Every mother wants the best for her child. What's best for a child during the first eighteen months of life? A recent study suggests that prospective mothers who eat fish while pregnant and who later breastfeed their infants are rewarded by seeing better physical and mental development in that early, important period of their child's life.

Children who were breastfed and children whose mothers ate the most fish while pregnant showed significantly better cognitive and motor skills.

The new study is based on data from the Danish National Birth Cohort, an epidemiological study which collected data about pregnancies, births and early child development. The DNBC covered nearly 30% of all births in Denmark between 1997 and 2002. Prospective mothers filled out detailed dietary questionnaires six months into their pregnancy. Early child development was assessed from interviews with the mother both six and eighteen months after birth of the child. Developmental criteria used at six months included whether the child could sit up, respond to voices or crawl. At eighteen months, some of the abilities looked at were whether the child could walk unassisted, write or draw, put words together or drink from a cup.

In the current study, Dr. Emily Oken headed a joint team that included both researchers from the Harvard Medical School and Danish investigators. The team sifted through the DNBC data, looking for factors which were beneficial in early child development. They focused on the over 25,000 mothers who completed the entire Danish study, including the interviews at eighteen months postpartum. The results were published in the September 2008 issue of the American Journal of Clinical Nutrition.

Children who were breastfed (the longer, the better) and children whose mothers ate the most fish while pregnant showed significantly better cognitive and motor skills. The benefits of fish consumption and breastfeeding were independent of each other.

The women who ate the highest amount of fish in the study ate about 3.5 servings per week. In 2004 FDA issued an advisory warning pregnant women to eat no more than two servings (twelve ounces) of fish or seafood a week. This advisory was issued due to concerns about toxic compounds, chiefly mercury, present in fish.

How do you balance concerns about mercury with the benefits of fish consumption? Dr. Oken suggests that "... [W]omen should continue to eat fish — especially during pregnancy — but should choose fish types likely to be lower in mercury."

The types of fish to be careful about are generally those highest on the food chain — big fish. These are the fish with the highest levels of mercury — tilefish from the Gulf of Mexico, shark, swordfish and king mackerel, for example. Salmon and sardines are very low in mercury. A detailed listing of the mercury levels in commonly eaten fish can be found at http://www.cfsan.fda.gov/~frf/sea-mehg.html.

Why do breastfeeding and maternal fish consumption spur early child development? This correlational study can't answer this question; more research is needed. It may be that the beneficial effect of fish consumption is due to certain fatty acids present in the fish. However, it is difficult to pin down the cause because there is a great deal passing between mother and infant during gestation and breastfeeding. What the study does show is more practical: both behaviors are beneficial to early child development.

It's one more piece in the puzzle of how to be a good parent.