Logically speaking, this is the only thing possible.
Ashwatthama attacks uttara the day after the war. So on 19th day.
But Krishna does his miracle, several months later.
So did the baby die immediately after the attack and uttara was carrying a corpse for 10 months that Krishna revives.
Or the baby does not die even after the attack but dies only after it is born. Then Krishna revives it.
Or the astra remains in suspension in the womb till the child is born and then attacks it.
Why doesnt Krishna act immediately on the day uttara is attacked. Whatever miracle needed to be done would make sense if it is done on the day of attack not 10 months later