Hypertensive disorders of pregnancy and gestational diabetes mellitus by birth order: A descriptive study across lifetime reproductive history

Abstract Introduction Hypertensive disorders of pregnancy and gestational diabetes mellitus are known to be strongly related, and underlying maternal cardiometabolic dysfunction has been proposed as a mechanism for both complications. A comparison of the trends by birth order for the two complications may reveal insights into their shared or different causes. Material and Methods We used data from the Medical Birth Registry of Norway from 1999 to 2020 with an aim to compare risk by birth order for hypertensive disorders of pregnancy and gestational diabetes mellitus. Visualizations of cross‐sectional risks by birth order are stratified by mother's age, calendar year, and mother's body mass index. Results The risk of gestational diabetes mellitus appeared to increase by birth order, while hypertensive disorders of pregnancy were most common in first birth. GDM was more common in one‐child mothers than mothers with 2 to 4 lifetime births (OR 1.96, 95%CI 1.83, 2.10). For hypertensive disorders, we observed above 50 per 1000 births in first birth, but below 50 per 1000 births for later births, regardless of covariates. We no longer observed this trend by birth order for gestational diabetes (with risk around 60 per 1000 births for 40 year old mothers at any parity, for instance). Higher risk of diabetes in subsequent pregnancies reflects the older age of mothers. Mothers' body mass index is a known risk factor for both complications. Conclusions The increase in gestational diabetes mellitus by birth order closely reflects trends in other chronological variables like year and age. One explanation for the distinctive rise in gestational diabetes prevalence in later births is secular trends, while secular trends do not fully explain trends by birth order for hypertensive disorders of pregnancy. Different relationships to birth order and secular trends indicate different contributing mechanisms besides maternal cardiometabolic dysfunction in these two complications.