OBJECTIVE: Blood pressure (BP) increases both in winter and in the last trimester of pregnancy. Some interaction seems to exist between season and gestational age. The present study observed home BP values during pregnancy with adjustment for seasonal variation and gestational age. METHODS: We observed 10 353 home BP measurements from 101 normal pregnant women attending a maternity hospital in Japan. Home BP values were examined by mixed linear model adjusting for meteorological data and gestational age. RESULTS: The lowest home BP values were observed in the second trimester [mean (±standard deviation) systolic/diastolic BP, 101.8 ± 7.9/59.8 ± 5.8 mmHg at gestational week 20]. In the last trimester, home BP values gradually increased and the values after gestational week 26 were significantly higher than those at gestational week 20 (110.1 ± 9.7/66.8 ± 7.7 mmHg at gestational week 40). A 10°C increase in daily minimum outdoor temperature was associated with a mean reduction of 2.5/2.5 mmHg (Δ systolic BP/Δ diastolic BP: 95% confidence interval, 2.3/2.4 to 2.6/2.7 mmHg) in home BP with adjustment for gestational age. The largest and smallest estimated home BP changes during pregnancy were 12.8/12.5 and 3.1/3.0 mmHg in pregnant woman who delivered in January and July, respectively. CONCLUSION: Interactions among BP, season and gestational age should be considered when evaluating BP in pregnant women. Risks associated with high BP might be underestimated in pregnant woman in summer who will deliver in winter.
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