Thieves fled with cash and cigarettes in a sack after storming into the West Midlands store and threatening staff.
The mother-to-be was working in the shop when she was pushed to the floor by the masked criminals.
She was not injured but was left shaken, with a full check-up later confirming there were no immediate concerns for her unborn baby.
Det Insp Kerrie Martin, overseeing the investigation, said: “Clearly this was a terrifying ordeal for those involved, made all the worse by the fact that one of the victims is pregnant.
“Thankfully, she was not seriously injured. But I’d urge those who may know the identity of the people responsible to examine their conscience and think how they would feel if a loved one of theirs was pregnant and subjected to such an awful ordeal.
“Similarly, I’d urge anyone who saw the cars in the area, or who may have been offered cigarettes for sale which they suspect were stolen, to call.”
Detectives believe about three gang members raided Marston Green store Norwich Express, off Norwich Court, on Friday.
At least two of the perpetrators were carrying guns during the ordeal, which took place at about 5.15pm.
A white Ford C-MAX and a white VW Golf were recovered nearby and are believed to be linked to the gang.
One offender was described as white, skinny, and about 5ft 7ins tall.
He wore a grey hooded jacket with dark bottoms and had fresh scratches on his face, West Midlands Police said.
Police are now trawling through CCTV and are appealing for help to identify the culprits.
CCTV and other enquiries are being carried out, and officers are now appealing for help to identify the gang.
Contact police through Live Chat between 8am and 12am at west-midlands.police.uk, call 101, or ring Crimestoppers anonymously on 0800 555 111, quoting crime number 20SH/260846E/18.