A Tennessee couple who haven't been able to have a baby and a Nashville medical professor who wants to help them yesterday urged the government to lift its moratorium on test-tube creation of human embryos.

This is "the only chance my husband and I have of having a child," pleaded 31-year-old Mrs. Dianne Grills.

She is one of scores of infertile American women who have told their doctors that they like Britain's Mrs. Lesley Brown - are eager to have a baby conceived in a laboratory.

James Gaither, chairman of a new federal advisory board named to consider the subject, said it may be next February before Dennis and Dianne Grills can expect any kind of answer.

Even then, he said at the first of two days of hearings, the 13-member Ethics Advisory Board that he heads will only make a recommendation to Health, Education and Welfare Secretary James A. Califano Jr.

The HEW secretary will make the final decision on ending what has been in effect a three-year ban on any federally funded efforts to join human sperm and human ova - female eggs - in the laboratory.

The ban applies whether the ova are fertilized for implantation of the resulting embryo in a prospective mother - as British doctors did for Mrs. Brown - or merely for observation and study of embryo growth.

Study is all that the Grills' medical counselor, Professor Pierre Soupart of Vanderbilt University, proposes to do until he feels resonably sure the resulting embryo will grow into a normal child. It is just this first step that he is asking HEW to finance at present. But he made it clear, in testimony at the National Institutes of Health yesterday, that he also wants to help Mrs. Grills and other like her have children as soon as possible.

"I could not conceive of a more ethical approach," he told the new advisory body as it began nationwide hearings and deliberations on the ethics of any test-tube conception and whether it would be safe for a child.

Soupart is only one of a number of American scientists who want to proceed with such research. But creating babies in the test has been attacked by some critics as a first step toward a future in which technology-mad governments could use such techniques for selective reproduction of submissive slaves or a superrace.

Test-tube fertilization also has been attacked because, in achieving it, doctors almost inevitably must discard some unsuitable embryos, even though they represent the beginning of life.

And, like Mrs. Brown's doctors, they would examine the baby during the pregnancy - 16 weeks or so after the three or four-day-old test-tube embryo is transferred to the mother - to seek any defects. If there were any, they would at least offer the mother the choice of abortion.

In a memorandum to the advisory group, Califano avoided such questions. But he ordered "the widest possible public involvement an debate," including "public hearings throughout the nation" on the many "serious moral and ethical questions."

"Will this research lead to selective breeding," he asked, "to attempts to control the genetic makeup of offspring or to the use of 'surrogate parents,' where . . . rich women might pay poor women to carry their children?"

Dennis Grills, a Dupont chemical enginner who lives in a Nashville suburb, told the board test-tube fertilization promises "untold benefit to mankind," providing not only babies for infertile women like his wife, but knowledge that could lead to elimination of many birth defects.

Dr. John Biggers of Harvard Medical School said 560,000 American women may have faults of the fallopian tubes that prevent ordinary conception, as do Mrs. Brown and Mrs. Grills. He said nature itself discards or kills one developing child in every two, in the embryonic or fetal stages, mainly because of genetic defects.

Dr. LeRoy Walters, director of the Center for Bioethics at Georgetown University's Kennedy Institute of Ethics, said he "leans toward" the view that an embryo need not in scientific fact be regarded as a "human subject" until it is about 14 days old, the age at which it stops floating freely in the womb or its approaches and becomes attached to the mother.

Dr. Joseph Shulman of NIH also urged such research, and human application as soon as possible, since this work "has been paralyzed in the United States for three to four years" and "every year of further delay . . . confines thousands of couples to permanent infertility."

Dr. Luigi Mastroianni of the University of Pennsylvania, however, urged much more research in animasls befor human application.

Professor John Gorby of John Marshall Law School in Chicago, maintained that discarding unsuitable embryos would be murder, and Ted Howard of the Peoples Business Commission, a Washington-based group, announced a national campaign to prohibit "any and all 'test-tube baby' experiementation."