A pregnant woman was sucker-punched in the back of the head Thursday afternoon — and may be the latest victim of the creepy “knockout game,” law enforcement sources said.

Jannatul Ferdous, a 34-year-old Brooklyn resident who is six months pregnant, was walking down a sidewalk in Bedford-Stuyvesant alongside her sister when Willie Stephens, 33, allegedly sneaked up behind her and socked her in the head around 3:45 p.m. Thursday, sources said.

Ferdous was knocked unconscious during the sudden attack, but neither she nor her child was critically injured.

Ferdous’ sister immediately called the cops and gave them a description of the attacker, who fled. Officers found Stephens three blocks away and arrested him. The alleged assailant, who does not have a criminal record, was charged with assault.

Cops believe the attack could be connected to the “knockout game,” in which criminals try to punch their victims unconscious with a single blow. The attack appeared to be random and officers believe Stephens did not know his victim, sources said.

Ferdous was taken to Woodhull Medical Center in Brooklyn.