The research, part of the wider Avon Longitudinal Study of Parents and Children (ALSPAC) study, also known as Children of the 90s, is aimed at identifying ways in which to optimise the health and development of children. Some 14,000 mothers were enrolled during pregnancy in 1991-2 and both children and parents have been followed by the researchers in minute detail ever since.
Previous studies had suggested that in communities with a high intake of fish - such as the Faroe and Orkney Islands - babies tend to be heavier. In the past, scientists have attributed this to the n-3 fatty acids in marine foods, which may prolong gestation. In Denmark, meanwhile, researchers reported a strong connection between low fish consumption, pre-term birth and low birthweight.
The Children of the 90s researchers, based at the University of Bristol, analysed the diets of 11,585 women at 32 weeks of pregnancy, and divided them into five groups according to how much fish they ate.
The women were asked to report whether they ate white fish, shellfish or oily fish. Oily fish is particularly rich in n-3 fatty acids and includes sardines, herring, mackerel and salmon.
At first sight there seemed to be a significant link between the amount of oily fish a mother ate and the size of her baby. On average, the group with the highest fish intake had babies between 70 and 80g heavier than those who ate no fish at all.
However, after taking into account factors such as social class and whether the mother smoked there was no longer any difference between the groups. The only significant finding was a larger proportion of very small-for-dates babies born to mothers who ate no fish compared to those who ate plenty.
The report's author Dr Imogen Rogers said: "Being very small-for-dates at birth has been associated with increased risk of high blood pressure and other problems in late or middle age.
"This work adds to the evidence that fish is an important part of the human diet and reinforces the recommendation that pregnant women should include at least two servings of fish a week. Including oily fish is a good habit to encourage."
Not all fish is recommended, however. The UK Food Standards Agency advises pregnant women and prospective mothers to avoid eating shark, marlin and swordfish because they contain relatively higher levels of mercury. They may also need to limit the amount of tuna they eat.