In the image the "Real Housewives of Atlanta" star is posing in front of a mirror while wearing a black and white bikini.
"Going swimming with the kiddos! The pool makes me feel so much lighter! This belly #twins #excited #blessed #nofilter," she wrote as the caption.
Zolciak announced she and husband Kroy Biermann were expecting twins in August.
"I can't believe I am going to be a mom to six children," the 35 year old -- who already has two sons with Biermann and two daughters from previous relationships -- told E! News. "It's going to be very busy, but it's a dream come true."