Recent evidence has shown that pregnant mothers who were given 4,000 IU (ten times the usual dose) of vitamin D daily had only half the risk of giving birth to premature babies as those who were not supplemented.  That amount can also be produced by about 20 minutes of full- body exposure to non-burning sunlight at midday.
The importance of this information is shown in the fact that annually, half of all premature babies die in the first month after birth, according to the March of Dimes. In North America, about 500,000 premature births occur annually. If vitamin D supplementation could prevent half of these deaths, that would save the lives of 250,000 babies per year. Worldwide, the lives saved might be as many as 7 million, since approximately 13 million babies are born prematurely each year. We also know that the average cost for each premature baby in the first year of life is about $49,000. http://www.marchofdimes.com/aboutus/22684_55250.asp
Premature babies, of course, are also low-birth-weight babies in most cases, which present an additional problem. Here is one more reason for mothers to get back in the sunlight: the potential for low birth weight in their babies.
Low birth weight is associated with poor mood, anxiety, depression, high blood pressure and other problems during childhood and afterward. Recent research shows that low birth weight is related to exposure by pregnant women to winter temperatures during a critical developmental time for the fetus.  This could indicate vitamin D deficiency of the pregnant mother during “vitamin D winter,” the time of year in northern latitudes when the sun is too low in the sky to produce vitamin D. The answer, of course, is to use a tanning bed or take vitamin D3 supplements (3,000-5,000 IU) during the winter. Remember never to burn!
Those who make a living frightening people out of the sunlight are responsible for much of the vitamin D deficiency in the population of North America. Don’t expect them to change. Non-burning sunlight is a wonderful gift for health, and we must stop the insanity that is causing vitamin D deficiency. The child needs every possible advantage prior to birth, and one of the advantages is a mom with high vitamin D levels. The only source of vitamin D for the fetus is the mother’s body, and the only natural way to obtain vitamin D is exposure to sunlight.
 Hollis, B. and Wagner C. Report from an international conference on vitamin D in Bruges, Belgium.
 March of Dimes statement Oct 4, 2009, based on World Health Organization (WHO) statistics.
  Elter K, et al. Exposure to low outdoor temperature in the midtrimester is associated with low birth weight. Aust N Z J Obstet Gynecol 2004;44:553-7.
 Murray, L. et al. Links of Season and outdoor ambient temperature: effects on birth weight. Obstet Gynecol. 2000 Nov;96(5 Pt 1):689-95.