Vitamin D receptors (proteins which actually do the work in the cell but need vitamin D to dock onto them so they can change in shape and convert to the active form) have been found in all fertility-related organs and cells, including sperm, ovaries and placenta. Vitamin D was found to affect sperm motility. Vitamin D was found to improve embryo quality and positively affect the endometrial receptiveness, area now really hot in reproductive research. Vitamin D seems to improve egg quality and this may have to do with the last complex meyotic step which takes place in the ovulating egg, but many details still need to be worked out. Also in IVF patients, vitamin D blood levels proved to be a better predictor of the cycle success then any other conventionally used clinical parameter.
By strengthening the immune system, vitamin D works against unexplained infertility. And here in this paper you can read that vitamin D protects the body against Polycystic Ovarian Syndrome (PCOS), which is one of the leading causes of infertility in women. Please continue reading here more on how vitamin D helps improve egg quality.