Vitamin D receptors are present and differently expressed in murine endometrium and ovary throughout the estrous cycle , whereas knock-out experiments have shown that vitamin D receptor null mice experience uterine hypoplasia and impaired folliculogenesis.

Only few retrospective studies examining the role of vitamin D levels in infertile patients have been published up to date, whereas results are strongly contradictory, with some supporting that maternal vitamin D deficiency is associated with lower pregnancy rates and others demonstrating that vitamin D deficiency does not affect final reproductive outcome.

Finally, a recent retrospective study postulated that vitamin D deficiency may negatively affect pregnancy rates with an effect mediated through the endometrium, given that vitamin D deficiency was not correlated with ovarian stimulation characteristics or with markers of embryo quality in this study.

In order to examine a potential negative effect of vitamin D deficiency on pregnancy rates, mediated through the endometrium, the aim of the current study was to examine the impact of vitamin D levels on pregnancy rates only in an infertile population undergoing embryo transfer of frozen-thawed embryos.

Inclusion Criteria:

• All women undergoing a frozen ET with 1 or 2 Day 5 (blastocyst stage) embryo
• Age 18-39

Exclusion Criteria:

• Women > or = 40 years old
• IVM ET
• Uterine abnormalities

Infertile women undergoing a frozen ET with 1 or 2 Day 5 (blastocyst stage) embryo