Latin, the mother tongue

Among the many ailments - I mean, changes - that affect a woman's body during pregnancy, PP is keen on one in particular: the linea nigra.

The linea nigra appears around now (22 weeks) connecting the belly button to the rectum. (Note how modern use of Latin has been relegated to euphemisms [originally a Greek word] to avoid our blushes.) It's an umistakeable sign of pregnancy - though I would have thought a figure like a rugby ball was something of a giveaway.

PP's linea nigra (dark line to you and me) hasn't appeared yet, and her belly button is still an inny. What's the Latin for inny?