Age-Specific Preterm Birth Rates after Exclusion of Risk Factors - An Analysis of the German Perinatal Survey

Aim: A description of preterm birth rates - specified according to maternal age - after the exclusion of anamnestic risk factors. Material and Methods: Data for this study were taken from the German Perinatal Survey of 1998-2000. We analysed data from 492 576 single ton pregnancies and determined preterm birth rates according to maternal age after a stepwise exclusion of anamnestic risk factors. Results: There was a U-shaped dependence of preterm birth rates on maternal age. The low est preterm birth rate (without excluding women with anamnestic risk factors) was 5.6 % at a maternal age of 29 years. The prevalence of some anamnestic risk factors for preterm birth, such as previous stillbirths, spontaneous and induced abortions, and ectopic pregnancies, increased with maternal age. Excluding women with anam nestic risk factors lowered the preterm birth rates substantially. The lowest preterm birth rates were found in women with one previous live birth, without any anamnestic risk factors, and with a body mass index (BMI) of 25.00-29.99. With these restrictions, we found preterm birth rates of under 2 % for women aged 24-31 years. Conclusions: The magnitude and age-dependence of the preterm birth rate can to some extent be explained with the age-dependent prevalence of anamnestic risk factors for preterm birth. Excluding women with anamnestic risk factors from our study population lowered the preterm birth rates substantially.