Having or making a finger baby is simply the act of fingering as performed by a lesbian to another. Lesbians are women and they do not have penises so they cannot actually make a baby. Rather, they finger each other, thus colloquially sex can be called making a finger baby, or often, making finger babies. See the "finger babies" entry.

When you finger a woman so well that it gets her pregnant.

When you have finger to vaginal intercourse you make finger babies, because when you make a real baby you have penis to vaginal intercourse.

When you come on your hand, in which there's a cut on your finger thus resulting in a finger baby.

A super hot girl. She has really little fingers that everyone wants to hold. She bags mad dick. Her fingers = the shit. She's got the cake too...