title: Tracking of 25-hydroxyvitamin D status during pregnancy: the importance of vitamin D supplementation.
abstract: BACKGROUND
  The role of maternal 25-hydroxyvitamin D [25(OH)D] in fetal development is uncertain, and findings of observational studies have been inconsistent. Most studies have assessed 25(OH)D only one time during pregnancy, but to our knowledge, the tracking of an individual's 25(OH)D during pregnancy has not been assessed previously.
  
  
  OBJECTIVE
  We determined the tracking of serum 25(OH)D from early to late pregnancy and factors that influence this.
  
  
  METHODS
  The Southampton Women's Survey is a prospective mother-offspring birth-cohort study. Lifestyle, diet, and 25(OH)D status were assessed at 11 and 34 wk of gestation. A Fourier transformation was used to model the seasonal variation in 25(OH)D for early and late pregnancy separately, and the difference between the measured and seasonally modeled 25(OH)D was calculated to generate a season-corrected 25(OH)D. Tracking was assessed with the use of the Pearson correlation coefficient, and multivariate linear regression was used to determine factors associated with the change in season-corrected 25(OH)D.
  
  
  RESULTS
  A total of 1753 women had 25(OH)D measured in both early and late pregnancy. There was a moderate correlation between season-corrected 25(OH)D measurements at 11 and 34 wk of gestation (r = 0.53, P < 0.0001; n = 1753). Vitamin D supplementation was the strongest predictor of tracking; in comparison with women who never used supplements, the discontinuation of supplementation after 11 wk was associated with a reduction in season-corrected 25(OH)D (β = -7.3 nmol/L; P < 0.001), whereas the commencement (β = 12.6 nmol/L; P < 0.001) or continuation (β = 6.6 nmol/L; P < 0.001) of supplementation was associated with increases in season-corrected 25(OH)D. Higher pregnancy weight gain was associated with a reduction in season-corrected 25(OH)D (β = -0.4 nmol · L(-1) · kg(-1); P = 0.015), whereas greater physical activity (β = 0.4 nmol/L per h/wk; P = 0.011) was associated with increases.
  
  
  CONCLUSIONS
  There is a moderate tracking of 25(OH)D status through pregnancy; factors such as vitamin D supplementation, weight gain, and physical activity are associated with changes in season-corrected 25(OH)D from early to late gestation. These findings have implications for study designs and analyses and approaches to intervention studies and clinical care.
authors.affiliation: Medical Research Council Lifecourse Epidemiology Unit, University of Southampton, Southampton General Hospital, Southampton, United Kingdom; Paediatric Endocrinology, University Hospital Southampton National Health Service (NHS) Foundation Trust, Southampton, United Kingdom;
authors.forename: Rebecca J
authors.initials: RJ
authors.lastname: Moon
authors.affiliation: Medical Research Council Lifecourse Epidemiology Unit, University of Southampton, Southampton General Hospital, Southampton, United Kingdom;
authors.forename: Sarah R
authors.initials: SR
authors.lastname: Crozier
authors.affiliation: Medical Research Council Lifecourse Epidemiology Unit, University of Southampton, Southampton General Hospital, Southampton, United Kingdom;
authors.forename: Elaine M
authors.initials: EM
authors.lastname: Dennison
authors.affiliation: Paediatric Endocrinology, University Hospital Southampton National Health Service (NHS) Foundation Trust, Southampton, United Kingdom;
authors.forename: Justin H
authors.initials: JH
authors.lastname: Davies
authors.affiliation: Medical Research Council Lifecourse Epidemiology Unit, University of Southampton, Southampton General Hospital, Southampton, United Kingdom;
authors.forename: Sian M
authors.initials: SM
authors.lastname: Robinson
authors.affiliation: Medical Research Council Lifecourse Epidemiology Unit, University of Southampton, Southampton General Hospital, Southampton, United Kingdom;
authors.forename: Hazel M
authors.initials: HM
authors.lastname: Inskip
authors.affiliation: Medical Research Council Lifecourse Epidemiology Unit, University of Southampton, Southampton General Hospital, Southampton, United Kingdom; National Institute for Health Research (NIHR) Southampton Nutrition Biomedical Research Centre, University of Southampton and University Hospital Southampton NHS Foundation Trust, Southampton, United Kingdom; and.
authors.forename: Keith M
authors.initials: KM
authors.lastname: Godfrey
authors.affiliation: Medical Research Council Lifecourse Epidemiology Unit, University of Southampton, Southampton General Hospital, Southampton, United Kingdom; National Institute for Health Research (NIHR) Southampton Nutrition Biomedical Research Centre, University of Southampton and University Hospital Southampton NHS Foundation Trust, Southampton, United Kingdom; and cc@mrc.soton.ac.uk.
authors.forename: Cyrus
authors.initials: C
authors.lastname: Cooper
authors.affiliation: Medical Research Council Lifecourse Epidemiology Unit, University of Southampton, Southampton General Hospital, Southampton, United Kingdom; National Institute for Health Research (NIHR) Southampton Nutrition Biomedical Research Centre, University of Southampton and University Hospital Southampton NHS Foundation Trust, Southampton, United Kingdom; and.
authors.forename: Nicholas C
authors.initials: NC
authors.lastname: Harvey
mesh_terms: D015652:25-Hydroxyvitamin D 2 / Q000097:blood; D000328:Adult; D002112:Calcifediol / Q000097:blood; D015331:Cohort Studies; D019587:Dietary Supplements; D004739:England / Q000453:epidemiology; D005260:Female; D005583:Fourier Analysis; D006801:Humans; D008137:Longitudinal Studies; D039382:Maternal Nutritional Physiological Phenomena; D008954:Models, Biological; D009752:Nutritional Status; D011247:Pregnancy; D011248:Pregnancy Complications / Q000097:blood / Q000453:epidemiology / Q000517:prevention & control*; D011255:Pregnancy Maintenance; D011446:Prospective Studies; D012306:Risk; D012621:Seasons; D014807:Vitamin D / Q000627:therapeutic use*; D014808:Vitamin D Deficiency / Q000097:blood / Q000453:epidemiology / Q000517:prevention & control*; D055815:Young Adult
chemical_list: D014807:Vitamin D; D015652:25-Hydroxyvitamin D 2; D002112:Calcifediol
keywords: epidemiology; osteoporosis; pregnancy; supplementation; tracking; vitamin D