title: Seasonal variation in the prevalence of pregnancy-induced hypertension in Japanese women.
abstract: OBJECTIVE
  Whether prevalence rate of pregnancy-induced hypertension (PIH), including gestational hypertension and pre-eclampsia, is lower for summer delivery than for winter delivery remains to be studied in Japanese women.
  
  
  METHODS
  The prevalence rate of PIH was retrospectively determined according to month of year at delivery and at conception in 301510 Japanese women who gave birth to singleton infants on or after gestational week 22 between 2005 and 2009.
  
  
  RESULTS
  The overall prevalence of PIH was 4.6% (13848/301510). The prevalence rate of PIH, ranging 3.6-4.6% for women with deliveries in July-August was consistently lowest during the 5-year study period. The mean ± standard deviation prevalence rate of 4.2 ± 0.3% for delivery in July-August was significantly lower than those for any other month (January-February, 4.7 ± 0.4%; March-April, 4.9 ± 0.4%; May-June, 4.6 ± 0.2%; September-October, 4.6 ± 0.1%; and November-December, 4.6 ± 0.2%). The prevalence rate of PIH was highest (4.9%) for women with conception in March and April and lowest (2.9%) for those with conception in November and December.
  
  
  CONCLUSIONS
  The prevalence rate of PIH was higher for delivery in winter and early spring and lowest for summer delivery among Japanese women.
authors.affiliation: Department of Obstetrics, Hokkaido University Graduate School of Medicine, Sapporo, Hokkaido, Japan.
authors.forename: Mamoru
authors.initials: M
authors.lastname: Morikawa
authors.forename: Takashi
authors.initials: T
authors.lastname: Yamada
authors.forename: Takahiro
authors.initials: T
authors.lastname: Yamada
authors.forename: Kazutoshi
authors.initials: K
authors.lastname: Cho
authors.forename: Shoji
authors.initials: S
authors.lastname: Sato
authors.forename: Hisanori
authors.initials: H
authors.lastname: Minakami
mesh_terms: D000328:Adult; D015331:Cohort Studies; D036861:Delivery, Obstetric; D005260:Female; D005306:Fertilization; D006801:Humans; D046110:Hypertension, Pregnancy-Induced / Q000453:epidemiology* / Q000208:ethnology; D007564:Japan / Q000453:epidemiology; D011247:Pregnancy; D015995:Prevalence; D012042:Registries; D012189:Retrospective Studies; D012621:Seasons
keywords: gestational hypertension; outdoor temperature; pre-eclampsia; seasonal variation; sunlight