If my baby has a massive poo inside my body where does it go?

A fetus will not have a massive poo inside their body before birth because they don't eat solid food- they just get nutrients supplied through the umbilical cord. If the fetus is overdue sometimes they could expel meconium into the amniotic fluid, which is the earliest stool of a mammalian infant, but that is rare. That case is very bad and would require medical attention immediately.