Kim K. & Amber Rose Once You Go Black ...

1/1/2013 12:10 PM PST

Kanye West's baby mama and his ex-gf Amber Rose have something in common -- both pregnant women are into black ... dresses.

Kim Kardashian was decked out in a tight lacy black gown last night in Las Vegas -- and about 4 months ago Amber was wearing ... a tight lacy black gown.

It's an odd coincidence, especially because Kim and Kanye were making their first appearance since their baby announcement -- just like Amber and Whiz Khalifa were back in September.

At least the dudes weren't matching.

Still, we gotta ask ...