Risks of major birth defects during the first year of life progressively increase with a mother’s overweight and obesity severity

The results of BMJ study suggest that efforts should be made to encourage women of reproductive age to adopt a healthy lifestyle and obtain a normal body weight before conception, say the authors.

Obesity has reached epidemic proportions globally and is now a major health concern in pregnancy in both high- and low-income countries, they argue.

It is already known that maternal obesity increases risks of congenital malformations, but it is not yet clear if risks are increased in offspring of overweight mothers and if risks increase with increasing severity of obesity.

A team of researchers, led by Martina Persson and Martin Neovius from the Karolinska Institute in Sweden, set out to investigate whether risks of major congenital malformations increase with severity of obesity in mothers.

They analysed information on more than 1.2 million live singleton births in Sweden recorded in the medical birth register between 2001 and 2014.

Data on maternal and pregnancy characteristics were recorded. Details of any major congenital malformation, and several organ specific malformations, diagnosed during the first year of life were also recorded.

The researchers defined underweight mothers as having a body mass index of less than 18.5, normal weight mothers (BMI 18.5 to 24), overweight mothers (BMI 25 to 29), and mothers in obesity classes I (BMI 30 to 34), II (BMI 35 to 39), and III (BMI 40 or over).

A total of 43,550 (3.5%) offspring had any major congenital malformation. Heart defects were the most common malformation, followed by defects of the genital organs, limbs, urinary system, digestive system, and nervous system.

Several factors that could have influenced the results were taken into account.

The researchers found that, compared with offspring of normal weight mothers (risk of malformations 3.4%), the proportions of any major congenital malformation among the offspring of mothers with higher BMI were: overweight, 3.5%; obesity class I, 3.8%, obesity class II, 4.2% and obesity class III, 4.7%.

The overall risk of any major congenital malformation was higher in boys (4.1%) than in girls (2.8%). The risks of congenital heart defects, malformations of the nervous system, and limb defects also progressively increased with BMI from overweight to obesity class III.

Malformations of the genital and digestive systems were also increased in offspring of obese mothers.

The authors point out that this is an observational study, so no firm conclusions can be drawn about cause and effect, and they outline some limitations which could have introduced bias. However, key strengths include the large sample size and ability to adjust for important confounders.

They say their results “underline the importance of having a maternal BMI in the normal range before pregnancy” and conclude that “efforts should be made to encourage women of reproductive age to adopt a healthy lifestyle and to obtain a normal body weight before conception.”