Culture that Diamond cites in Collapse did not have access to abortion (at least not efficacious abortion), hence infanticide, even of healthy babies. It was an agrarian culture that resorted to such means during periods of prolomnged drought, if I remember correctly (i.e., each new mouth decreases the chances of other members of the poulation surviving).
If they had access to technology like in utero genetic screening I would assume that yes, they might do as you say, based on their value system.
Realize these comments may be a bit off topic, but there seems to be a part of this thread about society's right (or not) to take a human life, and just wanted to make the point that the argument about when the taking of another's life is sanctioned is highly dependent on the cultural/societal context, and not absolute.