 H. stebingeri laid roughly spherical eggs of 20 by 18.5 cm (7.9 by 7.3 in), with embryos 60 cm (24 in) long. Hatchlings were around 1.7 m (5.6 ft) long. Young and embryonic individuals had deep skulls with only slight expansion in the bones that would one day form the crest. Growth was faster than that of an alligator and comparable to ratite growth, for several years, based on the amount of bone growth seen between lines of arrested growth (analogous to growth rings in trees). Research by Lisa Cooper and colleagues on H. stebingeri indicates that this animal may have reached reproductive maturity at the age of 2 to 3 years, and reached full size at about 10 to 12 years old. The circumference of the thigh bone at postulated reproductive maturity was about 40 % that of its circumference at full size. The postulated growth rate of H. stebingeri outpaces those of tyrannosaurids (predators of hypacrosaurs) such as Albertosaurus and Tyrannosaurus; rapidly growing hypacrosaurs would have had a better chance to reach a size large enough to be of defensive value, and beginning reproduction at an early age would also have been advantageous to a prey animal. Secondary cartilage has been found in the skull of a hatchling specimen of H. stebingeri.
