Lack of vitamin D almost halves the chances of conceiving for women undergoing IVF treatment, a study has found.
Researchers in Italy looked at the success rates of assisted fertility procedures in 154 women who were deficient in the vitamin and 181 with sufficient levels in their blood.
The second group was nearly twice as likely to conceive as the first.
A "sufficient" amount of vitamin D was defined as at least 20 nanograms (ng) per millilitre of blood - less than the recommended level of 30 ng.
Study author Dr Alessio Paffoni, from the Maggiore Policlinico Hospital in Milan, said: "Our work is the largest study to date to examine how vitamin D affects fertility in women who are undergoing IVF.
"We found that women who had sufficient levels of vitamin D were more likely to produce high-quality embryos and more likely to become pregnant than women who were deficient in vitamin D."
The vitamin may assist both the production of high-quality eggs in the ovaries and the implantation of embryos in the womb, said the researchers writing in the Journal of Clinical Endocrinology & Metabolism.
Dr Paffoni added: "Since vitamin D supplementation is an inexpensive with few relevant side effects, additional study in this area has the potential to markedly influence the way infertility is treated."