The latter half of the cycle and the endometrium

Because it's been a while since any of us has blogged, I thought I would post a quick teaser regarding our AAPA and HBA abstracts. Following up from a recent paper in Anthropological Science on rural Polish women, we have found in Canadian women that luteal phase length is correlated to endometrial thickness through the implantation window. The implantation window is six to twelve days after ovulation (the middle of the cycle), and the average luteal phase is twelve to sixteen days long (the second half of the menstrual cycle). Before menses endometrial thickness begins to decline, so as you might expect, those with shorter luteal phases have thinner endometria through the window of implantation.

For now this simply demonstrates variation in endometrial thickness through what has been demonstrated to be the period during which the vast majority of implantations occur. This also means that there is significant variation in luteal phase length, even in ovulatory cycles in well-fed, urban women, in a way that has the potential to be biologically meaningful. Are there factors that are producing variation in luteal phase length (timing of menstruation) that in turn impact endometrial thickness through the implantation window? Or is the endometrium breaking down early for some reason (not a timing so much as a maintenance issue), leading to the shorter luteal phase? How much of this variation is genetic?

This demonstrates that normal women are incredibly variable in their reproductive physiology. Thus, deviation from the "norm" is not immediately suggestive of pathology.