Early Jewish migrants (19th and early 20th century) had large families (high fertility rates). With the epidemiologic transition , Russian Jewish family sizes became smaller. Current feritlity rates of Jewish Women in the US are estimated between 1.4-1.9 children per woman, which is at the level below replacement.

To read more about epidemiologic transition, please visit http://en.wikipedia.org/wiki/Epidemiological_transition