That wastes money and could harm women with unnecessary care, said Mona Saraiya of the Centers for Disease Control and Prevention, who led the research.
The findings, reported in the journal Obstetrics & Gynecology, show women have to be savvy to ensure they are getting the right checkups.
"It's extremely discouraging," says Debbie Saslow, gynecologic cancer director at the American Cancer Society, who has had to argue with her doctor against testing too often.
Cervical cancer grows so slowly that Pap smears, which examine cells scraped from the cervix, usually find it in time to treat, or even to prevent when precancerous cells are seen and removed.
For decades, Paps were the only way to screen for cervical cancer. Now doctors know that certain strains of HPV, the human papillomavirus, cause most cervical cancer. HPV testing isn't a replacement for the Pap. But it can provide extra information on risk - if it's used correctly.
The CDC study found 60 percent of doctors and clinics say they give a routine Pap-plus-HPV test to women too young, under 30, for that combination.
Why the age limit? Saslow says HPV is common, especially in younger women. But their bodies usually clear the infection on their own. Learning a 20-something has HPV increases the odds of more invasive testing, which can leave her cervix less able to handle pregnancy.
Then there's the question of which test to use. Only a few so-called high-risk strains of HPV cause cervical cancer, the strains doctors are supposed to test for. But an old test for other strains is still on the market.
The American College of Obstetricians and Gynecologists recommends that:
Routine Paps start at 21.
Most women in their 20s get a Pap every two years.
Women 30 and older wait three years between screenings if they' hae had a negative Pap and negative HPV test, or three clear Paps in a row.
If a Pap is inconclusive at any age, HPV testing may help rule out who needs further examination and who can repeat a Pap in a year.
Anyone who has been immunized against HPV still must follow Pap screening guidelines for her age group.
Higher-risk women, such as those with HIV or previous cervical abnormalities, need more frequent screening.