Pregnant women who eat a lot of fish could be causing health problems to their unborn child and not even realize it.

A new study has found that women who ate fish more than three times per week during pregnancy were more likely to have a child that became overweight between 4 and 6 years of age than mothers who did not — and it’s worse if the child is a girl, according to a Los Angeles Times report.

It’s a surprising finding, but scientists think they may have an idea why this is the case. Fish are rich in omega-3 fatty acids, which may may cause fetal stem cells to differentiate into fat cells. Ither that, or pollutants in the fish could disrupt fetal hormones related to metabolism. Still, there’s no solid proof that this is the case.

The research is a big deal because it may mean federal guidelines need some tweaking. The Food and Drug Administration advised in July 2014 that pregnant women eat two to three services of fish each week, while avoiding fish that are known to have mercury in them, like swordfish and king mackerel, opting for salmon, tuna, cod, shrimp, and similar seafoods.

The study involved 26,184 pregnant women who gave birth between 1996 and 2011 in both the United States and Europe. The women then answered questionnaires about the foods they ate. Scientists observed the growth patterns and weights of the children up to age 6.

Fish have long been a difficult subject for pregnant women. While fears of mercury harming the fetus have caused many women to steer clear, public health campaigns have urged more fish consumption due to beneficial omega-3 fatty acids as essential for proper brain development.

The findings were published in the journal JAMA Pediatrics.