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Human endometrium is a highly regenerative tissue of the body similar to the bone marrow, hematopoietic tissue and epidermis. It undergoes nearly more than 400 cycles of regeneration, following menstruation in a women’s reproductive lifespan. After each cycle of shedding of the endometrium during menstruation, increasing levels of estrogen influences rapid proliferation of the endometrium. The quality of the endometrium during the embryo transfer cycles determines the success of the implantation. Endometrium is responsible for majority of failed implantation after embryo transfer.

Parts of Endometrium

1. Stratum Functionalis: This constitutes the upper two third layer of the endometrium. It contains glands supported by stroma and is supplied by spiral arteries. It is this layer which takes part in the event of menstruation.

2. Stratum Basalis: This constitutes the lower one third of the endometrium. It contains lower most branching portions of the glands, dense stroma with radial arteries at the origin. It also contains mesenchymal stem cells (MSCs) which has a high potential to differentiate and multiply under the influence of the hormone estrogen which is produced by the ovarian follicles.

What are the causes of thin endometrium?

Thin endometrium could be caused by several mechanisms including aging, hormonal imbalance etc. It could be a temporary cause where only the superficial layer is affected due to medications or a more serious cause when there is destruction of basalis layer of the endometrium. Below are the some of the various causes of thin endometrium

a. Clomiphene citrate: The drug causes impaired epithelial cell proliferation and delayed glandular maturation due to suppression of the endometrial components of epithelium, glands, stroma and vasculature.

b. Surgical procedures: Procedures such as repeated curettage, polypectomy, laparoscopic and hysteroscopic myomectomies, hysteroscopic septal resection and lateral wall metroplasty could cause destruction to the basal layer of the endometrium. Unlike superficial layer, destruction to basalis layer is difficult to treat.

c. Infections: Pelvic inflammatory diseases and genital tuberculosis could destroy basalis layer of the endometrium. In around 60% of genital tuberculosis patients, endometrium is affected. Tuberculosis in endometrium is initially focal, however in later stages, there is development of caseous necrosis and ulceration. The classical findings of endometrial tuberculosis on hysterosalpingography are lead pipe, coiled and blocked tubes with smaller and irregular uterine cavity. Sometimes, there could be intravasation of dye into the uterine vessels.

d. Asherman’s syndrome: It is one of the main reasons of secondary infertility in women who have undergone, surgical interventions to pregnant uterus. Repeated medical termination of pregnancy by suction and evacuation or vigorous curetting could lead to intrauterine adhesions. Intrauterine adhesions often cause scanty menses, dysmenorrhea and sometimes secondary amenorrhea.

When the basalis layer is damaged, it can be seen as thin endometrium with reduced blood flow on sonography. In such cases, usually the glands, radial arteries and stroma are damaged which poses a real challenge in regeneration of the endometrium.

Assessment of the Endometrium for embryo transfer

The quality of the endometrium can be assessed by transvaginal sonography. The main parameters studied during vagino-sonography are

1. Endometrial thickness
2. Endometrial pattern
3. Endometrial volume
4. Endometrial and sub-endometrial blood flow

Endometrial Thickness:

During the embryo transfer procedure in IVF, the minimum endometrial thickness is required to be 7mm. Endometrial thickness < 7mm is inversely related to the IVF outcomes. It has been proposed that the oxygen concentration in the basalis layer is higher in patients with thin endometrium, which interferes with embryo implantation. When endometrial thickness is > 9mm, there is increased clinical pregnancy and live birth rate

Endometrial pattern:

Endometrial pattern can be classified into

Pattern A: Triple layer characterized by a central hyperechogenic line representing the empty uterine cavity and hyperechogenic endometrium on either side.

Pattern B: Isoechoic endometrium with poorly defined outer walls and central echogenic line

Pattern C: homogeneous hyperechoic endometrium

The triple line endometrium is associate with higher pregnancy rate in comparison to a homogenous and hyperechogenic endometrial pattern

Endometrial and peri endometrial blood flow

The color doppler study of endometrial and peri endometrial areas can help in identifying the endometrial receptivity. These areas can be divided into the following zones

Zone 1: A 2mm thick area surrounding the hyperechoic outer layer of the endometrium

Zone 2: The hyperechoic layer which represents the outer layer of the endometrium

Zone 3: The hypoechoic layer which represents the inner layer of the endometrium

Zone 4: The endometrial cavity

Good endometrial vascularity can lead to better placentation resulting in a higher chance of live birth rate following embryo transfer. The pregnancy rates are higher when there is multifocal vascularity in zone 3.

Conclusion:

A thin endometrium significantly affects the implantation rate, pregnancy rates and increases the risk of miscarriage. Understanding the causes, and methods of diagnosis is important and essential to provide timely interventions. Appropriate treatment enhances the chances of success in IVF treatments. Continuous advancements in the field holds promise in improving outcomes for patients with thin endometrium.

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