Successful IVF: How Important Is Endometrial Thickness?
Endometrial receptivity is an important part of IVF success. Today, IVF laboratories are capable of supporting excellent embryonic development under in-vitro conditions. However, in order for a successful pregnancy to be achieved, the embryo must implant, and establish a connection to the maternal circulation. This process requires that the endometrium must be receptive. The so-called “window of receptivity” of the endometrium and the interaction between the embryo and the uterine lining is only beginning to be understood. We have observed over the years that thin endometrial lining, as measured by transvaginal ultrasound, results in decreased prospects for successful embryonic implantation, while thicker endometrial development results in higher implantation rates.
A recent publication in “Reproductive Biology and Endocrinology” 2014,12:96, divided patients into 4 distinct groups based upon the endometrial thickness on the day that HCG was administered. The groups were segregated as follows:
Group 1 – less than 7mm
Group 2 – 7mm – less than 8mm
Group 3 – 8mm – less than 14mm
Group 4 – greater or equal to 14mm
All of these patients were normal responders to ovulation stimulating medications. All patients in this study received an antagonist stimulation protocol.
The clinical pregnancy rate was significantly lower in patients whose endometrial thickness was less than 7mm, and no pregnancies were seen in patients with endometrial thickness below 6mm. The results are summarized below:
Ongoing Pregnancy Rate Miscarriage Rate
Group 1 - 13.79% 20%
Group 2 - 32.10% 11.29%
Group 3 - 38.23% 10.30%
Group 4 - 47.62% 9.09%
The clinical pregnancy rate increased with increasing endometrial thickness. The lowest clinical pregnancy rate was seen in Group 1 (endometrium less than 7mm). Miscarriage rates were highest in the group with the thinnest endometrial thickness (Group 1), although this data did not achieve clinical significance. The ongoing pregnancy rate was highest in the group with the thickest endometrium (Group 4).
The findings of this study is in agreement with a number of other published studies indicating that when the endometrium is less than 7mm, implantation and ongoing pregnancy rates are reduced. Implantation rates were lowest in Group 1 (10.17%), compared with Groups 2, 3, and 4 (23.92%, 29.80%, and 38.17%). Thicker endometria were associated with better implantation rates.
While we do not have a comprehensive understanding of endometrial receptivity, at the extremes of endometrial thinness (i.e. less than 7mm), implantation and ongoing pregnancy rates are significantly reduced. While it was not specifically addressed in this study, it seems reasonable that when circumstances allow the clinician to exercise some control of endometrial thickness, such as in egg donor and frozen embryo transfer cycles, it would be prudent to make every effort to achieve endometrial thickness of greater than 7mm before planning embryo transfer.