Draco's hands felt heavy as he held the egg that Hikari had laid. He had completely forgotten that she - and truthfully, he as well - were actually Dragons. Dragons didn't give birth to their children in human forms, but laid eggs like any other serpent.
He had assumed that since they had sex in human form, she would give birth in human form as well.
However, he had forgotten that while he had started out as a human who had gained his Dragon bloodline, in Hikari's case it was the other way around. She only took on the form of a human on Draco's behest.
The fact that she had taken on a half-dragon form while alone, clearly showed that if it were up to her she would remain a dragon, yet she did not want to fully disobey her Dragomate's wishes.
Nevertheless, it was reasonable for her to have returned to her most natural form to not complicate the laying of her very first egg in the anomaly realm.