She drives with her baby on top of car

A 21-year-old US woman who left her baby in a car seat on top of her car has escaped jail and was sentenced to 16 years of probation instead.

In June 2012, Catalina Clouser drove about 19km on an Arizona highway before realising her two-month-old tot was not inside the vehicle but on top, Britain's Mirror reported.

The tiny baby was discovered by a passer-by, miraculously unharmed.

Clouser, then 19, subsequently pleaded guilty to child abuse.

She told police she had taken marijuana before driving off and did not realise her son was on the roof or that he was missing until she got home, Britain's Mail Online reported.

Police said that Clouser had smoked "one or two bowls of marijuana" with friends.

When Clouser realised her son was missing and retraced her steps, she was nabbed by Phoenix police, who had been notified by the passer-by who saw the baby.