Endometrial Thickness

Definition - What does Endometrial Thickness mean?

Endometrial thickness refers to the depth of the inner lining of the uterus called the endometrium. The endometrium is the site where the embryo (fertilized egg) embeds itself within the uterus.

To achieve a successful pregnancy, it is necessary for the endometrium to be thick and ready for embryo implantation. An endometrium that measures at least 7 mm thick increases the odds of a successful conception. Conversely, a thin endometrium lowers the chances of a successful pregnancy.

Endometrial thickness is measured with the help of a routine gynecological ultrasound, ideally, one done through the vagina (transvaginal).

FertilitySmarts explains Endometrial Thickness

The endometrial thickness changes throughout a normal menstrual cycle:

  • In the late follicular (proliferative) phase (around day 3 to day 13), the endometrium grows and increases in thickness under the influence of estrogen produced by the growing ovarian follicles.
  • Following ovulation (around day 14), the mature follicle wears off and is transformed into a yellow mass called corpus luteum (around day 15 to 28). The endometrium is thickest during this luteal phase. If a fertilized egg implants during this period (the window of implantation), pregnancy will occur while the endometrial lining continues to grow thicker for supporting the developing embryo. Otherwise, the endometrium will shed with resultant menstrual bleeding.

An endometrium which is less than 7 mm thick at the time of the embryo implantation is known to have poor outcomes in assisted conception. A thin lining during IVF may require delaying the transfer of the embryos via freezing them.

These embryos are subsequently transferred (known as the frozen embryo transfer or FET) during the hormone replacement therapy (HRT) cycle, referred to as HRT – FET cycle. This strategy appears to yield better results. This is because HRT-FET cycles tend to maximize the endometrial receptivity (which is the ability of the endometrium to be receptive to — and accept — the developing embryo).

Alternative treatment modalities to improve the endometrial thickness include:

1. Intra-uterine granulocyte colony-stimulating factor (IU G-CSF) – G-CSF is a protein that stimulates the bone marrow to produce stem cells. This modality is based on the fact that human endometrium contains a small percentage of stem-like cells that may play a role in endometrial growth and reconstruction. Reduced number of stem cells in the endometrium may hinder its growth. The success rate of instilling G-CSF into the uterus is, however, limited. Most of the studies performed so far failed to show a significant improvement in pregnancy rates using G-CSF infusion into the uterus.

2. Extended estrogen support – Endometrial thickness may be improved by prolonging the period of estrogen treatment for 14–82 days in HRT-FET cycles.

3. Human chorionic gonadotropin (HCG) priming in the follicular phase – Giving HCG units daily for 7 days starting from day 8 to 9 of estrogen therapy may further drive endometrial growth, increasing its thickness.

4. Medications that stimulate blood flow to the endometrium – Giving medications like Pentoxyfilline and tocopherol daily over several months, sildenafil daily as a vaginal pessary, l-arginine, or low dose aspirin can optimize the endometrial thickness.

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