When I was pregnant with my first child, friends and family inevitably asked about my diet.
"Are you sticking to vegan food?" they wondered, with variable admiration or anxiety.
For some, the curiosity was about cravings. One mother told me she was irresistibly (and unexpectedly) drawn to hot dogs her third trimester; another confessed her sudden fascination with red meat. Surely, I'd crave things beyond the vegan sections of Whole Foods? (I didn't. Mostly.)
But for others, the curiosity was tinged with worry — a worry about safety. Was a vegan diet good for me? Was a vegan diet good for the baby?
My (Berkeley) OB-GYN didn't bat an eye, and the nutrition and pregnancy books I'd read supported common sense: Having a nutritious diet matters (whether or not you're pregnant), but that can come in many forms. There are more and less healthful ways to be vegan, just as there are more and less healthful ways to eat a diet that includes meat or fish.
But when I looked for quantitative data on vegan pregnancies, the pickings were slim. First, there simply wasn't much research. And, second, what research did exist was correlational. That's a problem because mothers who are or aren't vegan could vary in many ways beyond the way they eat, including their level of education, socio-economic status and exercise habits. That makes it hard to attribute any differences between vegan and non-vegan pregnancies to diet, in particular.
Five years after my initial search, there's been some progress, but not much — about half a dozen published studies (still correlational) and, earlier this year, a systematic review of the literature published in BJOG: An International Journal of Obstetrics and Gynaecology, the journal of the Royal College of Obstetricians and Gynaecologists. The article assesses the medical literature on the effects of vegan and vegetarian diets on maternal-fetal outcomes and offers the following encouraging — though qualified — summary of the evidence to date:
"The evidence on vegan–vegetarian diets in pregnancy is heterogeneous and scant. The lack of randomised studies prevents us from distinguishing the effects of diet from confounding factors. Within these limits, vegan–vegetarian diets may be considered safe in pregnancy, provided that attention is paid to vitamin and trace element requirements."
These conclusions are based on an analysis of the 22 studies that met the authors' criteria for inclusion in the review. For instance, they considered studies reporting on only a minimum of five cases, and where a vegetarian or vegan diet was freely chosen, not the result of privation.
Nonetheless, the 22 studies were quite diverse in their size and aims. For instance, the largest study involved data from a sample of 7,928 children (3,211 born to vegetarian or vegan mothers), and focused on the role of phytoestrogens in the development of the male reproductive system. A more recent study with a sample of 1,257 women (114 vegetarian or vegan) investigated the relationship between dietary iron intake during pregnancy and the baby's size at birth.
The variability across studies was, unfortunately, matched by some variability in results. For instance, five studies found that vegan/vegetarian mothers had babies with lower birth weight, but only one of these reported that the difference was statistically significant. And on the flipside, two studies found that vegan/vegetarian mothers had babies with higher birth weight, with one of these reporting statistical significance.
The nine studies that focused on nutritional deficits were somewhat more consistent, suggesting that pregnant women who are vegetarian/vegan may have higher levels of folate and magnesium relative to their omnivorous peers, but may also have an increased risk of vitamin B12 and iron deficiency — two of the nutrients vegetarians and vegans are generally advised to watch our for, whether or not they're pregnant.
Finally, the review found little evidence that vegetarian-vegan diets either reduce or increase the risk of what they call "severe, adverse pregnancy-related events," such as pre-eclampsia or major birth defects, provided B12 and iron levels are adequate.
The possible exception came from a single study that found an increased incidence of a condition called hypospadias — a condition in which the opening of the urethra is on the underside of the penis — in baby boys born to vegetarian/vegan women. The condition was also more prevalent in baby boys born to omnivorous mothers who took iron supplements or to mothers who had the flu during the first trimester of pregnancy. The authors of the review suggest further research is needed before attributing the increased incidence of hypospadias to diet.
Needless to say, the state of the evidence is still limited. Some of these limits are inherently difficult to overcome. For example, a lot of research regarding the effects of lifestyle on maternal-fetal outcomes is correlational rather than experimental because there are practical and ethical problems that arise in randomly assigning pregnant women to different lifestyles. That means it's especially difficult to draw conclusions about which factors play a causal role in bringing about good or bad outcomes.
On the other hand, there's a lot of correlational research that could be done — research with larger samples, a more fine-grained look at diets, a broader range of outcome variables, and additional measures to help rule out plausible confounds, such as exercise habits.
Many people have ethical or health-related reasons for following a vegetarian or vegan diet. The results, so far, suggest that pregnancy isn't a reason to reconsider: You can opt for the vegan hot dogs while waiting for that bun in the oven — just be sure to watch your iron and B12 intake, if you do.
Tania Lombrozo is a psychology professor at the University of California, Berkeley. She writes about psychology, cognitive science and philosophy, with occasional forays into parenting and veganism. You can keep up with more of what she is thinking on Twitter: @TaniaLombrozo