The Swiss Federal Statistical Office publishes data showing that the TFR of foreign women is much higher than for Swiss women. However, statistics from household registration (STATPOP) and from the Family and Generations Survey (FGS) indicate that foreigners have slightly smaller families than Swiss women. How can we reconcile this apparent contradiction? To do this we follow the fertility of cohorts of Swiss and foreign women through their reproductive life. In addition to birth registrations and population totals by age (the input data for calculating the TFR) we also include data on how many children women have at the time of their immigration, emigration and naturalisation. Using these input data, we compiled the fertility profiles of Swiss and foreign women aged 15-49 (cohorts 1965-2001); these correspond well with the FGS and household register data. Most immigrants arrive childless and start childbearing in the years following arrival; hence, younger foreign women in Switzerland have higher fertility than Swiss women. However, the ongoing inflow of low fertility women ‘dilutes’ the average fertility of older foreign women. Naturalisation–which is more frequent for women with children–significantly impacts the fertility profile of ‘Swiss’ and ‘foreign’ women. We confirmed that the TFR gives an inflated impression of the ultimate fertility of foreign women, and under-estimates that of Swiss women, because foreign women are only in the receiving country (Switzerland) for the most fertile portion of their reproductive career. Our comprehensive fertility model covering the entire reproductive life course better describes fertility differentials by age and nationality.