Background: When assessed in pregnancy studies, 25(OH)D is usually measured only once. However, it is unknown whether the ranking of an individuals 25(OH)D is maintained across pregnancy, which crosses several seasons. We therefore assessed the tracking of 25(OH)D from early to late pregnancy in a prospective motheroffspring study, the Southampton Womens Survey.
Methods: At 14 and 34 weeks gestation, serum 25(OH)D was measured, and diet and lifestyle questionnaires completed. We modelled seasonal variation in 25(OH)D separately for each time point using Fourier transformations, and then calculated the difference between actual 25(OH)D and the modelled value corresponding to the sampling date for each individual (denoted 25(OH)Ddev). We used Spearmans rank correlation to test tracking of 25(OH)Ddev from 14 to 34 weeks gestation. Multivariate linear regression was used to determine factors associated with alterations in an individuals 25(OH)Ddev ranking.
Results: 25(OH)D was available in 2060 and 2332 women at 14 and 34 weeks, respectively, with 1756 women included at both gestations. 25(OH)Ddev tracked moderately from 14 to 34 weeks (r=0.57, P<0.0001), although some women had marked changes in 25(OH)Ddev across pregnancy (median: −0.8; range: −150.1 to 129.6 nmol/l). 25(OH)D tended to fall with greater pregnancy weight gain (25(OH)Ddev β=−0.4 nmol/l per kg, P=0.02), and to rise with greater strenuous activity in late pregnancy (β=1.0 nmol/l per h per week, P=0.03). Vitamin D supplementation was the strongest influence on tracking: compared with women who never used supplements, discontinuing supplementation after 14 weeks was associated with negative change in 25(OH)dev (β=−7.2 nmol/l, P<0.001), whereas commencing (β=12.2 nmol/l, P<0.001) or continuing (β=8.0 nmol/l, P<0.001) supplementation were positively associated.
Conclusion: Stability of an individuals gestational 25(OH)D relative to the population is modest, and affected by weight changes, activity levels and vitamin D supplementation. These findings may explain some of the observed heterogeneity in studies relating maternal vitamin D status to offspring health.
Disclosure: K Godfrey has acted as a consultant to Abbott Nutrition and Nestle Nutrition. He is part of an academic consortium that has received research funding from Abbott Nutrition, Nestec and Danone. This work was supported by the Medical Research Council, British Heart Foundation, Arthritis Research UK, National Osteoporosis Society, International Osteoporosis Foundation, Cohen Trust, NIHR Southampton Biomedical Research Centre, and NIHR Musculoskeletal Biomedical Research Unit, University of Oxford and the Dunhill Medical Trust.