From the article:

It’s commonly believed that sun exposure is key to maintaining normal levels of vitamin D and therefore assumed that Mediterranean women are at lower risk of hypovitaminosis than those from Northern Europe. However in countries such as Spain, Italy, Greece and Turkey, Vitamin D deficiency occurs in up to 90 % of pregnant populations.

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They studied the effect of a number of different factors including, age, body mass index, race, socioeconomic status, skin types, period of gestation, sun exposure, calcium and vitamin D intake, smoking status, time of year of birth and pregnancy complications. They found that the best predictors of maternal vitamin D deficiency were dark skin, race and dress habits.