I voted for "Spartan infanticide is ok within 7 days after birth", because I don´t see any tangible moral difference between aborting a baby at X weeks, with modern ultrasound screening data at hand, or getting the same information after birth without any high tech, and making the decision there and then.
If I were to write a law about this, I would define that the parents´ right to abortion ends when the parents bring the baby into the society from the hospital, or in any case 7 days after birth.
I think you and I both agree that there is no moral difference between killing a newborn infant, and killing an infant that is still in the womb, but is soon to be born.
There certainly is nothing really that magical about birth, that what exists right after birth is terribly different from what exists right before.
I would go farther, of course. There is no moral difference between killing a newly-conceived human zygote, and killing a newborn infant, or a toddler, or an adolescent, or an adult, or an elderly person who is moments short of natural death. In all these cases, what you are killing is a human being. And unless some drastic situation exists that justifies the killing of a human being, what you do by intentionally doing so is to commit murder, which is among the most serious evils that a human being can commit.
It seems to me that any consistent placing of the point after which a child has the right not to be arbitrarily killed, needs to be a point where one can clearly see that something different now exists, that didn't exist at all before that point.
There is only one such point. That is conception. Before conception, you have two haploid gametes. (Actually, you have a bunch of them, but only two are going to actually be relevant.) After conception, you have a new diploid organism, that didn't exist before, comprised of the joined DNA of the two haploid gametes that produced it. From this point, for the rest of this organism's life, it will be the same organism that it is at this point. There is no other magic point where you can say that it is a human being after that point, but not before. Every change that this organism will undergo, from this point, until its death, will be a matter of gradual change and development, and not ever again of the creation of anything new that wasn't there before.
Your position that “spartan infanticide” is OK is a natural progression of the acceptance of abortion as being in any way acceptable. There is no logic that ever purports to justify killing a child before it is born, that won't easily extend to killing it for some time after it is born.