Jonah Hill says watching a woman give birth on a New York street was the "craziest thing" he had ever seen.

The 28-year-old funnyman was going for a stroll in the Big Apple when he saw came across the pregnant lady in the park.

TMZ's cameraman also happened to be there and captured Jonah taking a picture of the woman's placenta that was laying there after she had been loaded into an ambulance.

The woman apparently went into labour while sitting on a park bench leading to the messy scene.

Although Jonah made a hasty exit when he realised he was being photographed, he later took to Twitter to explain what happened.

"Craziest thing I've ever seen," he wrote on the social networking site. "A woman gave birth next to me in a park. I took a picture of the placenta and TMZ caught me. Embarrassing."

The actor was recently seen in movies 21 Jump Street and The Watch and has five other films in production.