OC13.05: Developing pre‐eclampsia: does gestational age matter?

To study the risk of pre-eclampsia in ongoing pregnancies according to gestational age. We also studied whether the gestational age specific risk of developing pre-eclampsia has changed across the last decades. In this retrospective cohort study, we used data from the Medical Birth Registry of Norway, and included all singleton pregnancies with delivery between gestational week 21 and 43 in Norway during the years 1984-2019 (n = 1 955 082). We calculated the proportion of pregnancies with pre-eclampsia at each gestational week. The numerator was pre-eclamptic pregnancies with delivery, and the denominator was all pregnancies still ongoing at the gestational week being studied. We performed analyses for the study period as a whole and separately for ten-year intervals within the study period. The total proportion of pregnancies with pre-eclampsia was 3.2%. The risk of developing pre-eclampsia increased by gestational age. At gestational week 28, the risk was 0.2 in 1000 ongoing pregnancies. The risk was 5.2 in 1000 ongoing pregnancies at gestational week 38, and 13.6 in 1000 ongoing pregnancies at gestational week 42. We found no large changes in gestational age specific risk of pre-eclampsia across the study period. The risk of developing pre-eclampsia increased almost exponentially by increasing gestational age. Thus, the reported decline in cumulative incidence of pre-eclampsia from some countries, could partly be explained by an increase in labour inductions and thereby shorter duration of pregnancy. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.