Can a grandparent get custody of a unborn

The child must be born first before any custody can be determined.

An unborn child is not a legal entity where custody can be granted. If its determined that the mother is a danger to the fetus, it's possible that the parent can have control of the mother to protect the fetus, but its unusual.

It would also be unusual for a parent to gain custody of the grandchild immediately after birth. Again, it would require showing the mother to be unfit.

Are you perhaps trying to stop your child from choosing adoption or abortion?

If so, you have no legal ground to stand on. While you may have legal control of YOUR child (if he or she is a minor), in all matters concerning his or her child, he/she is emancipated and can make all legal decisions regarding that child.