Seasonal variations in coitus and other risk factors, and the outcome of pregnancy.

Data were analyzed from 36,068 U.S. pregnancies to determine whether any of the more frequent maternal risk factors are associated with seasonal variations in perinatal mortality rates. These mortality rates decreased below the yearly mean in January and rose sharply above the mean during the spring. The perinatal mortality rate in May/June was 30% above the rate in January/February (P less than 0.001). This and other variations in perinatal mortality rates closely followed changes in maternal coital activity. The proportion of women having coitus was 26% greater in May/June than in January/February (P less than 0.001). Seasonal variations in the frequencies of maternal hypertension, acetonuria and low pregnancy weight gain did not correlate with changes in perinatal mortality rates.