This is some wacky logic.
A child is dependent on other people for a long, long time. Not just prenatally.
And yes, that means losing some autonomy, doesn't it?
Try to square some examples with your way of thinking:
Mommy wants her body to be in Aruba for 2 weeks. So she leaves her 6-month old baby at home to starve. Oh well, Mommy does what she wants with her own body.
Mommy puts 1-year-old baby in the bath. Mommy's body is tired. Mommy decides she wants her body to go to sleep now. Mommy sleeps all night; baby drowns. Oh well, Mommy does what she wants with her own body.
Mommy has a 20-week-old baby in her womb. Mommy decides she doesn't want to be pregnant. She pays a doctor to pull the baby's body out of her and kill it. Oh well, Mommy does what she wants with her own body.
In all of these situations, a loss of the mother's autonomy was necessary to preserve the baby's life.
Yet I doubt you'd argue that the first two ought to be legally acceptable.
What makes the third example any different?