The baby started to cry.

Akane woke up and threw Ranma out of the bed, hitting him with a water balloon
before he landed on the floor.

Akane:  "Its your turn to feed the baby."

Ranma grumbled to himself as he sleepily made his way to the crib.  No other
father had to deal with this, he thought to himself as he lifted his shirt to
feed his daughter.

Nani?! Well... one possible explanation would be that Ranma is such a 
loving father (or mother) that he took hormone injections to allow his 
female form to lactate... after all, Akane might not want to do it all 
the time...