Normal range of maternal serum vitamin d at 11-13 weeks' gestation.

Objective: To establish a normal range of maternal serum levels of total vitamin D at 11-13 weeks' gestation and examine factors from maternal characteristics and obstetric history affecting these levels.

Methods: Serum 25-hydroxyvitamin D [25(OH)D] was measured by liquid chromatography-tandem mass spectrometry in 1,000 singleton pregnancies with normal outcome. Multiple regression analysis was used to determine the factors from maternal characteristics and obstetric history affecting the measured 25(OH)D levels and the regression model was used to convert each measurement into a multiple of the median (MoM).

Results: Serum 25(OH)D increased with maternal age, decreased with body mass index (BMI), it was higher in the summer than other months and when conception was assisted than spontaneous and it was lower in cigarette smokers and in women of African and Asian racial origin compared to Caucasians. In Caucasian, non-smoking women, with BMI below 25, conceiving spontaneously and sampled in the summer months the estimated 5th, 10th, 50th, 90th and 95th percentiles of 25(OH)D were 10.8, 14.7, 30.7, 48.2 and 55.7 ng/ml, respectively.

Conclusion: Maternal serum 25(OH)D concentration at 11-13 weeks is affected by season of blood sampling and maternal characteristics.