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 Maternal weight linked to birth defects

Worldwide obesity levels are increasing at an alarming rate, and are linked to many chronic health problems. A recent study showed that the risk of major congenital malformations (birth defects) increases with maternal weight and severity of obesity during early pregnancy.

Healthcare professionals are concerned about the rapid increase in obesity levels. Obesity during pregnancy is associated with major birth defects, which can cause stillbirth, infant mortality, and long-term morbidity. However, it is unknown whether increased maternal weight during early pregnancy, the period when organs form, can influence the risk of developing major birth defects.

A recent report in the British Journal of Medicine investigated the effects of obesity on major birth defects in 1 246 957 live-born infants diagnosed during the first year of life. This population-based cohort study was conducted in Sweden between 2001-2014. Maternal socioeconomic characteristics and maternal/neonatal health data were obtained from various Swedish databases. Maternal BMI was recorded during the first prenatal visit and mothers were classified as: underweight (BMI <18.5), overweight (BMI 25-29), obese class I (BMI 30-34), obese class II (BMI 35-39), obese class III (BMI ≥ 40) or normal weight (BMI 18.5-24). Major birth defects included those affecting the heart, limbs, genital organs, urinary system, digestive system, orofacial clefts, eye, nervous system, and others, according to the European Surveillance of Congenital Anomalies. Newborns with major birth defects with known causes such as chromosomal aberrations, genetic syndromes, malformation syndromes, and viral infections were excluded from the main analysis.

The authors calculated the risk of the association between increasing maternal weight and major birth defects, taking into account various maternal characteristics and the offspring’s sex. They then looked at whether the associations were sex-specific and the effects of gestational and/or pregestational diabetes.

Overall, a total of 43 550 infants (3.5%) had a major birth defect. The risk of any major birth defect increased with increasing maternal weight and severity of obesity from 3.4% in underweight mothers to 4.7% in obese type III mothers. Congenital heart defects were the most common organ-specific defect (1.6%), but the largest risk was seen with nervous system defects. The overall risk of major birth defects was greater in boys than girls (4.1% vs 2.8%), even though the risk for both sexes increased with maternal weight. Diabetes did not influence the effects of increasing weight on birth defects.

The authors note that pregnancies are often terminated when major defects are detected; however, this may not be the case in obese women as antenatal detection of birth defects can be more difficult for these women. Therefore, it is possible that the risk of birth defects in the offspring of obese women was overestimated in this study. In addition, folic acid deficiency, which is associated with neural tube and congenital heart defects, is common in obese women and may have affected the results. Despite these limitations, this study emphasizes how important it is for women to maintain a BMI in the normal range before pregnancy.

Written by  Natasha Tetlow, PhD


  1. Persson M, Cnattingius S, Villamor E, Soderling J, Pasternak B, Stephansson O, et al. Risk of major congenital malformations in relation to maternal overweight and obesity severity: cohort study of 1.2 million singletons. BMJ. 2017;357:j2563. Available at: doi:10.1136/bmj.j2563.


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