In inventing a test to check for nicotine usage, University of Minnesota scientists made an interesting discovery: Plenty of expecting mothers lie about smoking.

Testing blood samples from newborns, they found that 12 percent had traces of a chemical that the body produces when exposed to nicotine, meaning their mothers smoked before giving birth. Trouble is, 41 percent of those mothers had not told their doctors that they had ever smoked.

The numbers “were similar to other studies of socially disapproved behaviors during pregnancy,” said Logan Spector, a U epidemiology researcher who led the study.

We wrote recently about Minnesota teenagers lying to doctors about their level of sexual activity, which results in undertesting for sexually transmitted diseases. But adults may be no better. A 2004 WebMD survey found that 13 percent of patients lied and 32 percent “stretched the truth” to doctors.

Some lie because they don’t want to let down their doctors, while others want to avoid lectures. Illicit drug users fear being reported. Others might be manipulating doctors into prescriptions or diagnoses that their health plans cover.

But lying has consequences.

“We could be making decisions … that could adversely affect you because we don’t know you are under the influence, or on this drug, or doing this substance, said Dr. Krista Skorupa, a HealthEast physician in Roseville. “We could do more harm not knowing the full story.”

Health care providers are taught to double patient estimates: Drinkers have twice as much booze as they admit; flossers clean their gums half as much as they claim.

Skorupa often suspects when patients are lying but said it backfires to interrogate them in the exam room. Eventually, the truth comes out over multiple visits.

“If you’re not going to create an environment that feels safe to them,” she said, “there is no incentive for them to be honest with you.”