Research
Obstetrics
Seasonal variation in pregnancy hypertension is correlated with sunlight intensity

The Figure was presented on a poster at the Annual Meeting of the International Society for the Study of Hypertension in Pregnancy, Oxford, UK, Sept. 11-12, 2009.
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Objective

To examine seasonality of pregnancy hypertension rates, and whether they related to sunlight levels around conception.

Study Design

Data were obtained for 424,732 singleton pregnancies conceived from 2001 through 2005 in Australia. We analyzed monthly rates of pregnancy hypertension and preeclampsia in relation to monthly solar radiation.

Results

Pregnancy hypertension rates, by month of conception, were lowest in autumn (7.3%) and highest in spring (8.9%). Higher sunlight intensity before delivery, but not around conception, was associated with decreased pregnancy hypertension (r = −0.67). Increased sunlight around conception may correlate with decreased rates of early-onset preeclampsia (r = −0.51; P = .09).

Conclusion

The correlation between sunlight after conception and pregnancy hypertension was opposite to that hypothesized; however, sunlight levels before delivery did correlate with lower hypertension rates. For sunlight or ambient temperature to explain seasonal variation, the plausible exposure window is the period before delivery, but this may not apply to early-onset preeclampsia.

Section snippets

Materials and Methods

Data on all women giving birth in hospital (>99% of births) in New South Wales (NSW), Australia, were available from anonymized linked population databases. The Midwives Data Collection (MDC) is a legislated surveillance system of all births in NSW. The Admitted Patient Data Collection (APDC) has discharge summaries of all hospital admissions (public and private) and includes International Classification of Disease-10 (ICD-10) diagnostic codes related to the admission. A MDC delivery record was

Results

There were 424,732 singleton pregnancies included in the monthly conception cohorts in the study period (January 2001 to December 2005) that were delivered at ≥20 weeks' gestation. The mean annual pregnancy hypertension rates and preeclampsia rates were 8.2% and 2.8%, respectively. The mean monthly pregnancy hypertension rate peaked at 8.9% for conceptions in October (midspring in the Southern Hemisphere); whereas, mean monthly preeclampsia rates were high for conceptions from October through

Comment

We found a definite seasonality to the rate of pregnancy hypertension in our study population. Seasonality existed whether pregnancy hypertension rates were calculated based on cohorts by month of conception or month of delivery. Both increased sunlight and increased ambient temperature in the month(s) before delivery were associated with decreased rates of pregnancy hypertension; whereas, increased sunlight and temperature around conception were associated with increased rates of pregnancy

Acknowledgments

We thank the NSW hospital staff who collected the data and the NSW Department of Health who maintains the databases.

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    This work was supported by an Australian National Health and Medical Research Council (NHMRC) Project Grant (570903). Dr Roberts is supported by an NHMRC Senior Research Fellowship (457078) and Dr Ford by an NHMRC Capacity Building Grant in Population Health and Health Services Research (573122).

    Reprints not available from the authors.

    Cite this article as: Algert CS, Roberts CL, Shand AW, et al. Seasonal variation in pregnancy hypertension is correlated with sunlight intensity. Am J Obstet Gynecol 2010;203:215.e1-5.

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