Rob and Blac Chyna: Massive Cover-Up at Vegas Pool Party

Rob Kardashian and Blac Chyna hosted a Vegas pool party ... but due to extenuating circumstances neither dressed for the occasion.

BC and Rob didn't venture out from the VIP area at Sky Beach Club -- Rob sipped a cocktail while Chyna sought cover from the blistering heat.

Las Vegas ... not so much fun when you're pregnant ... or just not quite ready.