Changes in risk factors for unfavorable pregnancy outcome among singletons over twenty years

Abstract

To examine the change in the prevalence and relative effect of pre-pregnancy risk factors for low birthweight and/or stillbirth and/or neonatal mortality over a period of twenty years. Two prospectively collected one year birth cohorts. The two northernmost administrative districts of Finland. A birth cohort for 1966 comprising 11905 singleton births and a cohort for 1985-86 containing a further 9247. Risk factors for poor pregnancy outcome. In twenty years the prevalence of low birthweight infants among singletons decreased from 4.2% to 3.1% and that of stillbirths and neonatal deaths from 2.4% to 0.9%. The impact of single marital status decreased and while maternal age < 19 years was not an independent risk factor, the age > or = 35 years related more to poor pregnancy outcome. The proportion of multiparous women was halved but multiparity had a protective influence on pregnancy outcome meanwhile the impact of primiparity decreased markedly. In spite of more effective health education, the prevalence of heavy smokers was 4 times higher in the 1980s as compared with the 1960s, and its impact on adverse outcomes increased, with odds ratio (OR) 1.72 (confidence interval (CI) 1.14-2.61) in 1966 and OR 2.15 (CI 1.61-2.88) in 1985-86. The number of mothers with a previous low birthweight baby was halved but the impact of low birth weight was still great in the latter cohort (OR 2.47; CI 1.64-3.71). The impact of an earlier stillbirth and/or neonatal death increased significantly, with an OR 1.53 (CI 1.10-2.14) in 1966 and OR 2.95 (CI 1.81-4.81) in 1985-86, but the number of mothers concerned was halved from that in 1966. The prevalence of pregnancies with an adverse outcome decreased markedly, as did the prevalence of many of the risk factors. The proportion of > or = 35 years old parturients and of heavy smokers increased. The impact of primiparity decreased, while that of a previous outcome as assessed from pre-pregnancy factors has not improved in these twenty years, especially where primiparous parturients are concerned.