The research found that incisor teeth grow quickly in the early stages of the second trimester of a baby's development, while molars grow at a slower rate in the third trimester. This is so incisors are ready to erupt after birth, at approximately six months of age, when a baby makes the transition from breast-feeding to weaning.
Weaning in humans takes place relatively early compared to some primates, such as chimpanzees. As a result, there is less time available for human incisors to form, so the enamel grows rapidly to compensate.