The study is part of the Norwegian Mother and Child Cohort Study (MoBa) at the NIPH and includes 62 099 women.
The study shows a slight increase in birth weight for each gram of fish and seafood the mothers ate. Compared with women who rarely or never ate fish, birth weight increased by about 10 grams per weekly meal up to two to three meals per week, then there was no further increase. The results show that it was the intake of lean fish in particular that gave an increase in birth weight.
The study found no increased risk of having children with a high birth weight (birth weight over 4500g) even if the mother has a high intake of fish.
There are a number of factors that affect birth weight and maternal diet is only part of the explanation. Gestational age, twins, maternal health, genetic factors and ethnicity are also important factors.
What is the significance of low birth weight?
Low birth weight can affect the child's health and is associated with an increased risk for diseases such as diabetes and cardiovascular disease later in life. Internationally, the proportion of children with low birth weight is used as a measure of health status in a population. A high proportion of children with birth weight below 2500 grams may indicate malnutrition or other health problems in the population. Several studies show a correlation between low birth weight (1500-2500 g), birth weight for gestational age and the child's cognitive function, i.e. learning and memory skills. Favourable living conditions after birth are more important than birth weight in terms of intellectual development.
What is the advice about eating fish during pregnancy?
The study supports the dietary advice from the Norwegian Directorate of Health that fish should be an integral part of a varied diet during pregnancy. Fish is especially important for pregnant women because it is rich in omega 3 fatty acids, vitamin D, selenium and iodine. These nutrients are found in much smaller amounts in other foods. Pregnant women are advised to eat fish two to three times a week, varying between lean and fatty fish and using fish as both sandwich spreads and for dinner. However, there are certain types of fish that pregnant women should avoid.
The Norwegian Directorate of Health recommends that all pregnant women should take vitamin D supplements. An earlier study from the NIPH showed that women who took supplements of vitamin D equivalent to a dessert spoon (10 ml) of cod liver oil had a lower risk of developing pre-eclampsia.
About the study
This study is part of the Norwegian Mother and Child Cohort Study at the NIPH and includes 62 099 women. The women answered a comprehensive questionnaire about their diet in the middle of pregnancy. In the questionnaire, there were detailed questions about consumption of seafood (lean fish, fatty fish and shellfish) and questions about dietary supplements. Data about birth weight, length and head circumference were obtained from the Medical Birth Registry of Norway, also based at the NIPH.
The study took into account a number of factors that may influence the results, such as total energy intake, use of dietary supplements, body mass index, age, education, gestational age and more. The study can only demonstrate that there is an association, not a definite dose-response effect.
The findings resemble results from a similar study in Iceland. There are also studies that have found no association between fish consumption and birth weight. We believe that this can be explained by differences between countries when it comes to the types of fish that are eaten and fish sources.
Maternal seafood consumption and infant birth weight, length and head circumference in the Norwegian Mother and Child Cohort Study. Brantsæter AL, Birgisdottir BE, Meltzer HM, Kvalem HE, Alexander J, Magnus P, Haugen M. (2011) Br J Nutr. 18:1-9.