It is an uncomfortable question that, in today's world, is often asked by expectant mothers who had more than one male partner at the time they became pregnant. Who is the father?
With more than half of births to women under 30 now out of wedlock, it is a question that may arise more often.
Now, blood tests are becoming available that can determine paternity as early as the eighth or ninth week of pregnancy, without an invasive procedure that could cause a miscarriage.
Besides relieving anxiety, the test results might allow women to terminate a pregnancy if the preferred man is not the father -- or to continue it if he is. Men who clearly know they are the father might be more willing to support the woman financially and emotionally during the pregnancy, which some studies suggest might lead to healthier babies.
And if the tests gain legal acceptance, some lawyers say, women and state governments one day might pursue child support payments without having to wait until the birth. Under current law, "until and unless the pregnancy produces a child, any costs associated with it are regarded as the woman's personal problem," said Shari Motro, a University of Richmond law professor.
The testing itself can be awkward, however, because at least one of the possible fathers must contribute a blood sample.
Courtney Herndon, after breaking up with her boyfriend, had a brief relationship with a man whom she regarded more as a friend. She found herself pregnant at age 19, without knowing which man was the father.
The friend also wanted to know, so he agreed to the testing. He turned out to be the father, and the two agreed on child support even before the baby was born.
"I got the test done and was able to go on with my life," said Ms. Herndon of Fort Polk, La.
Estimates of the extent of paternal uncertainty vary. Studies have found a discrepancy rate -- when the presumed father is not the biological father -- of anywhere from 0.8 percent to 30 percent, with the median being 3.7 percent, according to one review of such studies.
Another study found that about 9 percent of birth certificates in Florida, even excluding births to teenage mothers, did not list the full names of the father, although it was not clear how much of this was related to uncertainty. Infant mortality was higher in those cases than if the father's name was on the birth certificate.
It has already been possible to determine paternity during pregnancy using amniocentesis or chorionic villus sampling, the same medical procedures used to test a fetus for Down syndrome. But those methods are invasive and carry a small risk of inducing a miscarriage, so they are rarely used for paternity testing.
By contrast, the new tests require only blood samples from the pregnant woman and the potential father. And doctors generally do not have to be involved.
That could vastly expand testing, said Sara Katsanis of Duke University's Institute for Genome Sciences and Policy. She is planning a study with one of the testing companies to see if prenatal paternity testing can reduce a pregnant woman's stress.
Some noninvasive paternity tests have been offered over the Internet for about a decade, and there have been various complaints about inaccurate or even fraudulent results.
But experts say the technology has advanced to the point that such testing can now be done reliably. A brief paper describing one such test, developed by a company called Ravgen, was published recently in the New England Journal of Medicine.
"I have no doubt that these tests will work clinically," said Mark I. Evans, a Mount Sinai School of Medicine professor and director of Comprehensive Genetics, a New York City medical practice that specializes in prenatal testing.
The tests analyze fragments of DNA from the fetus that are present in the mother's blood in tiny amounts. The same approach is now also being used to noninvasively determine the gender of the fetus or whether it has Down syndrome. And last week, researchers demonstrated that they could even determine a fetus's entire genome this way.
Ravgen, a small company in Columbia, Md., has been offering its test on a limited basis and charges $950 to $1,650, depending on the circumstances, said chief executive Ravinder Dhallan.
Another test, developed by a Silicon Valley firm called Natera, is marketed by DNA Diagnostics Center, a leading provider of conventional paternity tests. Thousands of the prenatal tests have been ordered since going on sale in August, executives say. The price is $1,775, compared with around $500 for a conventional postbirth paternity test.