So lets talk about this:
Infant mortality used to be very very high. It's low now, but over history, that's a lot of dead babies. Both prenatal and postpartum, it far outnumbers the quantity of dead adults.
Christians often believe that dead babies go to heaven. This means heaven is full of babies who don't know Jesus. Does this really square with the biblical depictions of heaven?
So for the Christian either,
1) Babies don't go to heaven,
2) Heaven is packed with babies and as a result heaven is filled with people who never knew the name of Jesus
3) Life doesn't begin at conception (this reduces the number of babies in heaven significantly, but still leaves us with a lot of 1 and 2 year olds in heaven)
4) Life doesn't begin even after birth.
So pick your poison. I think most Christians would pick #2 because it's most emotionally appealing despite being unbiblical. But is #1 really so hard to believe if you're an annihilationist?