Pregnant Woman Resting On Back
13 July 2017

New Study: Obesity Increases Risk Of Major Birth Defects

Researchers have long known that obese women appear to deliver more children with congenital malformations than pregnant women who maintain lower body weights. A new study in the British Medical Journal, however, has expanded our understanding of the link between maternal weight and birth defects.

Maternal Weight Elevates Likelihood Of Congenital Malformations

According to an international group of researchers from Sweden, Denmark and the US, the likelihood of delivering a child with major birth defects seems to increase in step with a mother’s weight. Overweight women are more likely to have children with birth defects than women at normal weights. Likewise, obese women are more likely to deliver children with congenital abnormalities than overweight women.

Pregnant Woman With Baby Shoes

Interviewed by The Guardian, study co-author and professor at Stockholm’s Karolinska Institute Martin Neovius interpreted the results frankly, “in terms of risk, it is better to be normal weight than overweight and much better to be more normal weight than obese.”

Graduated Risk

Hailing from institutions like Stockholm’s Karolinska Institute and the University of Michigan, the team of epidemiologists analyzed data from over 1 million singleton (ie not twins, triplets or other forms of multiple pregnancy) deliveries in Sweden between 2001 and 2014. Around 43,500 of these children were born with major birth defects, but infants born to women who were classified as either overweight or obese at their first prenatal visit were significantly more likely to be affected by congenital anomalies:

  • Overweight (BMI 25 to 29.9) – 0.05% increased risk
  • Obesity Class I (BMI 30 to 34.9) – 12% increased risk
  • Obesity Class II (BMI 35 to 39.9) – 23% increased risk
  • Obesity Class III (BMI 40+) – 37% increased risk

These increased risks appeared to hold for all forms of major birth defects. As the body weight of a mother increased, the study found progressive growth in the rate of congenital heart defects, orofacial clefts (cleft lip and / or cleft palate) nervous system abnormalities, limb anomalies, genital defects and digestive system problems. The correlation was strongest, though, in relation to disorders of the nervous system (so-called neural tube defects like spina bifida and anencephaly). Dr. Raul Artal, chairman emeritus of the obstetrics department at Saint Louis University, says congenital heart defects are “by far” the most frequent “birth defect among mothers that have obesity and diabetes.” Artal spoke with CNN in a recent interview.

How A “Dose-Response” Relationship Works

Epidemiology is a field of medical research that studies factors that explain the presence or absence of diseases between different populations. Epidemiological research helps us understand how many people have a specific disorder, as well as why certain people seem to get the disorder more or less than other people.

In the recent British Medical Journal article, we’ve seen that overweight and obese women appear to give birth to children with birth defects at a higher rate than women with normal weights. What’s more, we’ve found evidence that this increased risks grows even more as a mother’s weight does. In medical circles, this is known as a “dose-response relationship” – as an exposure changes, so too does the effect on a person.

Confounding Variables & Limitations

The study’s data analysis also confirmed several causal relationships that have been extensively-documented in the birth defect literature. As in previous research, the British Medical Journal article noted a weak correlation between maternal age and major birth defects. Congenital malformations were more common in male children than females and more likely to be delivered by smokers, women with low socioeconomic status and mothers who did not live with a partner during pregnancy. After controlling for these factors, the link between maternal weight and birth defects came into clearer focus. Several potential confounders, however, could not be ruled out. Alcohol use, for example, a well-understood contributor to birth defects, was not considered for the study.

While compelling, the report’s findings are not conclusive. Body mass index (BMI), for example, is calculated using two figures: a person’s weight and their height. To calculate BMI numbers for the study, the research team used a relatively-objective standard, a mother’s weight as it was measured during a first prenatal visit. For height, on the other hand, the researchers had to rely on self-reports, which may skew the data somewhat.

Obesity’s Complex Impact On Pregnancy

Moreover, their dataset was limited solely to live births. Still births and induced abortions, which may have changed the picture, were not included. This isn’t an idle objection, since many parents choose to terminate a pregnancy precisely because their unborn children have been diagnosed with serious congenital abnormalities. Complicating the picture further, the researchers write, a number of studies have found that obesity may increase the risk for birth defects so serious that spontaneous abortion occurs. If true, this hypothesis could mean that the study’s evidence for a link between maternal weight and congenital anomalies is, if anything, an underestimation.

Weight Gain Guidelines During Pregnancy

You can find a simple BMI calculator at the National Heart, Lung and Blood Institute. The study’s lesson, however, is not that all pregnant women should strive to lose a dangerous amount of weight, an extremely difficult task in and of itself. Since maternal weight and birth defects appear to be linked on a “sliding scale,” reducing one’s weight by any amount could show major benefits for a child’s health. Guidelines for weight gain during pregnancy, created by the American College of Obstetricians and Gynecologists, suggest:

  • 28 – 40 pounds for women who are underweight
  • 25 – 35 pounds for women who are normal weight
  • 15 – 25 pounds for women who are overweight
  • 11 – 20 pounds for women who are obese

Increased maternal weight has been shown to alter hormonal levels, increase inflammation in the body and decrease the amount of nutrients that can reach a fetus. All of these factors could play a role in the association, the article’s authors report. Blood sugar control may also be a problem, the study’s findings held true even after women with diabetes were excluded from the data set. Obesity is a growing problem, especially in developed nations and among women of reproductive age. One-third of the world’s population can now be classified as obese, according to recent research in the New England Journal of Medicine.