Bottom Line: Being over- or underweight before pregnancy was associated with a small increased risk of severe maternal illness or death.
Why The Research Is Interesting: Unhealthy weight during pregnancy is associated with adverse birth outcomes. Less is known about the association between unhealthy weight before pregnancy and maternal complications.
Who and When: 743,630 women who gave birth in Washington State, 2004-2013.
Exposure: Women's before-pregnancy body mass index
Outcome: Severe maternal illness or death, defined as life-threatening conditions or conditions leading to serious consequences, or complications requiring intensive care unit admission, or maternal death during the hospitalization.
How (Study Design): This is an observational study. In observational studies, researchers observe exposures and outcomes for patients as they occur naturally in clinical care or real life. Because researchers are not intervening for purposes of the study they cannot control natural differences that could explain study findings so they cannot prove a cause-and-effect relationship.
Authors: Sarka Lisonkova, M.D., Ph.D., University of British Columbia and the Children's and Women's Hospital and Health Centre of British Columbia, Vancouver, and coauthors
Results: Compared to women with normal body mass index [BMI; 18.5-24.9), absolute risks of severe maternal illness or death per 10,000 women were:
28.8 more for underweight women (BMI less than 18.5);
17.6 more for overweight women (BMI 25.0-29.9);
24.9 more for obese women with a BMI of 30.0-34.9;
35.8 more for obese women with a BMI of 35.0-39.9;
61.1 more for obese women with a BMI of 40 or greater.
Study Limitations: Despite the large study size, death and a few very rare complications occurred in only a small number of women to be able to assess associations between unhealthy BMI and these specific outcomes. Information on BMI was self-reported and potentially inaccurate.
Study Conclusions: Among pregnant women in Washington State, unhealthy pre-pregnancy BMI, compared with normal BMI, was associated with a small absolute increase in severe maternal morbidity or mortality.