Seasonal variation of serum 25(OH) vitamin D levels in maternal and umbilical cord blood in Japanese women.

We aimed to demonstrate that the serum 25-hydroxyvitamin D (25(OH)D) level in maternal and umbilical cord blood has a seasonal variation in Japanese women. The study cohort comprised 256 healthy Japanese women with a singleton pregnancy who delivered after 36 gestational weeks between 2012 and 2015. The season at delivery was categorized for 3 months and recorded as "spring", "summer", "autumn" and "winter". Subjects were divided into four groups according to season. A sample of peripheral venous blood at 35-36 gestational weeks and blood from the umbilical vein at delivery were taken. The mean serum 25(OH)D concentration (ng/mL) in maternal blood for each season (spring, summer, autumn and winter) was 18.0 (+/- 6.7), 17.1 (+/- 5.1), 21.6 (+/- 8.0) and 16.0 (+/- 5.1), whereas that for umbilical cord blood was 8.8 (+/- 3.6), 8.6 (+/- 2.6), 10.7 (+/- 3.5) and 8.6 (+/- 2.1), respectively. The mean serum 25(OH)D concentration of maternal and umbilical cord blood in autumn was higher than that for the other three seasons. In pregnant Japanese women, the mean serum 25(OH)D concentration in maternal and umbilical cord blood was affected by the season of delivery, with both being highest in autumn. Regardless of the season, the maternal serum concentration of 25(OH)D was low in Japan.