Karltin actually didn't know much about dragons. He only knew the basics he had been taught in his magical studies. He had no idea how to ensure a dragon egg hatched, nor how long it would take. But he did know that dragons had stronger instincts than other creatures. They knew everything they needed to know without being taught. If he transformed himself into a dragon, surely he would have that information. Specifically, if he became a female dragon, he would know. He wasn't sure if male dragons were involved in egg brooding, and thus whether they would have the information needed for it.
Karltin carried Pearl's egg to a natural cave near his cabin. He set it down and then cast a transformation spell on himself. There was a flash of light. When it cleared, Karltin had become a remarkably petite golden dragoness. So much smaller than average was she that she could be mistaken for an adolescent. But she was, in fact, fully grown. And now, Karltin knew everything she needed to tend to the egg.
Karltin created a nest and curled around the egg to keep it warm. She now knew it would take two months to hatch. She didn't have enough magic to be transforming back and forth constantly, so she would remain a dragon until Pearl hatched. Pleased, Karltin closed her eyes and fell asleep around the egg.