Seasonal variation in birth weight

Hope Weiler and colleagues noted that it was difficult to explain their observation that newborns with vitamin D deficiency were heavier and longer and had greater head circumferences than newborns with adequate vitamin.¹ However, their findings are consistent with several studies examining the impact of season of birth on neonatal anthropometry.² For example, our group found that birth weight, birth length and head circumference all fluctuated across the seasons, with peak values in children born during the winter and spring, when hypovitaminosis D is most prevalent.³

Vitamin D suppresses cell proliferation and promotes cell elimination via apoptosis in a variety of tissues.⁴ Thus, its absence in the prenatal period could lead to inappropriately high cell numbers, which could subsequently influence the size of the offspring. Evidence from animal experiments indicates that these mechanisms do have an impact on fetal growth. For example, the newborn offspring of normocalcemic rats deprived of vitamin D were significantly heavier than those of control animals and there were subtle changes in the shape of their brains.⁵ Guinea pig fetuses exposed to low levels of vitamin D had expanded growth plates in their long bones.⁶ If similar mechanisms operated in humans, we would predict precisely what Weiler and colleagues have found: the newborns of mothers with hypovitaminosis D should be heavier (due to increased cell number) and longer (due to wider growth plates in the lower limb bones).

John J. McGrath Thomas H. Burne Darryl W. Eyles Queensland Centre for Mental Health Research University of Queensland St. Lucia, Australia