Matt Sayles, file, Associated Press
FILE - In this Nov. 19, 2009 file photo, Levi Johnston arrives at US Weekly's Hot Hollywood party in West Hollywood, Calif. Johnston writes about his relationship with Bristol Palin and the Palin family in his upcoming book "Deer in the Headlights," set for release on Sept. 20.

NEW YORK — Levi Johnston writes in his upcoming book his ex-girlfriend Bristol Palin was so angry about her mother's pregnancy with son Trig she wanted to get pregnant, too.

The book is called "Deer in the Headlights." It comes out Sept. 20. The Associated Press bought a copy Monday.

Johnston says when Bristol found out her mother, former Alaska Gov. Sarah Palin, was expecting a baby she responded she should be having a baby, not her mother. He says she told him in March 2008, "let's get pregnant."

He says they had begun dating in 2006 and had been sexually active.

Johnston says he'd "been too dumb" to use protection while dating Bristol but knew having a baby was "what she wanted."

She had their baby at age 18.