A great deal of selection happens prior to birth, whereby many viable embryos do not make it to term. Antenatal selection is responsive to environmental factors and affects maternal and fetal health outcomes. Since male embryos tend to be more frail than their female counterparts, previous studies have used sex ratio at birth to establish that selection on the XY chromosomes exists prior to birth. However, the genetics of antenatal selection beyond the effects of XY chromosomes are not well understood. The challenge lies in the fact that many of the embryos which undergo selection are never observed. In this study we develop a novel approach to understanding the genetics of survival to term using within-family data.