Bristol: Do as I say …

Bristol Palin says that if girls knew how tough it was to be a mom, they’d think twice about having sex.

The 19-year-old daughter of former Alaska Gov. Sarah Palin says she’s now practicing abstinence herself, until marriage.

Palin gave birth to a son, Tripp, in December 2008 and says she “wasn’t prepared at all” for the dramatic changes in her life.

“I don’t think anyone realizes how difficult it really is until you actually have a screaming baby in your arms and you’re up all night,” she says.

“It changes literally every aspect of your life, and if girls realized how hard it was to be a teen mom, they would think twice about having sex without the proper education and proper knowledge.”

Last year, she was named an ambassador for The Candie’s Foundation, a division of the apparel brand Candie’s, which has been raising awareness about teen pregnancy since 2001. She’ll appear in a new public service announcement called “Pause Before You Play.”

Palin also recently taped a cameo on “The Secret Life of the American Teenager,” about a high school student who unexpectedly got pregnant and is now a young mom.

“I’m very blessed to have an extremely supportive family and just a good support system, which other girls out there, most of them, don’t have,” she says.

Despite the challenges, Palin hopes to have more kids.

“Eventually,” she says. “Long down the road, though.”

The birthday bunch

Musician Herbie Hancock is 70. Actor Ed O’Neill is 64. Talk-show host David Letterman is 63. Singer-actor David Cassidy is 60. Actor Andy Garcia is 54. Country singer Vince Gill is 53. Singer Art Alexakis (Everclear) is 48. Actress Shannen Doherty is 39. Actress Claire Danes is 31. Actress Jennifer Morrison (“House M.D.”) is 31.