Test-tube babies are now being produced in large numbers by the innovative British doctor who pioneered the effort, which is good news for couples who can't conceive normally.

Dr. Patrick Steptoe, who achieved the first successful artificial conceptions, has told American doctors that 48 women are now pregnant, carrying babies conceived in his laboratory.

He credits major improvements in the methods that produced the first two such infants in England three years ago. The advances promise to make artificial impregnations, which heretofore have been a chancy process, routine.

"This news is revolutionary--numbers like these offer hope to a large number of couples who can't have babies by other ways," said Dr. John Marlow of Washington's Columbia Hospital for Women, where Steptoe spoke recently.

In all the world probably fewer than 20 such babies have been born. This includes eight Steptoe patients and nine more in Melbourne, Australia, where an additional nine women are now pregnant. About a half dozen more are pregnant after laboratory conceptions at the Eastern Virginia Medical Center in Norfolk, this country's first such clinic. One is due to give birth sometime during the holidays.

Among the newly pregnant Britons is Mrs. Lesley Brown, who gave birth to the first test-tube baby on July 25, 1978.

The term "test-tube baby" is something of a misnomer. The baby is conceived in a small laboratory dish, not a test-tube. And the developing embryo, the union of the mother's egg cell and the father's sperm, is soon transferred to the mother's womb to be carried and born.

Before the birth of the Brown baby, Steptoe and collaborator Robert Edwards failed to achieve implantation in the womb in 80 patients.

But as of Oct. 31, Steptoe told doctors at Columbia Hospital and at the American Association of Gynecological Laparoscopists convention in Phoenix, he had successfully fertilized 338 eggs in 436 attempts.

He attempted implantation in 337 cases and made 74 women temporarily pregnant until the new embryos were rejected. But as of Oct. 31, eight babies had been born, 48 women were still pregnant and Steptoe was continuing to transfer fertilized eggs into many of the 1,000 women on his waiting list.

"In short," said Dr. Marlow, a friend of Steptoe's, "77 percent of all egg cells obtained were fertilized, and 21 percent successfully implanted themselves. Of these, the loss during pregnancy those lost or aborted was only around 16 percent. More embryos and fetuses are lost in natural pregnancy!"

To create a test-tube baby, a doctor first takes an ovum or egg cell from an ovary of a woman who is infertile because of blocked, damaged or missing Fallopian tubes, the tubes through which a fertilized egg moves to the womb, or one whose husband produces too few sperm for ordinary conception.

To get the egg, Steptoe inserts a laparoscope, a slim foot-and-a-half tube, in effect a skinny telescope, into the woman's abdomen, entering through the navel. On finding the ripening egg cell, he removes it with a fine suction needle.

He joins the egg with the husband's sperm in the laboratory. Within hours, egg and sperm marry and begin to multiply to become a collection of growing cells. Within a day or two, this new incipient being is placed in the woman's womb with a long, flexible catheter.

All these things were being done in 1978. But there have been important refinements at Steptoe's new laboratory at Bourn, England, near Cambridge:

* There is hour-by-hour monitoring of the woman's hormonal condition so the doctors can retrieve her ripening egg just when it is "ready."

* The husband's sperm cells are tested and washed to reduce the chance of infection, and used in a precise quantity to try to make sure one cell successfully penetrates the waiting egg.

* There is careful control of the growing embryo's laboratory environment: the kind of solution it grows in and artificial air in its container.

* The embryo is usually placed in the mother's uterus in late afternoon or early evening, often around 5:30 p.m. Steptoe has found that the womb is generally most receptive then.

Other factors, according to Marlow, include the quiet, stress-free site of Steptoe's present work, its easy access to scientific collaborators in Cambridge and the fact that he can give his full time. A former government health service gynecologist, Steptoe retired and last year opened his own clinic at a country estate, Bourn Hall.

Some scientists think human test-tube conception should wait until there has been more animal research to make sure there won't be too many birth defects. Some doctors have tried to make the technique work in their patients, without success.

American doctors in general have been slow to move, for fear of being labeled unethical. The Norfolk effort was bitterly attacked by some religious leaders as a rash effort to manipulate life.