Baby Weight?!

I've noticed myself that sims can gain weight during pregnancy, but it's much more common when they give birth to more than one child, or have children in adulthood rather than young adulthood. For example, I've had young adult and adult sims have more children, but when one of my sims already had previous children from young adulthood and got pregnant and had another baby in adulthood, she had gained almost as much weight after that third child than both of the previous children's baby weight combined. Though I think all sims gain baby weight to an extent after pregnancy, it's much less common in young adulthood and first births.