Expectant moms who get a lot of iron in their first trimester have heavier, although still healthy-weight, babies, English researchers say.
They didn't find the opposite trend, that insufficient iron intake in early pregnancy led to lower birth weights, however.
Specifically, the more daily iron women got from food or supplements, the bigger their babies, according to the report published in the journal Human Reproduction.
US guidelines recommend that pregnant women get 27mg of iron a day. In the UK, all women are urged to get at least 15mg a day.
The 15mg equals about three pounds of roasted chicken breast, 15 cups of raw spinach, or a half-cup of iron-fortified breakfast cereal. Most prenatal vitamins contain at least 18mg of iron, and cost between $3 (about R21) and $40 (about R280) a month.
Not meeting base iron requirements
The majority of pregnant women in the UK aren't meeting even the general iron intake recommendation, study co-author Dr Nisreen Alwan, research fellow at the University of Leeds School of Food Science and Nutrition said. Most pregnant women get about 11.8mg of iron per day from both food and supplements.
The researchers surveyed about 1,260 women on what they ate and whether they took iron supplements during all three trimesters of their pregnancies.
They found that for every 10mg/d increase in total iron intake, there was around a 34g increase in their babies' birth weights.
The average baby in the study weighed about 3,400g, and only about one in 25 babies fell into the low birth-weight category.
"It was actually a very small effect that they picked up," said Dr Laura Caulfield of The Johns Hopkins University Bloomberg School of Public Health, who wasn't involved in the study.
The researchers did find that most British women get their iron from eating vegetables rather than meat, which hasn't been shown before, she said.
There is some previous evidence that iron intake is associated with a greater birth weight, Dr Caulfield said.
However, the current study doesn't prove that the pregnant moms' higher iron intake is what caused their babies to be bigger, only that there's some association between the two.
Still, that doesn't necessarily mean women should take iron supplements during pregnancy, Dr Alwan cautioned.
She suggests that pregnant women be informed about how to get enough iron in their diets, "and also how they can maximize the benefits by eating a varied diet, including vitamin C-rich fruit and vegetables."
(Reuters Health, Leigh Krietsch Boerner, March 2011)