In ancient Egypt, women mixed their urine with wheat seeds. If the plant grew upright, the legend went, she would bear a son, if it leaned she would have a girl and if the seed failed to sprout, she wasn't pregnant.

Hardly scientific, but the Egyptians had the right idea. A pregnant woman's urine contains a special hormone, HCG (human chorionic gonadotropin). Modern lab tests rely on the urine's interaction with animal antiserum and hormones to determine pregnancy.

Last year, drug companies began touting an in-home, miniaturized version of this test that sells for about $10. TV and magazine advertisements assure women that they can now answer "the most important question a woman can ask herself . . . in the privacy of your own home . . . in just a few hours."

What they don't mention is that "the at-home test is only a method of making a preliminary diagnosis, instructions must be followed strictly and false readings may create or disguise other medical problems," says Jacqueline Maio of the Food and Drug Administration.

Sold over the counter in drug stores, the in-home test has been available in Europe for years, according to Dr. Arthur Flanagan, vice president of medical affairs for Warner/Chilcott, which last year introduced this country's first such test kit, E.P.T. (Early Pregnancy Test).

When used by "typical consumers" E.P.T. results in 97 percent accuracy on positive readings and 80 percent accuracy in negative readings, Dr. Flanagan claims. Women who get a negative reading, however, and do not menstruate within a week are instructed to buy another kit and repeat the test, after which the negative accuracy rate goes up to 91 percent.

If a woman gets a positive result she is urged to consult her doctor for proper medical care. If she gets two negative results, she is instructed to consult her physician for advice on why she isn't menstruating. So why sink $10 or $20 into a kit if you have to see a doctor anyway?

"The most important benefit of the home pregnancy test is for the large segment of women who, for whatever reason, would like to do her own test and be knowledgeable of her condition first before she decides to share it," says Dr. Flanagan. "It has sold very well, attesting to a large interest and demand. But it is not a substitute for good medical care and for people who want the security of having a doctor perform the test."

Some health-are professionals applaud the test for simply putting women in touch with their bodies.

"I believe home pregnancy tests help de-mythify medicine by emphasizing self-responsibility," says community health specialist Mina Bender, a nurse who runs two clinics in New York. "People should realize that they don't have to be geniuses to know about medicine and how their bodies work."

People who are too scared or embarrassed to tote a urine sample to a doctor or clinic may test themselves, said Ilene Wolcott, project director of the Washington-based Women and Health Roundtable.

"But the privacy of an in-home test can be counterproductive," added Wolcott, who noted that pregnancy tests are free at most abortion clinics and $5 at Planned Parenthood. "The people most likely to utilize these tests are teen-agers who may be fearful of going to a physician, and these same people have the greatest need of counseling that goes along with tests done in clinics. Ads always show women thrilled at being pregnant, while the whole population out there may not be thrilled."

"We neither condemn nor applaud the tests," says Dr. Ervin Nichols, director of practice activities for the American College of Obstetrics and Gynecology. "There is no harm in the patient doing the test, but we do have concern for false results."

In the case of a false positive a woman might run to a storefront abortionist, says Dr. Nichols. A false negative result could hinder early detection of an ectopic (tubal) pregnancy which, if untreated, could result in death.

"Results of this type of (in-home) test can be easily, though inadvertently, misinterpreted or botched," notes FDA spokeswoman Maio. A residue of detergent or dirt in the container, slight movement (such as a refrigerator hum) or reading the test too early or too late may alter results.

"Though products stress early diagnosis (7 to 9 days after a missed period), product-accuracy claims are actually based on tests run 15 or so days after a missed period," she says. "While an at-home pregnancy test can give an accurate diagnosis, consultation with a doctor is essential to assure proper medical care."