Strong boy-bias and its consequences for young and unborn girls have been widely documented for Asia. This paper considers a country in Sub-Saharan Africa and finds that parental gender preferences do affect fertility behavior and shape traditional social institutions with negative effects on adult women's health and well-being. Using individual-level data for Nigeria, the paper shows that, compared to women with first-born sons, women with first-born daughters have (and desire) more children and are less likely to use contraceptives. Women with daughters among earlier-born children are also more likely to have shorter birth intervals, a behavior medically known to increase the risk of child and maternal mortality. Moreover, they are more likely to end up in a polygynous union, to be divorced, and to be head of the household. The preference for sons is also supported by child fostering patterns in which daughters are substitutes for foster girls, while the same does not hold for sons and foster boys. These results can partly explain excess female mortality among adult women in Sub-Saharan Africa.