Several people have alreadt given good answers to your position on infanticide, but they haven't mentioned what is in my opinion the crucial concept involved here: Schelling points.

We are all agreed that is is wrong to kill people (meaning, fully conscious and intelligent beings). We agree that adult humans beings are people (perhaps excluding those in irreversible coma). The law needs to draw a bright line separating those beings which are people, and hence cannot be killed, from those who are not. Given the importance of the "non-killing" rule to a functioning society. this line needs to be clear and intuitive to all. Any line based on some level of brain development does not satisfy this criterion.

There are only two Schelling points, that is obvious, intuitive places to draw the line: conception and birth. Many people support the first one, and the strongest argument for the anti-abortion position is that conception is in fact in many ways a better Schelling point than birth, since being born does not affect the intrinsic nature of the infant. However, among people without a metaphysical commitment to fetus personhood, most agree that the burdens that prohibition of abortion place on pregnant women are enough to outweigh these considerations, and make birth the chosen Schelling point.

There is no other Schelling point at a later date (your ten-month rule seems arbitrary to me), and a rule against baby infanticide does not place so strong burdens on mothers (giving for adoption is always an option). So there is no good reason to change the law in the direction you propose. Doing it would undermine the strengh of the universal agreement that "people cannot be killed", since the line separating people from non-people would be obscure and arbitrarily drawn.