Q. My sister recently found out that she was a chronic carrier of hepatitis B and, what's worse, that she had passed this infection on to her newborn baby while pregnant. We just learned that there is a test for this infection, and treatment to prevent it. Why wasn't she tested for this when she was pregnant?

A. Currently, routine testing for hepatitis B virus -- which can cause severe liver disease and cancer in chronic carriers -- is not recommended in pregnant women. But because early detection and treatment can prevent serious complications, the Centers for Disease Control is considering changing its recommendations to include testing for this infection during pregnancy.

Hepatitis B virus is transmitted through blood products and sexual contact -- much like the AIDS virus, only more easily. Once infected, a person usually develops hepatitis -- liver inflammation -- with symptoms like fever, nausea, fatigue and yellow jaundice. In most cases, the body fights the infection, which gradually goes away.

In some people, however, the virus can linger in the body for years, causing further liver damage. In such cases, the person remains infected and can transmit the infection to others; mothers who are chronic carriers can pass the infection on to their unborn baby.

Once infected, most newborn infants become chronic carriers of the virus themselves. About one out of four such infants will go on later in life to develop fatal liver cirrhosis (scarring) or liver cancer. In addition, they can transmit the virus to their own newborns, perpetuating this cycle of life-threatening disease.

Currently, testing for hepatitis B infection is recommended only for high-risk groups -- those who have an increased risk of coming into contact with infected blood or body fluids -- including health care workers, patients on dialysis, intravenous drug users, hemophiliacs, and sexual contacts of people who are chronic carriers of hepatitis B virus. In addition, certain ethnic groups have a high incidence of hepatitis B infection in their native countries, including Southeast Asians, Alaskan Eskimos and Haitians.

Because of difficulties in detecting all high-risk people, many of the 16,500 chronic carriers who become pregnant each year are not identified, and their infants do not receive treatment. But if doctors know a pregnant woman is a chronic carrier of hepatitis B, they will treat her newborn infant with hepatitis B vaccine and an injection of hepatitis B immune globulin -- antibodies against hepatitis B virus. This treatment prevents the newborn from becoming a chronic carrier and developing serious liver disease in more than 90 percent of cases.

If you're in a high-risk group for hepatitis B infection, I recommend getting vaccinated against this virus. If you're planning on becoming pregnant, I recommend getting tested for chronic infection. And although routine screening is not generally done during pregnancy, this test may soon be added to the other preventive tests done at the first prenatal visit.

Follow-Up: Yawning

At the end of a recent column on yawning, I asked readers to send in their pet theories about this peculiar phenomenon. Here's what I heard.

Several readers said they yawned more with exercise, especially during aerobics classes.

Although yawning may have something to do with making your lungs work better, I really can't explain this reaction.

For one reader, a sudden string of yawns seemed to be the first clue to serious internal bleeding from an ulcer. She reasoned that yawning was her body's way of trying to get more oxygen into her bloodstream.

Another reader associated yawning with being hungry or having low blood sugar, since she could stop her yawns with some candy or gum.

Thinking in terms of evolution, a reader suggested that yawning served as a group signal among primitive man to bed down for the night.

Another felt that yawning served as a tension reliever, much like laughing or crying.

And puzzling over the "contagiousness" of yawning, a reader wondered why her dog's yawning made her yawn, but not the other way around.

And some readers said that just writing about yawning made them yawn the entire time.

Jay Siwek, a family physician from Georgetown University, practices at the Fort Lincoln Family Medicine Center and Providence Hospital in Northeast Washington.

Consultation is a health education column and is not a substitute for medical advice from your physician. Send questions to Consultation, Health Section, The Washington Post, 1150 15th St. NW, Washington, D.C. 20071. Questions cannot be answered individually.