Objective To study early pregnancy characteristics as possible risk factors associated with preterm birth.
Design Retrospective analysis of prospectively collected maternity data.
Population 21,069 singleton deliveries with record of a specified last menstrual period and a midtrimester dating scan.
Setting Catchment area of tertiary centre serving a general maternity population.
Methods Univariate and multivariate analysis. Variables included: maternal age; height; weight at first visit; parity; ethnic group; cigarette smoking and alcohol consumption recorded in early pregnancy; history of abortion; history of preterm birth; and discrepancy between menstrual dates and ultrasound dates.
Main outcome measures Adjusted odds ratios for factors associated with preterm birth, stratified according to parity (nulliparae vs multiparae) and gestational age (early preterm, 24–33 weeks; late preterm, 34–36 weeks; all preterm, < 37 weeks). Population attributable risk (aetiologic fraction) of the significant variables for preterm birth.
Results The overall preterm (< 37 weeks) delivery rate according to scan dates was 7.0%. Preterm birth was associated with young (< 20 years), short (≤ 155 cm) and underweight (≤ 52 kg) mothers, non-Europeans, cigarette smokers, previous abortion or previous preterm delivery, and a prolonged menstruation-conception interval. Preterm births which followed the spontaneous onset of labour (72%) had results which were similar to the overall group, while there were too few iatrogenic preterm deliveries for separate analysis. Logistic regression showed that associations varied in different parity and gestational age groups. For nulliparae, smoking was not associated with preterm birth, but it was strongly associated with multiparous women (adjusted OR 1.8, 95% CI 1.6–2.1). A past history of premature delivery had the highest risk for birth before 34 weeks in the index pregnancy (adjusted OR 5.1, 95% CI 3.4–7.6). A discrepancy between menstrual and scan dates of greater than +7 days, suggestive of a prolonged interval between last menstruation and conception, was present in 23.3% of all pregnancies, and was associated with an increased risk of preterm delivery in all gestational age categories for nulliparae (adjusted OR 1.5, 95% CI 1.3–1.8) and multiparae (adjusted OR 1.9, 95% CI 1.6–2.2). Because of its high prevalence, this variable constituted a relatively high population-attributable risk for premature birth for both nulliparae (10.7%) and multiparae (16.6%).
Conclusions A discrepancy of more than +7 days between menstrual and scan dates, indicating a prolonged interval between last menstruation and conception, is a significant predictor of preterm birth. This effect is independent of other factors such as maternal age, height, weight and smoking which are also associated with prematurity. In a maternity population with ultrasound scan dates and recorded last menstrual period, this variable can be easily calculated and used as a marker for increased surveillance.