The proportion of UK families with three or more children is high in European perspective. Commentators often assume that high rates of progression to third, fourth and higher order births result from the higher fertility of women who migrated to the UK from regions of the world where large families are common. Less attention has focused on the role of serial partnering where repartnered couples cement their commitment with a ‘shared child’. This paper provides new empirical evidence as to the relative importance of increased international migration and increased experience of multiple partnerships in fostering higher order births among British women now aged 45+ who were born 1940-1969. We conclude that migration from high fertility regions, and the experience of multiple partnerships are both associated with progression to third and fourth birth. However, the likelihood of going on to have a third or fourth birth has remained similar across these birth cohorts for women who were born in the UK (to UK-born parents), and those who have only been married once, suggesting that additional factors encourage large families in the UK.