I do care about welfare, and the difference between infants and zygotes is sentience.

Most medical professionals and ethicists consider 24 weeks to be the reasonable cutoff for abortion because this is when the fetus starts to develops sentience.

The reason this is relevant is because that is the first stage of development capable of having an identity relationship with the future person that fetus/infant will become.

Animals don't have to be self-ware to suffer. Not introspectively self-aware at least.