Endometrial thickness: how it changes during the cycle (and when it is favorable for implantation)

How much do you measure the thickness of the endometrium before and after the period? When is it considered normal and how much should it be to favor embryo implantation? What is trilaminar endometrium?

The endometrium  is the mucous membrane that lines the inner walls of the uterus. This covering layer is essential for the establishment of a  pregnancy:  it is precisely in the endometrium, in fact, that the implantation of the embryo takes place a few days after conception.

To create a favorable environment for the engraftment of the embryo (or rather the blastocyst), the endometrium changes during the menstrual cycle , changing in thickness thanks to the action of two hormones: estrogen and progesterone.

In this article, we will try to explain as simply as possible how the thickness of the endometrium changes in the various phases of the cycle and what it looks like on ultrasound .

Endometrial thickness before and after period

Endometrium in the proliferative phase: 5-11 mm

After menstruation , the endometrium enters the so-called proliferative phase (which corresponds to the follicular phase of the cycle). During this stage, the mucous membrane thickens and is enriched with blood vessels under the action of  estrogen , starting to prepare itself to host a possible pregnancy.

The thickness of the endometrium in the proliferative phase ranges from 5 to 11 mm.

Endometrium in the secretory phase: 12-13 mm

Once ovulation has occurred, we move on to the secretory phase (which corresponds to the luteal phase of the cycle). Under the influence of another hormone,  progesterone , the endometrium continues to thicken and is further enriched with vessels and glands, to create a more favorable environment for implantation and for nourishing the developing embryo.

In the secretory phase, the endometrium reaches an average thickness of 12-13 mm, which is maintained until the beginning of the next cycle. The maximum thickness is reached about a week after the LH peak (the one that precedes ovulation by a few hours). 

Endometrium in the menstrual phase: 2-4 mm

If conception does not occur, the concentration of estrogen and progesterone decreases and  the endometrium flakes off giving rise to the menstrual flow . At this stage, the thickness measures approximately 2-4 mm .

Endometrial thickness: when is it normal?

On average, an endometrium less than 14 mm thick is considered normal, regardless of menstrual phase.

Endometrial thickness: why is it important?

Endometrial thickness is one of the key factors influencing the success of the implant and therefore the onset of pregnancy.

For this reason, endometrial thickness is one of the parameters that are taken into consideration in the case of couple infertility .

Furthermore, it is a factor to be monitored when carrying out assisted reproduction treatment . An inadequate thickness, in fact, could compromise the implantation of the blastocyst and cause the failure of the PMA.

Specifically, to favor the implantation of the fertilized egg, the thickness of the endometrium should be 6-10 mm around the 19th-21st day of the cycle (counting from the first day of menstruation).

This time interval is called the implantation window and represents the period in which the endometrium is receptive to implantation of the embryo.

Endometrial thickness: how is it measured?

The exam used to evaluate the thickness of the endometrium is transvaginal ultrasound . From an ultrasound point of view, the endometrium appears as a line of variable thickness which is called the endometrial rhyme .

If ultrasound is not sufficient to analyze the thickness and appearance of the endometrium, the gynecologist may proceed with a hysterosonography and possibly a hysteroscopy .

Ultrasound appearance: the trilaminar endometrium

In addition to measuring the thickness, transvaginal ultrasound also makes it possible to analyze another important parameter for implantation purposes, namely the appearance of the endometrium .

Depending on the phase of the menstrual cycle, in fact, the inner lining of the uterus changes and takes on the following ultrasound characteristics:

• in the first part of the proliferative phase the endometrium appears as a clear and well-defined line with respect to the surrounding muscle tissue (called myometrium ) which instead appears to have a darker appearance (hypoechoic);
• shortly before ovulation (late proliferative phase) , the endometrium assumes a trilaminar appearance , which means that it appears divided into 3 alternating light (hyperechoic) and 2 dark (hypoechoic) stripes. This aspect is also called “coffee bean”.
• after ovulation (secretory phase) the endometrium loses its trilaminar aspect and from an ultrasound point of view it appears homogeneous and hyperechoic .

What is sought to increase the chances of successful implantation is a trilaminar endometrium . 

Endometrial thickness in the phases of the cycle: summary table

Phase of the cycle	Endometrial thickness	Ultrasound appearance
proliferative phase	5-11mm	Early: bilaminar appearance Late: trilaminar appearance
Secretory phase	12-13mm	Homogeneous and hyperechoic appearance
Menstrual phase	2-4mm	Thin white line (hyperechoic)

Questions and answers

Does an 8 mm endometrium with a trilaminar aspect guarantee implant success?

Unfortunately not. The process of implantation of the blastocyst is linked to multiple factors and even if the ultrasound parameters of the endometrium are all favourable, there is no certainty that it will be successful .

Is it possible to get pregnant with an endometrial thickness of 6 mm?

Yes, an endometrial thickness of 6-10mm is considered favorable for blastocyst engraftment .

Is it possible to get pregnant with an endometrial thickness of less than 6 mm?

It’s difficult. If the endometrial thickness is less than 6 mm, the gynecologist can prescribe treatments to stimulate the growth of the endometrium, such as taking estrogen and progesterone-based drugs .