Obese mums' birth defect worries

If you’re obese when you’re pregnant, your baby is more likely to suffer from birth defects, reported the Daily Mail, The Daily Telegraph and the BBC.

The Telegraph noted that this raises further concern over the growing obesity epidemic in Britain, “where one quarter of women are currently classed as obese”.

The BBC gave the findings more context by reporting that, “Researchers stress the risk of having a baby with birth defects is low, even for obese women. Among women of healthy weight, about three in 100 babies will have serious birth defects. That seems to rise to about four in 100 for obese mothers.”

The articles were about new research that suggests that being obese while pregnant is linked to a greater risk of the baby having birth defects including spina bifida, heart defects and malformed limbs. In addition, the research found that pregnant women themselves are at greater risk of fertility problems and miscarriages.

This research has been based on a study carried out to investigate whether infants with various birth defects also have obese mothers. Although, the study's findings appear to suggest a link, it cannot prove maternal obesity to be the cause. While maintaining a healthy weight during pregnancy does have health advantages, the vast majority of overweight women have healthy babies and the reports of these findings may cause undue concern.

Professor Michael Patton, the medical director of charity BDF (Birth Defects Foundation) Newlife, told the BBC, "At present there are no increased concerns for mothers who are overweight but the best advice is still to eat a healthy balanced diet before and during pregnancy."

Where did the story come from?

The study was carried out by Dr Kim Waller and colleagues at various medical centres and departments in the US. Funding was provided by the Centers for Disease Control and Prevention to the Texas Department of State Health Services’s Center for Birth Defects Research and Prevention.

The study was published in the peer-reviewed journal Archives of Pediatrics and Adolescent Medicine .

What kind of scientific study was this?

This was a case-control study, as part of the National Birth Defects Prevention Study, which examined the difference between mothers of babies born between October 1 1997 and December 31 2002 with birth defects (cases) and those mothers with healthy babies (controls).

Eligible mothers in eight US states who gave birth to babies with one or more of 30 possible birth defects were identified. In order to give a sufficient number for statistical comparisons, birth defects for which 150 or more cases could be obtained were selected for the study. Some rare conditions were therefore not included. Conditions that were possibly linked to genetic abnormalities or maternal diabetes were excluded. Mothers of healthy babies born in the same period were randomly selected from birth certificates and hospitals.

In total 10,249 cases and 4,065 healthy controls were selected. The mothers were then interviewed over the phone to ascertain their height and pregnancy weight, so that their body mass index (BMI) could be calculated. Calculations were then made to look at the links between the birth defects and BMI. Mathematical adjustments were made to take into account other potential contributing factors such as age, ethnicity, smoking history, folic acid use, etc.

What were the results of the study?

The researchers found that there were significant links between maternal obesity and risk of the infant having spina bifida, heart defects, limb reduction defects, hernia in the diaphragm, omphalocele (protrusion of internal organs through the umbilicus), anorectal atresia (absent or abnormal anal opening) and hypospadius in males (opening of the urethra on the underside of the penis). When adjustments were made for diabetes that had developed during pregnancy (gestational diabetes), however, a significant link remained only for spina bifida, heart defects and hernia of the diaphragm. An inverse link to obesity was found for the birth defect gastroschisis (the intestine bulging out of a weak spot in the abdominal wall near the umbilicus).

What interpretations did the researchers draw from these results?

The researchers concluded that this and other case control studies show a consistent link between seven out of 16 examined categories of birth defect. They suggest, however, that although the mechanisms of the link are not understood, it may be caused by undiagnosed diabetes.

What does the NHS Knowledge Service make of this study?

The study is fairly reliable and has collected information on a large number of women. It does not prove there to be a link between maternal obesity and birth defects and, as the authors themselves suggest, there may be other causes.

• We should not interpret from this study, as the news reports suggest, that birth defects are more common in obese mothers. This study only presents a possible link, and has found on closer examination that this may be due to other causes.
• The birth defects that were examined are varied and mostly unrelated. Although the researchers attempted to take into account other factors that had been identified, there may be many differing causes and links, such as other genetic links.
• We need to be careful when reading “seven of the 16 most common defects”.  The seven types of birth defect which were found to be linked to obese mothers reduced to three after diabetes was taken into account. The 16 most common defects are only those where the researchers found enough occurrence (150+) of the birth defect to include in the study.
• The controls (mothers of healthy babies) do not appear to have been matched to the cases (mothers of babies born with birth defects) for other factors, i.e. ideally they should have similar characteristics in terms of other things that could have an effect on pregnancy outcome, with only the BMI differing.
• The study relied on the mother’s recall of height and weight prior to pregnancy by phone interview. As some time would have passed since, this may be subject to recall bias.
• No information can be obtained for those cases of foetal defects where the mother underwent a termination.

Although maintaining a healthy weight during pregnancy seems the ideal, and clearly has health advantages, the reports of these findings may cause undue concern to the majority of overweight women who will have healthy babies. Further research is needed to identify the true causes of birth defects, of which there may be many.