Preeclampsia and birthweight by gestational age in singleton pregnancies in Flanders, Belgium: a prospective study

Recent studies have questioned the relationship between low birthweight and hypertension in pregnancy, especially in term pregnancies. We aimed to analyse the influence of chronic hypertension, preeclampsia, gestational hypertension and superimposed preeclampsia on birth weight in singleton pregnancies at different gestational ages.

Between January 1, 2001 and December 31, 2002 data on hypertension (subdivided in chronic hypertension, preeclampsia, gestational hypertension, superimposed preeclampsia and eclampsia) were collected prospectively for all deliveries in the region of Flanders, Belgium. Multiple pregnancies and patients with diabetes were excluded from analysis. Multiple linear regression was performed to construct a model for the prediction of birthweight and to determine the contribution of hypertension.

Hypertension was diagnosed in 5,284 of 111,007 (4.8%) singleton pregnancies, and of these 647 had chronic hypertension (0.6% of the total group), 2,253 (2%) gestational hypertension, 2,244 (2%) preeclampsia and 140 (0.1%) superimposed preeclampsia. Birthweight less than 2,500 g was most frequent in the preeclamptic group and less frequent in case of gestational hypertension, but in all hypertensive groups it was statistically more frequent compared to the normotensive pregnancies. Before 26 weeks' gestational age the presence of any kind of hypertension did not influence birthweight. From 26 weeks on preeclampsia contributed to a lower birthweight. Gestational hypertension resulted in a lower birthweight between 28 and 34 weeks, but not before or after this period. Superimposed preeclampsia only had an effect between 32 and 34 weeks and chronic hypertension only marginally contributed to birthweight. A relation with both a high birthweight (> 4000 g) and birthweight < 2500 g was found in term gestational hypertension and preeclampsia.

At an early gestational age (less then 26 weeks) hypertension is not a significant factor influencing birthweight. Uncomplicated chronic hypertension is not an important factor determining birthweight but preeclampsia is. Gestational hypertension influences birthweight in a limited period between 28 and 34 weeks of gestational age. When hypertensive pregnancies reach term they tend to result both in more babies weighing < 2,500 g and > 4,000 g.