More information on the Gona discovery from the Case Western Reserve News Center:
Making obstetrical measurements and comparing them to prior published specimens, Simpson determined that the birth canal in the pelvis had adapted to deliver a baby with a brain almost comparable to a modern-day baby at about 36 weeks and weighing approximately four pounds.
The H. erectus brain would have been up to 315 cubic centimeters (cc) at birth based on measurements of the width of the inlet and outlet of the pelvic fossils, making it slightly smaller than today’s babies born with brain sizes averaging about 380 cc.
Working at the Cleveland Museum of Natural History, he also made comparisons with the reconstructed pelvis of "Lucy," an Australopithecus afarensis specimen, which is about 3.2 million years old, to see how the pelvis was evolving as means to deliver offspring with larger brains.
Can you say "transitional fossil?"