Endometrial thickness is not independent of luteal phase day in a rural Polish population

The endometrium is important for luteal phase function, implantation, placentation, and gestation. Endometrial morphology has been shown to impact early pregnancy success in in vitro fertilization and egg donor cycles and has been implicated in conception success in spontaneous cycles. However, few studies have monitored endometrial morphology in normo-ovulatory women or examined possible population variation in its physiology or thickness. Further, the methodology of most studies of the endometrium is to take a single measurement close to ovulation, meaning the endometrium is not often measured during the window of implantation; the assumption has been that an individual's endometrial thickness (ET) is consistent through the luteal phase. Therefore, we have tested the hypothesis that ET is independent of luteal phase day in a study of urban-dwelling US and rural Polish women. We found that ET is not independent of luteal phase day in this sample of rural Polish women: ET is negatively associated with cycle day (P = 0.03). Compared to the US sample, the Polish sample also had a significantly later age at menarche (P = 0.002), lower midluteal progesterone concentrations (P = 0.0006), lower C-peptide concentrations (P = 0.0003), and higher energy expenditure (P = 0.03), suggesting greater energetic constraint. The relationship between ET and luteal phase length highlights the need to measure the endometrium serially over the menstrual cycle in order to better document inter-and intra-population variation in reproductive function.