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
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.