometrium is prepared in assisted reproductive techniques (ART), you can read the following article: Endometrial preparation for embryo transfer.

If, on the other hand, you are interested in learning more about the embryo transfer process, we recommend you to visit this link: Embryo transfer: when and how is it done?

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Bu Z, Sun Y. The Impact of Endometrial Thickness on the Day of Human Chorionic Gonadotrophin (hCG) Administration on Ongoing Pregnancy Rate in Patients with Different Ovarian Response. PLoS One. 2015 Dec 30;10(12):e0145703.

Enciso M, Carrascosa JP, Sarasa J, Martínez-Ortiz PA, Munné S, Horcajadas JA, Aizpurua J. Development of a new comprehensive and reliable endometrial receptivity map (ER Map/ER Grade) based on RT-qPCR gene expression analysis. Hum Reprod. 2018 Feb 1;33(2):220-228.

Moreno I, Codoñer FM, Vilella F, Valbuena D, Martinez-Blanch JF, Jimenez-Almazán J, Alonso R, Alamá P, Remohí J, Pellicer A, Ramon D, Simon C. Evidence that the endometrial microbiota has an effect on implantation success or failure. Am J Obstet Gynecol. 2016 Dec;215(6):684-703.

Moreno I, Simon C. Relevance of assessing the uterine microbiota in infertility. Fertil Steril. 2018 Aug;110(3):337-343.

Simón C, Gómez C, Cabanillas S, Vladimirov I, Castillón G, Giles J, Boynukalin K, Findikli N, Bahçeci M, Ortega I, Vidal C, Funabiki M, Izquierdo A, López L, Portela S, Frantz N, Kulmann M, Taguchi S, Labarta E, Colucci F, Mackens S, Santamaría X, Muñoz E, Barrera S, García-Velasco JA, Fernández M, Ferrando M, Ruiz M, Mol BW, Valbuena D; ERA-RCT Study Consortium Group. A 5-year multicentre randomized controlled trial comparing personalized, frozen and fresh blastocyst transfer in IVF. Reprod Biomed Online. 2020 Sep;41(3):402-415.

FAQs from users: 'Is endometriosis the same as endometritis?', 'Can endometrial thickness be improved wit natural remedies?', 'How much does the ERA test cost?' and 'What endometrial thickness is needed to perform IVF?'.

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Authors and contributors

 Alicia Francos Pérez
Alicia Francos Pérez
M.D., M.Sc.
Alicia Francos has a degree in Medicine from the University of Salamanca and a Master's degree in Mastology and Breast Pathology from the Autonomous University of Madrid and another Master's degree in Human Fertility from the Complutense University of Madrid. She also has a diploma in Gynaecological Endoscopic Surgery from the Université Clermont Ferrand. More information about Alicia Francos Pérez
License: 330840199
 Marta Barranquero Gómez
Marta Barranquero Gómez
B.Sc., M.Sc.
Graduated in Biochemistry and Biomedical Sciences by the University of Valencia (UV) and specialized in Assisted Reproduction by the University of Alcalá de Henares (UAH) in collaboration with Ginefiv and in Clinical Genetics by the University of Alcalá de Henares (UAH). More information about Marta Barranquero Gómez
License: 3316-CV
 Rut Gómez de Segura
Rut Gómez de Segura
Graduation in Medicine and Surgery from the University of Alcalá de Henares. Specialization in Obstetrics and Gynecology at the Hospital Costa del Sol in Marbella. Dr Rut Gómez de Segura currently works as medical director in the fertility center ProcreaTec in Madrid. More information about Rut Gómez de Segura
Licence number: 28/2908776
 Silvia Azaña Gutiérrez
Silvia Azaña Gutiérrez
B.Sc., M.Sc.
Graduate in Health Biology from the University of Alcalá and specialized in Clinical Genetics from the same university. Master in Assisted Reproduction by the University of Valencia in collaboration with IVI clinics. More information about Silvia Azaña Gutiérrez
License: 3435-CV
Adapted into english by:
 Cristina  Algarra Goosman
Cristina Algarra Goosman
B.Sc., M.Sc.
Graduated in Psychology by the University of Valencia (UV) and specialized in Clinical Psychology by the European University Center and specific training in Infertility: Legal, Medical and Psychosocial Aspects by University of Valencia (UV) and ADEIT.
More information about Cristina Algarra Goosman
Member number: CV16874

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