How does a plant-based diet affect pregnancy outcomes?

By Dr. Liji Thomas, MDJan 25 2024

As plant-based diets become more common because of increasing sympathy with environmental and animal rights concerns, exploring how this affects pregnancy has become more important. This is especially so for those diets that are extremely limited in their repertoire of acceptable foods. A new study in the journal Acta Obstetricia et Gynecologica Scandinavica recently reported on this aspect of health, examining outcomes in mother and baby when plant-based diets were followed during pregnancy.

Study: Adherence to different forms of plant-based diets and pregnancy outcomes in the Danish National Birth Cohort: A prospective observational study. Image Credit: Creative Cat Studio / Shutterstock

In Denmark, hardly anyone was vegan in 2010, but just 12 years later, the proportion surged to 3%. Among young adults below the age of 34 years, it is above 7%.

Since 70% of Danish vegetarians are women, the proportion of vegans in pregnancy may still be higher. While vegetarian diets have been linked to a lower risk of death from any cause, in some studies, more stringent evidence is required.

Highly restricted vegan and other plant-based diets have shown an increased risk of nutritional deficiency. This could affect overall health. However, the potential impact on birth weight and pregnancy outcomes is still more important.

Earlier studies have reported conflicting findings regarding birth weight in babies of vegan mothers. Recently, two studies, one Israeli and one American demonstrated lower birth weights and a higher risk of small-for-gestational-age (SGA) babies in this group. The lack of detailed nutritional information often hampers definitive conclusions.

As a result, public health guidelines are also unconvincing, being based on limited data. This can be improved only by carefully characterizing the dietary composition of vegetarians and vegans.

For the current study, the researchers used data from the Danish National Birth Cohort, including over 100,000 pregnancies from over 90,000 mothers. Of these, dietary and pregnancy outcome data in over 66,000 pregnancies were incorporated into the study.

What did the study show?

Of the whole group, less than 2% reported that they were non-omnivorous, compared to 1% who were vegetarians eating only fish or poultry, 0.3% who used only eggs or milk, and a tenth vegans.

Those on plant-based diets were more likely to be older, have other children, were not current smokers, and were at higher risk of being underweight, while overweight or obesity was less likely.

Though the overall proportion was minuscule, vegetarians were much more likely to have lower protein intake, with an absolute intergroup difference of 2%. While energy intake was similar between groups, vegans showed low protein intake at about 10% of total energy intake, 5% less than the 15% of energy intake in omnivorous mothers or the 13-15% in vegetarian mothers.

Vegetarian mothers had a lower fat intake, at ~30%, vs 32-33% in omnivorous or vegan mothers.

There were no significant differences between all three cohorts in terms of the mean birth weight or birth length, the period of pregnancy at which delivery occurred, and the prevalence of low birth weight. Again, all three were at similar risk of gestational diabetes mellitus, pre-eclampsia, and Cesarean section (CS).

However, the mean birth weight was about 250 g lower in the vegan versus omnivore cohort. The average length of pregnancy before delivery occurred was 5 days longer for vegan mothers. Thus, low birth weight (LBW) was more common in about 11% of vegan mothers vs <3% in omnivorous mothers.

Babies were born SGA in over a quarter of vegan pregnancies vs 10% or less in omnivorous and vegetarian mothers. Iron deficiency anemia before the 30^th week of gestation was observed in ~13% of vegetarian vs ~8% of omnivorous and ~6% of vegan mothers.

Pre-eclampsia occurred in 11% of vegan mothers vs ~3% of all vegetarian and omnivorous mothers. The incidence of CS among omnivorous and vegetarian mothers was ~15% vs <6% in vegans.

 All three diets showed equivalent quality when diet supplements were added to the nutritional content score. Vegans were more likely to have pre-eclampsia during pregnancy.

What are the implications?

Vegan diets during pregnancy were associated with lower average birth weight, and these mothers were more likely to develop pre-eclampsia versus either omnivorous or vegetarian. The latter categories formed a more or less homogeneous group, except that anemia was more common among vegetarians.

The reduction in protein intake may explain, at least in part, the decline in birth weight, given that micronutrient intake was not significantly different from that in the other groups. This corroborates earlier research showing that women who consumed below 60 g/day of protein were more likely to have low birth weight babies vs those who had 80-90 g/day.

Notably, the difference in birth weight of 240 g is similar to that between infants born to mothers who smoked daily vs nonsmokers in the same cohort.

Vegetarian diets may open the door to restrictive eating patterns, often to lose weight, and may even be a manifestation of eating disorders. This, in turn, affects birth weight. In the current study, none of the women reported any eating disorders.

However, vegetarian diets were not associated with adverse outcomes or inferior diet quality, a reassuring outcome given the large sample size.

Overall, earlier findings combine with this study to yield "quite consistent evidence suggesting that adherence to vegan diets during pregnancy may adversely affect fetal growth." While not causal in nature, such observations should encourage a focus on diet quality among vegan mothers and others on a plant-based diet in future research. This will help draw causal correlations and lead to more authoritative dietary guidelines during pregnancy.