Mothers on vegan diets during pregnancy may give birth to infants with lower mean birth weights than those of omnivorous mothers and may also have a greater risk of preeclampsia, a prospective study of Danish pregnant women suggests.
According to researchers led by Signe Hedegaard, MD, of the department of obstetrics and Gynecology at Rigshospitalet, Juliane Marie Center, University of Copenhagen, low protein intake may lie behind the observed association with birth weight. The report was published in Acta Obstetricia et Gynecologica Scandinavica.
While vegan-identifying mothers were very few in number, the authors conceded, their babies were more likely to weigh less on average than those of omnivorous mothers — 3441 g vs 3601 g — despite a mean gestation 5 days longer.
Prevalence rates of low birth weight (< 2500 g) in the two groups were 11.1% and 2.5%, respectively, and the prevalence of preeclampsia was 11.1% vs 2.6%. The mean birth weight of infants in the maternal vegan group was about 240 g lower than infants born to omnivorous mothers.
“The lower birth weight of around 240 g among vegans compared with omnivorous mothers in our study strengthens our observation that vegans may be at higher risk of giving birth to low-birth-weight infants. The observed effect size on birth weight is comparable to what is observed among daily smokers relative to nonsmokers in this cohort,“ Dr. Hedegaard and colleagues wrote. “Furthermore, the on-average 5-day longer gestation observed among vegans in our study would be indicative of reduced fetal growth rate rather than lower birth weight due to shorter gestation.”
These findings emerged from data on 66,738 pregnancies in the Danish National Birth Cohort, 1996-2002. A food frequency questionnaire characterized pregnant subjects as fish/poultry-vegetarians, lacto/ovo-vegetarians, vegans, or omnivores, based on their self-reporting in gestational week 30.
A total of 98.7% (n = 65,872) of participants were defined as omnivorous, while 1.0% (n = 666), 0.3% (n = 183), and 0.03% (n = 18) identified as fish/poultry vegetarians, lacto/ovo-vegetarians, or vegans, respectively.
Those following plant-based diets of all types were slightly older, more often parous, and less likely to smoke. This plant dietary group also had a somewhat lower prevalence of overweight and obesity (prepregnancy body mass index > 25 [kg/m2]) and a higher prevalence of underweight (prepregnancy BMI < 18.5).
Total energy intake was modestly lower from plant-based diets, for a mean difference of 0.3-0.7 MJ (72-167 kcal) per day.
As for total protein intake, this was substantially lower for lacto/ovo-vegetarians and vegans: 13.3% and 10.4% of energy, respectively, compared with 15.4% in omnivores.
Dietary intake of micronutrients was also considerably lower among vegans, but after factoring in intake from dietary supplements, no major differences emerged.
Mean birth weight, birth length, length of gestation, and rate of low birth weight (< 2500 g) were similar among omnivorous, fish/poultry-, and lacto/ovo-vegetarians. The prevalence of gestational diabetes, preeclampsia, and cesarean section was similar across groups, but the prevalence of anemia was higher among fish/poultry- and lacto/ovo-vegetarians than omnivorous participants.
As for preeclampsia, previous research in larger numbers of vegans found no indication of hypertensive disorders during pregnancy. Some studies, however, have suggested a link between preeclampsia and low intake of protein, calcium, or vitamin D, but the evidence is inconclusive, and the mechanism is unclear.
The observed associations, however, do not translate to causality, the authors cautioned. “Future studies should put more emphasis on characterizing the diet among those adhering to vegan diets and other forms of plant-based diets during pregnancy,” they wrote. “That would allow for stronger assumptions on possible causality between any association observed with birth or pregnancy outcomes in such studies and strengthen the basis for dietary recommendations.”
This study was funded by the Danish Council for Independent Research. The Danish National Birth Cohort Study is supported by the March of Dimes Birth Defects Foundation, the Danish Heart Association, Danish Medical Research Council, Sygekassernes Helsefond, the Innovation Fund Denmark, and the Danish National Research Foundation. The authors had no conflicts of interest to declare.