Background
Observational studies have implicated low serum vitamin D (25‐hydroxyvitamin D (25(OH)D)) levels in the development of mood disorders. Postpartum depression (PPD) is an important public health issue, although little is known about its association with serum 25(OH)D.

Aims
To determine the association between 25(OH)D at delivery and the subsequent risk of PPD at six weeks and six months postpartum in a large cohort of Australian women.

Materials and Methods
Cord blood samples from 1040 women participating in the docosahexaenoic acid (DHA) to Optimise Maternal Infant Outcome randomised controlled trial were analysed for 25(OH)D by mass spectroscopy. Maternal PPD was assessed using the Edinburgh Postnatal Depression Scale at six weeks and six months postpartum. The association between standardised 25(OH)D and PPD was assessed, taking into account DHA treatment, social and demographic variables.

Results
There was no association between cord blood 25(OH)D concentration at delivery and PPD at either six weeks or six months postpartum. Cord blood 25(OH)D 25–50 and >50 nmol/L at delivery was associated with decreased risk of PPD at six weeks postpartum compared with 25(OH)D <25 nmol/L in the control group, but not the DHA group. There was no association between cord blood 25(OH)D <25 nmol/L at delivery and PPD at six months postpartum. Conclusions This largest study to date of 25(OH)D levels at delivery and PPD did not reveal a consistent link with PPD.