Pregnant woman weighing herself on a bathroom scale

    Don’t call it ‘fat shaming’ – there are good reasons obese women shouldn’t get pregnant

    4 July 2017

    A new study by Sweden’s Karolinska Institutet suggests that women are more likely to have babies with serious birth defects if they are obese.

    The researchers examined data from over a million pregnancies. There were approximately 44,000 birth defects detected. Other risk factors were excluded, allowing the association between obesity and birth defects to be explored directly.

    The background risk for birth defects in this study was approximately 3.5 per cent for women of normal weight. Obese women had a risk ranging from 3.5 per cent up to 4.7 per cent. Statistically this is insignificant, however it is an excellent demonstration of the fact that statistically insignificant data can be extremely significant clinically.

    So, does obesity itself cause birth defects, or is it just an associated factor?

    There are some anomalies with a direct link to obesity. Specifically neural tube defects like spina bifida (where a segment of spinal cord is exposed when the overlying skin or vertebra fails to form properly) or meningomyelocele (herniation of brain tissue through a defect at the back of the skull).

    The proposed mechanism is a relative deficiency of folic acid; it is known that folic acid deficiency can cause this anomaly, and the recommended dose of folic acid that pregnant women are advised to supplement with has to be increased for obese women as the regular dose will not reach a therapeutic level.

    Obesity associated with diabetes is also implicated in congenital anomalies; we know that hyperglycaemia (high blood sugar) is teratogenic – it causes birth defects. This applies to both obese and non-obese women with uncontrolled diabetes; the exact mechanism is still being studied but we do know that there is a causal pathway.

    There is a clear causal link between obesity and birth defects, but being overweight during pregnancy can cause other problems too.

    This study only looked at defects that were identified after birth, as opposed to antenatally (during pregnancy) via ultrasound. Individual anomalies of the heart, nervous system and limbs are almost always picked up, but ultrasound is not infallible. The overall pick up rate for all anomalies doesn’t usually exceed 75 per cent.

    Obesity makes it even more difficult. To put it crudely, the ultrasound beams can’t penetrate so many layers of tissue, and at a certain point we lose the ability to visualise anatomy clearly enough to identify any anomalies present.

    As a result, it is not unusual for birth defects to be identified in the babies of obese mothers that were not spotted on ultrasound. This explains the fact that obesity is more often associated with birth defects rather than the cause. 

    Obesity has a significant impact on pregnancy, from the pre-conceptual period (it’s more difficult to conceive if you’re overweight), early pregnancy (there’s a greater risk of miscarriage) as well as later pregnancy (higher risk of gestational diabetes, intra-uterine foetal death, preeclampsia) and the delivery process itself, during which there is a higher risks of emergency c-section, birth trauma, postpartum haemorrhage and severe tearing.

    It is imperative that women should be discouraged from trying to get pregnant until they have attended to any excess weight. In many cases, doing so will result in higher rates of spontaneous conception.

    Some will say that this is ‘fat shaming’, but it’s just what we do in the medical profession; the identification of risk and its amelioration. We can’t sacrifice the health of mothers and babies on the altar of political correctness.