Various factors comes into play with infanticide in ancient times.Historically speak
Or going back to the survival of the family as a whole. Resources for the whole
family’s survival, and often the community as a whole, not just the mother.
We also need to consider lack of birth control as a reason for what can be assumed were many unwanted births no? In societies where women don’t have a say over their sexual choices within a marriage. The only way to prevent infanticide is to refrain from sex and how would that go over with the husband? Let’s face it that would not of been an option for the woman anyway because if we are talking about infanticide in the context of limited encomonic resources, we have to consider the social, gender, and cultural make up of these communities where women probably didn’t have many rights to begin with.
I’m not saying that no woman has cruelly killed her kids for purely selfish reasons, like women who kill their kids to get with some guy or because they wanted to go on vacation and couldn’t find a babysitter (actual true case), but for this example of infanticide for economic reasons we have to consider the bigger picture and not limit it to just the woman’s survival. If it was just for the woman’s survival, she would kill all her kids and anyone else who took resources away from her.
Scarcity of resources being top.
Back when I was on a trip to Papua New Guinea, I've heard of stories related by the locals there of some tribes eating their own babies - committed by the mother. This was 20 odd years ago.
But in our modern society it's more about psychological infanticide, whereby the mother's survival (lifestyle) is paramount.