How does stimulation around a fetus’s mouth trigger an early reflex?

In the context of biology, a reflex is an inborn nervous system activity that produces a stereotyped, immediate response to a specific stimulus without involving consciousness.1

Regarding the development of these actions in the womb, the first reflexes can be observed as early as seven and a half weeks after conception. When stimulation occurs around the mouth of a fetus, it triggers an early reflex that causes the lips to turn toward the source of the stimulus.1

This process relies on a reflex arc, which is the anatomical pathway consisting of an afferent nerve that carries the sensory signal, interneurons within the central nervous system, and an efferent nerve that directs the muscular response. While these responses are described as stereotyped, they are not necessarily unchangeable; they can be influenced by factors such as habituation or sensitization over time.1