"It was terrifying to see," said Zeng, who saw two young men striking the pregnant woman repeatedly.
"I see the guy spit on her and then just start throwing punches," he said.
Quincy Police said the pregnant woman felt sick, and began throwing up.
Two men, identified as David Russell and Tyrese Johnson-Nurse, began to make fun of her, words were exchanged, and then the assault happened, police said.
Bystanders stepped in to try to help. The woman's boyfriend also tried to step in, but he was also beaten.
"Get your (bleeping) hands off my girl, bro," the boyfriend is heard saying to the attackers in the video.
Zeng also made an attempt to stop the beating.
"Yo, chill!" he is heard saying in the video.
"I told them to stop but then they didn't stop, then I (reached) for my phone," Zeng said.
Police say they're glad he did. They tell us the men initially faced assault and battery charges.
According to the Patriot Ledger, Russell was also charged with assault with a deadly weapon and was arraigned. The newspaper also reported that the woman and her fetus were evaluated at a hospital.
The pregnant woman's condition was not immediately known.
The suspects are reportedly due back in court on May 29.