Both men, wearing crash helmets, stamped on the woman's stomach, said police.
A pregnant woman who lost her baby after she was pushed to the ground and kicked may have been deliberately targeted, police have said.
The 21-year-old, who was 32 weeks pregnant, was attacked by two men wearing crash helmets in Peckham, south London at around 20:00 BST on Monday.
Both men "continually kicked and stamped on her stomach" before fleeing the scene, police said.
A 20-year-old man was arrested on Tuesday and remains in custody.
The victim was taken to hospital where she remains in a critical condition.
After the attack in Talfourd Place, the suspects, described as black and in their 20s, ran off in the direction of Denmark Road, said the Met.
Investigating officer Det Ch Insp Robert Pack, said officers were investigating whether the woman was targeted by the assailants.
"This was a truly abhorrent and vile attack on a heavily pregnant woman who has now, tragically, lost her baby.
"The men who attacked her must have known the impact their actions would have when they kicked her repeatedly in the stomach.
"It is sickening to think someone would deliberately target a pregnant woman in this way."
He said the two men were distinctive in their crash helmets and appealed for anyone with information to come forward.