Lynne Spears started to laugh and then cry when she learned that teenage daughter Jamie Lynn left her a note explaining she was pregnant.

The Nickelodeon star’s mum was in complete dismay when her youngest child handed her a note and told her to read it alone in the bathroom of the family's home.

It was the first time Lynne realised her youngest child was having a baby with her boyfriend Casey Aldridge - but she couldn't believe what she read.

"I thought it was a joke... I kept waiting for the punchline," says Lynne on America's Today show.

But the Spears' matriarch quickly realised it was no laughing matter when Casey refused to look at her as she came out of the bathroom.

"I was just truly in shock and then I started to cry. And then she (Jamie Lynn) started consoling me at that point," she added.

Jamie Lynn was only sixteen when she fell pregnant. She gave birth to a daughter, Maddie Briaan, in June.