The present study investigated whether the length of interbirth intervals between first- and secondborn children in a North American middle-class sample could be explained by paternal and alloparental support and firstborn children’s gender. The sample consisted of 225 families in which mothers were expecting their 2nd child. Parents reported on paternal and alloparental support (maternal kin, paternal kin, and nonkin support). The results showed that higher maternal kin support and having a firstborn son was linked with shorter interbirth intervals. Mothers’ longer work hours during the pregnancy with the second born was related to longer interbirth intervals. These results highlight the importance of maternal kin support and children’s characteristics in understanding the timing of birth when parents have a 2nd child.