Table 4.

Relationship between season of birth and the season coinciding with the three trimesters (A) and with the period of exponential mineralization of the femur in utero (B).

Season of birth	Conception	(A) Trimesters coinciding with summer and autumn	(B) Period of exponential femur mineralization in utero (week 20–36)
Jan–Mar (Q1)	Apr–Jun	2nd, partly 3rd	Sep–Mar (winter)
Apr–Jun (Q2)	Jul–Sep	Partly 2nd	Dec–Jun (spring)
Jul–Sep (Q3)	Oct–Dec	3rd	Mar–Sep (summer)
Oct–Dec (Q4)	Jan–Mar	3rd, partly 2nd	Jun–Dec (autumn)

The highest serum vitamin D levels in Danish women are found from July to October (Brot et al., 2001). The lowest average second trimester vitamin D levels in utero would be experienced by persons born in Q3 and most of Q2, and the lowest average third trimester levels by persons born in Q2 and most of Q1. In a person born in April (Q2), the period of maximum mineralization of the femur would begin in early December and proceed through spring where vitamin D levels in the population are at their lowest. By contrast, persons born in December (Q4) enter their period of exponential mineralization in August where vitamin D levels are at their highest.