title: Seasonal variation of dystocia in a large Danish cohort.
abstract: BACKGROUND
  Dystocia is one of the most frequent causes of cesarean delivery in nulliparous women. Despite this, its causes are largely unknown. Vitamin D receptor (VDR) has been found in the myometrium. Thus, it is possible that vitamin D affects the contractility of the myometrium and may be involved in the pathogenesis of dystocia. Seasonal variation of dystocia in areas with distinct seasonal variation in sunlight exposure, like Denmark, could imply that vitamin D may play a role. This study examined whether there was seasonal variation in the incidence of dystocia in a Danish population.
  
  
  METHODS
  We used information from a cohort of 34,261 nulliparous women with singleton pregnancies, spontaneous onset of labor between 37 and 42 completed gestational weeks, and vertex fetal presentation. All women gave birth between 1992 and 2010 at the Department of Obstetrics and Gynecology, Aarhus University Hospital, Skejby. Logistic regression combined with cubic spline was used to estimate the seasonal variation for each outcome after adjusting for calendar time.
  
  
  RESULTS
  No evidence for seasonal variation was found for any of the outcomes: acute cesarean delivery due to dystocia (p = 0.44); instrumental vaginal delivery due to dystocia (p = 0.69); oxytocin augmentation due to dystocia (p = 0.46); and overall dystocia (p = 0.91).
  
  
  CONCLUSIONS
  No seasonal variation in the incidence of dystocia was observed in a large cohort of Danish women. This may reflect no association between vitamin D and dystocia, or alternatively that other factors with seasonal variation and influence on the occurrence of dystocia attenuate such an association.
authors.affiliation: Department of Obstetrics and Gynecology, Aarhus University Hospital, Skejby, Aarhus, Denmark.
authors.forename: Christine
authors.initials: C
authors.lastname: Rohr Thomsen
authors.affiliation: Department of Obstetrics and Gynecology, Aarhus University Hospital, Skejby, Aarhus, Denmark.
authors.forename: Niels
authors.initials: N
authors.lastname: Uldbjerg
authors.affiliation: Department of Obstetrics and Gynecology, Aarhus University Hospital, Skejby, Aarhus, Denmark.
authors.forename: Lone
authors.initials: L
authors.lastname: Hvidman
authors.affiliation: Department of Pediatrics, Aarhus University Hospital, Skejby, Aarhus, Denmark.
authors.forename: Hjördís Ósk
authors.initials: HÓ
authors.lastname: Atladóttir
authors.affiliation: Department of Pediatrics, Aarhus University Hospital, Skejby, Aarhus, Denmark; Perinatal Epidemiology Research Unit, Institute of Clinical Medicine, Aarhus University Hospital, Skejby, Aarhus, Denmark.
authors.forename: Tine Brink
authors.initials: TB
authors.lastname: Henriksen
authors.affiliation: Perinatal Epidemiology Research Unit, Institute of Clinical Medicine, Aarhus University Hospital, Skejby, Aarhus, Denmark.
authors.forename: Ioanna
authors.initials: I
authors.lastname: Milidou
mesh_terms: D000328:Adult; D015331:Cohort Studies; D003718:Denmark / Q000453:epidemiology; D004420:Dystocia / Q000453:epidemiology*; D005260:Female; D006801:Humans; D011247:Pregnancy; D012621:Seasons; D055815:Young Adult
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