The way I remember the story, Zeus did swallow Metis to prevent her from having a child that would overthrow him like he had overthrown Kronos. She did have the kid and they had to chop Zeus’s head open to get her out. That is the origin of Athena, who popped out in full war armour. read more
There was a prophecy that Metis would bear a child greater than the father. Fearing this outcome, Zeus swallowed Metis. Some time later, Athena was born from Zeus' forehead; technically Metis isn't her mother. read more
Zeus had impregnated Metis and when the great goddess Gaia found out she issued a prophecy that any son of Zeus and Metis would usurp the throne of Zeus' father. Zeus wanted to prevent this from happening so naturally he decided to swallow Metis whole. read more
Athena leaped from Zeus's head, fully grown, armed, and armoured, and Zeus was none the worse for the experience. The similarities between Zeus swallowing Metis and Cronus swallowing his children have been noted by several scholars. read more