Immigrant women’s fertility often declines with their duration of stay. Competing hypotheses may explain this: According to the adaptation hypothesis, an immigrant woman will adapt over time to the fertility norms in the destination country. On the other hand, the family formation hypothesis predicts that fertility will be extra high the first years after migration simply because women often migrate to start a family.

When immigrants are from high fertility countries, it may be hard to determine which of these mechanisms are at work, because both hypotheses predict a falling fertility by duration of stay. However, when the migrants move from low to high fertility countries, it is easier to test the hypotheses: The adaptation hypothesis would predict an increasing fertility, whereas the family formation hypothesis would still predict elevated fertility right after migration.

Using data on immigrant women from Poland and Lithuania living in Norway, my first results show that even in these groups, fertility is highest right after migration - suggesting that the family formation hypothesis may explain these immigrants’ fertility patterns better than the adaptation hypothesis.