Risk of pre-eclampsia according to gestational age across three decades: population-based study.

OBJECTIVES

To determine the risk of pre-eclampsia in ongoing pregnancies according to gestational age. We also studied whether the gestational-age-specific risks of developing pre-eclampsia have changed over recent decades.

METHODS

This was a population-based study using data from the Medical Birth Registry of Norway. The study population included all singleton pregnancies in Norway delivered between 21 and 43 weeks' gestation during the period 1984-2019. We calculated the proportion of pregnancies with pre-eclampsia at each week of gestation. The numerator was the number of pre-eclamptic pregnancies that were delivered during the gestational week being studied, and the denominator included all pregnancies still ongoing at the same gestational week. We analyzed the study period as a whole and subdivided it into four time periods (1984-1989, 1990-1999, 2000-2009 and 2010-2019).

RESULTS

A total of 1 955 082 pregnancies were included, of which 3.2% developed pre-eclampsia. The risk of pre-eclampsia increased with advancing gestational age. The risk per 1000 ongoing pregnancies was 0.2 at 28 weeks, 5.2 at 38 weeks and 13.6 at 42 weeks. The gestational-age-specific risk of developing pre-eclampsia was lower at all gestational ages during the period 2010-2019 compared with 2000-2009, and the absolute decrease was greatest in term and post-term pregnancies.

CONCLUSIONS

The risk of developing pre-eclampsia increased almost exponentially with advancing gestational age. During the last decade of the study period, the risk of pre-eclampsia decreased, particularly in term pregnancies. This decrease may be explained by the use of obstetric interventions to shorten the duration of pregnancy among women at high risk of developing pre-eclampsia. © 2026 The Author(s). Ultrasound in Obstetrics & Gynecology published by John Wiley & Sons Ltd on behalf of International Society of Ultrasound in Obstetrics and Gynecology.