Nutrition Notes

Do Low Carb Diets Cause Birth Defects?

Health and nutrition news outlets are at it again: another day, another misleading and alarmist study about the potential dangers of low-carb diets during pregnancy. This is nothing new. Unfortunately, it seems to be a recurring theme for organizations competing for “likes” and pageviews to publish articles summarizing poorly conducted and even more poorly interpreted research. It happens so often that you have to wonder if the journalists reporting on these studies have even taken the time to read the full text, or if they’re basing everything on sound bites someone else published first.

As word spreads about the efficacy of low-carb diets for a multitude of different health conditions, not to mention weight loss, the nutritional “old guard” appears to be circling its wagons ever tighter, lest you begin to truly understand that no, fat really, really isn’t bad for you, and maybe basing your diet on starchy carbohydrates—as specifically recommended by decades of government nutrition advice—isn’t the best way for everyone, across the board, to go.

What sensationalist headlines are we talking about here?

How about:

But is this true?

Is it possible that a diet that’s highly effective for type 2 diabetes, metabolic syndrome, PCOS, GERD, and more, could be harmful for a developing baby? Let’s take a closer look at the study.

Owing to concern over the rate of babies being born with a particular birth defect called a neural tube defect (NTD), beginning in 1998 the US government mandated that cereals and grains be fortified with folic acid. (It was known that folic acid or folate deficiency was a major risk factor for NTDs.) Because women who follow low carb diets typically consume fewer carbohydrate-based processed foods that are fortified with folic acid, researchers speculated these women might have lower folate status compared to women who do not restrict carbohydrates. And they further speculated that this lower folate status might translate into increased risk for NTDs. So they designed a study to determine if women who follow low carb diets are at greater risk for producing offspring with NTDs than are women who don’t restrict carbs.

Contrary to the certainty with which this was reported on in the media, the study has several flaws and shortcomings.

First, the women’s carbohydrate consumption and folic acid intake were based on a food frequency questionnaire. The questionnaire contained only 58 items, so it was pretty limited to begin with, and women were supposed to recall what they’d consumed during the year before pregnancy. If you have a hard time remembering what you ate for lunch four days ago, try imagining how difficult it would be to give an accurate account of what you ate, how often, or how much, over the past year.

To their credit, the study authors do at least acknowledge that food frequency questionnaires (FFQs) are pretty shaky and unreliable. They wrote:

“As nutrient intakes were estimated based on self-reported frequency of specific food items using an abbreviated FFQ, we further caution against interpreting the estimated nutrient values in this study as absolute values; we cannot assume, for example, that all women classified in our study as having restricted carbohydrate intake actually consumed fewer than 95 g/day.”

For starters, we can’t even be certain these women were restricting carbohydrates. The FFQ is, at best, a guess. A hazy ballpark estimate.

An even more glaring issue is that maternal folate levels were not measured. Folate intake was estimated based on the FFQ; the researchers didn’t actually measure the women’s folate levels directly.

It’s true that women who consume grain products fortified with folic acid may be getting more synthetic folic acid in their diets than women who follow low carb diets and consume little to no grain. But low carb diets are hardly lacking in folate. Egg yolks, a common staple among low carb dieters, are a good source of folate, as are the leafy greens that typically make up a significant portion of low carb dieters’ vegetable intake. Spinach, broccoli, romaine lettuce, and Brussels sprouts are just a few examples. The FFQ used in the study is not disclosed in the publication, so it’s impossible to know which foods were even included, and whether or not they encompassed a large selection of folate-rich foods that are low in carbohydrate.

And the hits just keep on comin’. The FFQ used in the study did not pertain to the diet consumed during pregnancy. It pertained to the year before pregnancy. The researchers also acknowledge this shortcoming: “Another limitation is that the FFQ captured usual diet in the year before conception, and the extent to which diets may have differed around the time of conception or during the first few weeks of pregnancy is unknown.” At best—at best—the FFQ used in this study is a very loose guess as to which foods, and in which amounts, the women consumed during the stage of pregnancy during which the neural tube is formed.

To be fair, the researchers pointed out that neural tube development takes place very early on, before many women are even aware they’re pregnant, so it’s unlikely that they would have made dramatic changes to their habitual diets during that time. Even ceding this point, this is still incredibly unreliable evidence on which to come to conclusions about low carb diets being harmful for fetal development.

Additionally, based on the data provided in the study, there were a total of 1559 cases of NTD: 93 among the low carb group and 1466 among the non-carb restricted diet, so 94% of the occurrences of NTD were in the group not following a low carb diet. Granted, even though low carb diets have gained in popularity, relative to the population as a whole, the number of people following low carb diets is still very small. Since there are so many more women not eating low carb, it’s only logical that there would be more cases of NTD among offspring of these women. Nevertheless, if low carb diets were truly a major risk factor for NTDs, you would expect low carb moms to account for more than just 6% of the NTDs.

It’s unfortunate that what was emphasized in the news was “low carb diets cause birth defects,” rather than a much more cautious and measured interpretation of weak and highly suspect conclusions based on unreliable data. Sadly, we will likely only see more of this in the age of clickbait headlines and media outlets competing for attention.



  1. Desrosiers TA et al. Low carbohydrate diets may increase risk of neural tube defects. Birth Defects Res. 2018 Jan 25.
  2. Feinman RD, Volek JS. Carbohydrate restriction as the default treatment for type 2 diabetes and metabolic syndrome. Scand Cardiovasc J. 2008 Aug;42(4):256-63.
  3. Austin GL, Thiny MT, Westman EC, Yancy WS Jr, Shaheen NJ. A very low-carbohydrate diet improves gastroesophageal reflux and its symptoms. Dig Dis Sci. 2006 Aug;51(8):1307-12.
  4. Mavropoulos JC, Yancy WS, Hepburn J, Westman EC. The effects of a low-carbohydrate, ketogenic diet on the polycystic ovary syndrome: A pilot study. Nutrition & Metabolism. 2005;2:35.