There is an inverse association between income per adult and fertility among countries, and across households this inverse association is also often observed. Many studies find fertility is lower among better educated women and is often higher among women whose families own more land and assets. What do we know about the social consequences of events and policies that change fertility, if they are independent of parent preferences for children or the economic conditions which account for much of the variation in parent lifetime fertility? These effects of exogenous fertility change on the health and welfare of children can are assessed from Kenyan household survey data by analysis of the consequences of twins, and the effect of avoiding unanticipated fertility appears to have a larger beneficial effect on the body mass index or health status of children in the family than would be expected due to variation in fertility which is accounted for by parent education and household land.