Michael March banged the weapon on the floor to scare a couple walking past a pub in South Shields in October 2016.
Prosecutors said the offence was carried out during the "killer clown" craze.
The 18-year-old admitted possessing a bladed article at South Tyneside Magistrates' Court.
He was bailed and will be sentenced at Newcastle Crown Court on 6 February.
Paul Anderson, for the Crown, said March - who was 17 at the time - carried out the offence the night before Halloween.
He had waited for the couple outside the Sand Dancer pub and struck the floor with the axe to frighten them.
The woman, who was 22 weeks pregnant, picked up a brick and threw it at him.
Mr Anderson said: "[March] tried to explain it was a prank and ran off in something of a panic.
"He said it was all meant to be a joke that went badly wrong."
March, of Hopedene, Gateshead, was traced by police via CCTV and still had the axe and the mask in his backpack.
Magistrates heard he told police: "I'm sorry, I was not going to hurt her."
A teenager in a clown mask who brandished an axe at a pregnant woman left her so scared she threw a brick at him, a court heard.