Authorities said the victim, 66-year-old Allen Briscoe, was driving his pickup truck at a "low speed" when the accident occurred in Camden around 7:30 p.m. Monday.
The pregnant girl was knocked down but only suffered minor injuries.
Prosecutors say her boyfriend, 18-year-old Aleem Mayes, confronted Briscoe, who told him he had not seen the girl.
Then, according to investigators, Mayes punched Briscoe once in the face, knocking him to the ground and briefly leaving him unconscious.
"After realizing he had hit this young lady, he immediately stopped and exited his vehicle to see if she was okay. While that was happening he was punched in the face and knocked unconscious," said prosecutor Mary Alison Albright.
Within two hours Briscoe, a former school teacher and Vietnam veteran, was dead. Camden County prosecutors say an autopsy determined Briscoe suffered an aneurysm caused by blunt force trauma.
Prosecutors also say they have two witnesses and a confession.
On Wednesday Mayes, a football player and senior at Camden's Woodrow Wilson High School, appeared in court.
"This is a very strong case. We have eyewitnesses to the assault, both of whom know the defendant and we have the defendant's own videotaped admission," Albright said.
Mayes remains in jail on $750,000 cash bail.
"You got an 18-year-old who's just getting ready to start life and a 66-year-old man who retired less than a year ago, and his life shortened over tragedy," said Albert Briscoe, the victim's brother.
"We're saddened, we're hurt, but we'll also continued to pray for him," said Kimberly Gardner, Briscoe's niece, who was talking about Mayes. "He made a poor life choice."