I've read that a newborn human child's head size is constrained by balancing fitting through the mother's pelvis ("cephalopelvic disproportion") with ability to run. If human descendants evolve to scoot on hands and bottom (see video), the pelvis can become as wide as it needs to be to pass baby.

Or she walks on her hands and bottom like this

Tell me about it. In one episode of Weebles, Bumpus (mailman/inventor) body-slams a firefighter into a Super Springaling 3000 catapult.