The medically relevant vitamins ergocalciferol (vitamin D2) and cholecalciferol (vitamin D3) are obtained from food but can also be produced under the influence of UVB light. They act as prohormones and regulate the balance of calcium and phosphate.
The main sites of action are the intestines, bones, and kidneys. In ovaries, vitamin D increases the production of sex steroids, progesterone and estrogen, which are required for maintenance of normal menstrual cycle. Vitamin D is also required for the production of hormones which support implantation.
Women with higher blood levels of vitamin D have better reproductive capacity. Moreover, low blood levels of vitamin D also hinder in vitro fertilization, and women undergoing assisted reproduction should also take vitamin D supplements.
The recommended daily dose of Vitamin D during pre-conception and pregenancy is 20 micrograms or 4,000 IU.