Seasonal patterns in perinatal mortality and preterm delivery.

An investigation of possible seasonal patterns in preterm delivery and perinatal mortality utilized linked birth, infant death, and fetal death records from Minnesota for the years 1967-1973. Data included over 400,000 white singleton live births and stillbirths of 29 or more weeks completed gestation. Composite monthly cohorts of ongoing pregnancies were constructed for each month of the year and the probability of a preterm delivery and/or perinatal death was estimated. A statistically significant increase in the probability of a preterm delivery or perinatal death occurred during July, August, and September. The probability of a preterm delivery ranged from a low of 55 per 1000 pregnancies at risk in April to a high of over 59 per 1000 in July and August. In addition, although each assigned cause of death group showed a similar pattern, perinatal deaths due to infection in the mother or fetus showed a standardized mortality ratio of 65 in May and 155 in August and September, while the ratios of all other causes combined ranged from 94 to 108. Fetal deaths reported as having occurred before labor began showed a seasonal pattern nearly identical with that for all perinatal deaths, while those occurring during labor showed no seasonal pattern. The findings are consistent with published evidence that environmental factors likely involving ascending infections may play a larger role in preterm delivery and perinatal mortality than has been generally recognized.