Pregnancy rates following in vitro fertilization are reduced with a thin endometrium, but are unrelated to endometrial thickness above 10 millimeters

Abstract

To investigate the influence of endometrial thickness measured on the day of human chorionic gonadotropin (hCG) administration on the outcome of in vitro fertilization (IVF) treatment based on the largest patient sample size reported to date. Retrospective analysis. Cycles of IVF conducted between January 2002 and February 2004 were reviewed. All patients undergoing IVF with embryo transfer (ET) during this period were included in the analysis, using only the first treatment cycle per patient. Endometrial thickness was measured on the day of hCG administration to the nearest tenth of a millimeter (mm). Clinical pregnancies were defined by visualization of a gestational sac on ultrasound examination four to six weeks after oocyte retrieval. A total of 2145 patients were included in the analysis. Patient age ranged from 22.6 to 45.2 years (mean = 33.5, SD = 4.0). Endometrial thickness ranged from 3.1 to 22.8 mm (mean = 11.4, SD = 2.4). Linear regression analysis indicated a very weak but statistically significant decline in endometrial thickness with patient age (R-Square = 0.0084, p < 0.0001). Mean endometrial thickness declined from approximately 12 mm for patients in their mid 20's to approximately 11 mm for patients in their early 40's. There was a significant difference in endometrial thickness between cycles that resulted in pregnancy and those that did not (11.7 vs 11.1 mm, p < 0.0001, t-test). Pregnancy rates increased with increasing endometrial thickness through 10 mm. Multiple logistic regression analysis revealed a significant positive relationship between endometrial thickness and pregnancy rate among patients with an endometrial lining below 10 mm (p = 0.0013, n = 540), after factoring out the effects of patient age. The logistic regression model indicated an increase in pregnancy rates from 19% for a thickness of 6 mm to 50% for a thickness of 9.9 mm (based on a standardized age of 35.5 years, the mean age of the study population). However, there was no relationship between endometrial thickness and pregnancy rates with linings of 10 mm or greater (p = 0.29, n = 1605). The mean pregnancy rate per ET for cycles with an endometrium of 10 mm or greater was 51%. Increasing endometrial thickness is associated with increasing pregnancy rates with IVF treatment for relatively thin linings, below 10 mm in thickness. However, at 10 mm or greater, there is no relationship between endometrial thickness and the outcome of IVF treatment.