Children, pregnant women, and women who are planning to become pregnant should strictly limit their intake of fish and avoid some types altogether to avoid potentially harmful levels of mercury and PCBs, experts said on Thursday.

According to new guidelines released by the Physicians for Social Responsibility and the Association of Reproductive Health Professionals, women of reproductive age and children should never eat shark, swordfish, tilefish or king mackerel.

The guidelines also recommend that women of reproductive age and children should eat no more than one serving per week of fresh or canned albacore tuna, orange roughy, marlin, grouper and other types of fish.

Fish that is safe to eat up to twice per week includes squid, canned chunk light tuna, monkfish, bass, trout and Pollock, which is found in fish sticks.

“Everyone needs to be eating fish for a healthy diet. These are healthy options,” Susan West Marmagas of the Physicians for Social Responsibility said.

While fish and shellfish can be a good source of protein and omega-3 fatty acids, nearly all seafood contains traces of mercury. High levels of mercury in the bloodstream of fetuses and young children can impair development of the nervous system.

Millions of women in American currently have levels of mercury in their blood considered unsafe by the Environmental Protection Agency, and around 630,000 newborns are exposed every year to unsafe levels of the contaminant.

These guidelines, presented here Thursday at a press conference, closely resemble recommendations issued in March by the U.S. Food and Drug Administration and the EPA.

However, the latest version also suggests that young women and children limit their intake of fatty fish, which contains high levels of PCBs, which have also been shown to impair neurological development in fetuses and young children. According to the guidelines, to limit their intake of PCBs, women and children should eat no more than 1 to 2 servings per month of salmon, sardines, herring and bluefish.

Marmagas explained that there are two different types of canned tuna, chunk light and albacore. Research has shown that albacore contains around three times the amount of mercury as chunk light. So while tuna sandwiches are a staple for many children, make sure you are serving them the lower-mercury option, she cautioned.

Farm-raised salmon also appears to contain more contaminants than wild salmon, she noted.

Marmagas cautioned to Reuters Health that women of reproductive age and children should also be careful of fish oil supplements, which can contain high levels of PCBs and mercury.

She added that the best way to limit intake of mercury and PCBs is to prevent the pollutants from getting into fish and other foods in the first place by controlling what gets pumped into the environment from industrial activities. Limiting fish intake is “only a bandaid,” she said.

“The issue really is upstream,” she said.