Final answer:
The baby's hearing begins to develop around 18 weeks gestation, allowing them to detect sound vibrations. By the second trimester, the baby can start to respond to external sounds. The development of hearing continues after birth.
Step-by-step explanation:
When discussing with Anna when her baby's hearing will develop, you can inform her that the baby's hearing begins to develop around 18 weeks gestation. At this stage, the structures of the inner ear start to form, allowing the baby to detect sound vibrations. By the second trimester, the baby can start to respond to external sounds, such as the mother's voice or music played for them. It's important to note that the baby's hearing will continue to develop and refine after birth as well.