A Swedish population-based study found that, despite concerns of folate deficiency, gastric bypass reduced the risk of major birth defects.

This week, Sweden.

Ah, Sweden. Birthplace of Bjorn Borg, harvesters of delicious gummy fish, inventors of the dowel, I assume.

This Scandanavian paradise offers more than just meatballs. They have what may be the most comprehensive medical database of any country in the world. Sure sure, big brother and all that, but the power of having detailed medical data from birth to death on an entire population is an epidemiolgist’s dream. And this week, a Swedish study appearing in JAMA gives us an answer to a question that we really couldn’t get any other way. Does bariatric surgery increase the risk of the patient’s child having birth defects?

It’s an important question. There is ample data to suggest that gastric bypass surgery can lead to folate deficiency, and in theory that could increase the risk of neural tube defects. There are other malabsorptive syndromes as well that may have untold fetal effects. On the other hand, better control of metabolic syndrome, hyperinsulinemia, and blood glucose might actually reduce the risk of birth defects.

And here’s why you need Swedish data.  Let’s concede that no one is going to take obese pregnant women and randomize them to bariatric surgery or usual care while they are pregnant. You could potentially randomize people to surgery before they become pregnant, but that’s a pretty huge trial to ensure you’d capture enough pregnancies to say anything.

So we’re left with observational data. But this is tricky. Women who undergo bariatric surgery are different from women who don’t, making finding a control group really difficult. With Swedish data, though, well, check it out.

Researchers led by Dr. Martin Neovius identified all live births in Sweden from 2007 to 2014, and linked that to a registry of all women who had roux-en-y gastric bypass surgery in that time period because they could.

They then searched all other women who had given birth for potential controls matching on BMI, diabetes, delivery year, age, smoking, substance abuse, parity, psychiatric drugs and number of prescription drugs. I’m so jealous.

What they found was pretty clear. Women who underwent bariatric surgery prior to becoming pregnant had a lower risk of giving birth to a child with birth defects.

This association held examining just the first births after surgery and after excluding births with chromosomal abnormalities. Importantly, there were zero cases of neural tube defects in the surgery group and 20 in the control group.

In fact, the overall rate of major birth defects in the surgery group, 3.5%, was basically the same as the general population.

Now, we have to be a bit careful here. This study only examined live births. You could posit that surgery may have increased the rate of fetal loss, perhaps putting some selective pressure on “healthier” babies, but this feels like a stretch to me. Also, remember this study only looked at roux-en-y bypass, it doesn’t say anything about gastric banding or sleeves.

But basically, I think we have once again seen that bariatric surgery carries substantial benefit. Though it is essentially the riskiest weight-loss option, it may also be the most effective – for the patient and children.

This commentary first appeared on medscape.com.