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