Credit: Obtained by ABC News

A Phoenix, Ariz., woman put her 5-week-old baby on top of her car in his car seat and drove away, apparently forgetting he was there, not even noticing when the seat fell off the car and landed in an intersection, police said.

Neighbors discovered the baby on the roadway, still strapped to his safety seat, which was lying on its side. Luckily, the baby was unhurt.

The baby's mother, 19-year-old Catalina Clouser, who allegedly had been smoking marijuana, was arrested and charged with aggravated driving under the influence and child abuse, police said.

"We believe that whatever she was under the influence of was the deciding factor in what did happen to this child and we're extremely happy that for the baby this turned out well and the baby is going to be OK," Officer James Holmes of the Phoenix Police Depart said.

Clouser, her boyfriend and their friends had been smoking marijuana earlier in the evening at a nearby park, but the boyfriend was arrested on suspicion of aggravated DUI when they went to a store - with the baby in the car - to get beer, according to police.

Upset that her boyfriend was arrested, police said, Clouser went to a friend's home and smoked more marijuana.

Clouser left around midnight and apparently put the sleeping baby on the roof of the car and drove off, forgetting he was there, and continued driving after the baby fell off the car, Holmes said.

Leilani Gerlach, a homeowner who lives across from the intersection, notified police after she and another neighbor saw the baby in the car seat on the street.

"We were both rolling our eyes in astonishment that someone could forget their baby," Gerlach said. "The car seat was turned to the side so anyone passing by wouldn't be able to tell there was an actual baby inside. Thank god they stopped."

After police arrived, two people walked up and told them the baby was Clouser's. As the officers talked to the two, who identified themselves as friends of Clouser and her boyfriend, the young mother drove up, police said.

"[She] had gotten into her car, set the baby seat on top of the car and forgot that the child was up there. It does not appear that immediately she realized what happened," Holmes said. "I think that perhaps she got where she was going and realized that she did not have the baby in the car."

"There was damage to the car seat, it was scraped but there was no damage to the child," Holmes said.

The baby is now in custody of Child Protective Services.