Genetic factors

Intrauterine growth is certainly genetically co-determined. Whether this is mediated through the fetal genes or through maternal genes and conditions has not been fully established. The previously mentioned strong tendency to repeat similar birth weight, gestational length and weight-for-gestation could be explained either way. Studies of maternal and paternal half siblings might help to clarify the picture. Likewise, as data sets become available of pregnancy outcomes across generations we may be able to better separate genetical from environmental factors. In Norway a Medical Birth Registry has existed for 30 years, which means that it now becomes possible to study the outcome of the pregnancies of women included as newborns in the registry from 1967 onwards. Preliminary analysis based on these data reveals a tendency to repeat birth weight across generations that is similar to the tendency to repeat birth weight within sibships (Magnus et al, 1993). This suggests that genes play a considerable role in determining birth weight. The correlation of gestational ages across generations is weaker in spite of that fact that the tendency to repeat gestational ages within sibships is nearly as strong as the tendency to repeat birth weight. This might indicate that genetic factors play a smaller role in determining the length of pregnancy.