The influence of maternal age on very preterm birth of twins: differential effects by parity.

For singleton births, parity can modify the effect of maternal age on birth outcomes such as low birthweight and preterm birth; however, it is unknown whether this relationship exists for twin births. As the rate of twin births increases among older women, it is important to understand how parity may influence the relationship between maternal age and adverse birth outcomes. The NCHS Matched Multiple Birth Data Set, which contains all twin births in the USA from 1995 to 1998, was analysed. Parity was grouped into two levels (primiparous--no prior live births, and multiparous--at least one prior live birth), and maternal age was divided into the following groups: 20-24, 25-29, 30-34, 35-39, and 40 years or more. Very preterm birth was defined as births occurring before 33 weeks. Logistic regression was used to obtain odds ratios (OR) to estimate the risk of very preterm birth, and to determine the relationships between parity, maternal age, and very preterm birth. Among primiparae, women 40 years and older had a reduced risk of very preterm birth compared with women of 25-29 years (OR 0.74 [95% CI=0.66, 0.84]). Among multiparae, women 40 years and older had the same risk of very preterm birth compared with women of 25-29 years (OR 1.00 [95% CI=0.90, 1.12]). However, stratification by education revealed that the age gradient was limited to women with >12 years education among primiparae. The effect of maternal age on very preterm birth of twins differs according to parity. To some extent, that effect is further modified by education. Therefore, future analyses of maternal age and twin birth outcomes should account for measures of obstetric history and other factors, which may influence these results.