According to the National Institute for Health and Clinical Excellence, one in seven couples in the UK have problems conceiving. Most couples (about 80%) who are trying to get pregnant do so within a year and about 90% succeed in 2 years. About 50% of fertility problems are due to problems with the man, while at least 20% of cases of infertility have no known reason.
Scientists from the University of Stockholm have already shown that higher vitamin D levels in men are associated (association is not causation) with healthier sperm, faster sperm, sperm better able to penetrate the female egg. Randomized controlled trials of men, women, and couples are underway to see if vitamin D supplements help couples conceive, but the results will not be available for years.