Seasonal variation in the prevalence of pregnancy‐induced hypertension in Japanese women

Abstract Aim 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 301 510 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% (13 848/301 510). The prevalence rate of PIH , ranging 3.6–4.6% for women with deliveries in J uly– A ugust was consistently lowest during the 5‐year study period. The mean ± standard deviation prevalence rate of 4.2 ± 0.3% for delivery in J uly– A ugust was significantly lower than those for any other month ( J anuary– F ebruary, 4.7 ± 0.4%; M arch– A pril, 4.9 ± 0.4%; M ay– J une, 4.6 ± 0.2%; S eptember– O ctober, 4.6 ± 0.1%; and N ovember– D ecember, 4.6 ± 0.2%). The prevalence rate of PIH was highest (4.9%) for women with conception in M arch and A pril and lowest (2.9%) for those with conception in N ovember and D ecember. Conclusion The prevalence rate of PIH was higher for delivery in winter and early spring and lowest for summer delivery among J apanese women.