Association between postpartum depression and vitamin D.

The fitted fractional polynomial model of degree 3 (FP (3)) with 95% CI in black and the categorical estimates in blue. The odds ratios are from the fully adjusted model adjusting for maternal age, year of delivery, season, gestation week, parity, smoking, socio economic status, BMI group, physical activity and social support and multivitamin supplement. The reference concentration is 60 nmol/L. The fractional polynomial part of the final model is estimated to be: log Odds =  constant+0.46·X³+0.27·X³·ln(X) -3.26·X³·ln(X)², where X = vitamin D/100.