Robin Thicke is about to be a father again, with a girlfriend who is barely able to legally drink.

22-year-old April Love Geary is 3 months pregnant with a girl ... the baby is due March 1.

40-year-old Robin has been dating April for 3 years ... shortly after his wife, Paula Patton, filed for divorce.

LEAVING FOR HAWAII
TMZ.com

Robin and Paula are still not divorced, and they have had an on-and-off custody war over their 7-year-old son, Julian. By the way, we're told Robin and April told Julian this week and he's "super excited for a little sister."

March 1 would have been Alan Thicke's 70th birthday.