18th Communication: Preterm Birth Rates and Maternal Occupation – The Importance of Age and Number of Live Births As Confounding Factors

Background and Aim: Socio-economic status, as described by maternal occupation, is an important determinant of the rate of preterm births, as are age and parity. We aimed to investigate the influence on preterm birth rates of maternal occupation, age and number of live births, and the interaction of these factors. Material and Methods: We analysed data of 2130584 singleton pregnancies from the German perinatal statistics of 1995-2000. Preterm birth rates, stratified according to age and number of live births, were calculated for the total population and for two (of seven) categories of maternal occupation coded for in German perinatal statistics, "unskilled labourers" and "highly qualified employees and entrepreneurs". Differences between preterm birth rates of women in these two occupational categories were calculated and standardised according to age and number of live births. Results: Overall rates of preterm birth were 7% (total population), 7.8% (unskilled labourers), and 6.3% (highly qualified employees and entrepreneurs). Age and number of live births both exhibited a U-shaped influence on preterm delivery rates. Rates were lowest for women aged 25-29 (higher for younger and older women) and those giving birth to their second child (higher for primiparae and those with ? 3 live births). While the crude overall difference in preterm birth rates between the two occupational groups was only 1.5%, the overall difference standardised according to age and number of live births was 2.3%. Taking age and number of live births into account, the differences between preterm birth rates in certain subgroups can substantially exceed the overall difference between occupational categories. In women >= 40 with >= 4 live births, unskilled labourers have an 8.2% higher rate of preterm births (13.8 vs. 5.6%). Conclusions: When analysing the influence of maternal socioeconomic status (occupation) on preterm birth rates it is important to take account of confounding factors such as age and number of live births. The rates of preterm delivery presented here may serve as an aid in counselling women on the risk of preterm birth.