That infanticide is a logical corollary from abortion would have led, in a happier time, to people recognizing that abortion must therefore be every bit as wrong as infanticide is. Now, as a testimony to their darkened intellect, they argue in reverse, that infanticide must be morally licit because abortion is legal. This is what happens to cultures that apostatise from the Church, they sink back to their ancient levels of barbarism and depravity.
"The history of infanticide is gruesome. As hard as it may be to imagine today, throughout history infanticide was a common and endorsed practice. While it undoubtedly still occurs today, all governments outlaw it. And in the West at least, society and culture condemn it. So how did we get from there to here? From having Western societies that condoned and encouraged infanticide to having a Western society that condemns and discourages infanticide?
The short answer: Christianity."
http://christiancadre.org/member_contrib/cp_infanticide.html