Childbearing patterns for Swedish mothers of twins, 1961-1999

The Nordic population registers provide a unique possibility to study the demographic behavior of very small population groups and rare events. In this paper, we study the childbearing behavior of Swedish mothers of twins between 1961 and 1999, inclusive. Our most consistent finding is that mothers of twins wait noticeably longer than women with singletons before they have another child. This apart, mothers with twins at their first birth have next-birth fertility patterns very similar to women who have two singletons at their first two births. This commonality in child-bearing behavior does not extend to higher-order births. For mothers with a singleton and a pair of twins, the progression to a third birth depends very much on whether the twins came first or second. Beside these main results our fascinating material also provides a number of descriptive findings. The Swedish twinning rate has increased since the mid-1970s in response to a growing use of fertility-stimulating treatments such as in-vitro fertilization, in parallel with similar developments in many other countries. Such medical procedures are applied mainly to women beyond prime childbearing ages. Nevertheless, we find no simple age pattern in twinning rates. Even in recent years they do not just in-crease with the woman's age. By way of contrast, at parities beyond 3 twinning rates increase with parity when we control for calendar period, time since last previous birth, and (NB) the woman's own age.