"Kim and Kroy are engaged, and only a few people know about it," a source told RadarOnline. "They wanted to announce it in a national magazine, but Kim was so excited that she told a couple of close friends."
Biermann reportedly bought Zolciak a substantial ring which she has shown to a few friends but has yet to wear in public.
The 32-year-old reality TV star and her 25-year-old husband-to-be have been dating since May but only confirmed they are expecting their first child together last week after previously dodging pregnancy reports. They plan to marry before Zolciak gives birth, according to RadarOnline.
The new baby will be Zolciak's third child -- adding to her 13-year-old and nine-year-old daughters from a previous marriage.
"I was surprised! While it wasn't planned, God clearly has a bigger and better plan for us and we're excited," Zolciak told Life & Style last week in an interview in which she announced her pregnancy.
Zolciak is in her second trimester of pregnancy and she and Biermann are currently in talks for their own spinoff reality show, according to the RadarOnline source.
"And if [the spinoff] does not happen then she will want to re-negotiate her terms onThe Real Housewives Of Atlanta if her wedding and new baby were to be featured on the show," the source said.
"The other cast members are publicly backing Kim and Kroy but privately there are some jealousies because Kim is very much in the spotlight right now and not everyone is happy about that."