There are few things in this world more agonizing than wanting to have children--and not being able to.

"Don't talk to me about population control," snaps a young publicist who delayed her pregnancy to establish her career and who now has been unable to conceive.

"Believe me," she sighs, "when you're 24 or 25 it never occurs to you you won't be able to get pregnant when you want . . . you just don't think about that."

She has been thinking about little else for about three years now, but she is optimistic that after a laparascopic diagnosis and some microsurgery on her tubes, her chances have improved.

She is nearly 35 and she feels that time is working against her, but, the experts are saying, maybe not.

There is an explosion of new knowledge not only about what causes infertility, but about remedies--from something as simple as relaxation techniques to sophisticated hormonal manipulation, microsurgery and, as a last step, in vitro fertilization (IVF). These, along with genetic testing, "give women the election of having children even in their middle 50s," says infertility expert Dr. John L. Young, "with no more risk of having abnormalities than younger women, assuming she undergoes all the available testing."

(Now such age-related problems as Down's syndrome are identifiable through amniocentesis, and the potential for neural-tube defects can be screened by a series of blood tests with amniocentesis needed for only a tiny percentage.)

Most fertility experts agree that more couples than ever are seeking help in starting families--up to nearly 6 million married couples in this country, about 15 percent, are unable to conceive naturally--and that today's life styles account for at least some of the problems. Fertility in women begins to wane after 30 and in men after 40. So just postponing pregnancy can have some effect.

Endometriosis, a disease in which tissue from the uterine cavity implants in other parts of the female reproductive system, is one of the major causes of infertility--although only about 10 percent of those who have it become infertile as a result. It seems to be on the increase among young women. However, according to Dr. Richard Falk, chief of the reproductive endocrinology divisions at both Georgetown University Medical Center and Columbia Hospital for Women (a joint fertility program), it simply may be that it is being more easily diagnosed.

According to Falk, most infertility experts see tubal problems as the top cause of infertility in women along with ovulation problems. There are also more problems with male fertility than had been previously recognized. "Contrary to what used to be accepted gynecological dogma," says Young, "infertility is caused almost as much by male as by female problems." Falk says it has been his experience that male problems account for about 30 percent of infertility problems. Often there is a mixture of male and female factors. (Experts disagree on which problems rank in what order, all conceding there are vast areas yet to be explored.)

However, it is known that sperm are exquisitely sensitive to temperature, for example, and even the presence of a varicose vein in the scrotum--a not uncommon occurrence--can raise the temperature enough to damage the sperm or lower the count. (This can be surgically corrected.) Sperm levels--20 million per ejaculate is considered sub-fertile--also can be affected by smoking, alcohol, tension, even poor diet and fatigue. When semen samples are examined, the specialist looks for fast swimmers that look like they know where they're going. In collecting sperm for artificial insemination or IVF, in fact, the technique resembles nothing so much as a swimming contest.

Falk also notes that studies suggest that the average sperm count is dropping in this country, although no one can say for sure what impact that has on fertility, or why it is happening.

And it does not help that human reproduction is not particularly efficient to begin with. If the woman's cervical mucous is too thick or too watery, or if its temperature is too hot or too cold, even a healthy and determined-looking sperm cannot make it to the egg. The miracle, it is said, is that as many babies get made as are.

Infertility tests for women are mainly non-invasive in the early stages. The physician checks for proper ovulation, proper responsiveness of her uterine lining or for blocked fallopian tubes. Ovulation can be stimulated or regularized through the proper use of some new drugs, like Clomid or Pergonal, but the patient ought to be monitored constantly. It is not enough, specialists emphasize, to just give her some pills and send her away. (Both these drugs increase the likelihood of multiple births, although Pergonal is more likely to do so.)

After an elaborate radiological procedure called a hysterosalpingogram, there may be a laparoscopy, minor surgery to diagnose and possibly treat tubal blocking caused by previous infections or endometriosis. A laparoscopy will also determine if the woman's ovaries may be reached surgically to remove an egg if she is to become a candidate for an in vitro program.

Male problems sometimes can be treated with drugs or surgery. Occasionally the husband's sperm is incompatible with the wife's cervical mucous, and sometimes this, too, can be adjusted hormonally. Some new artifical insemination techniques may be indicated in which a concentrated amount of the best sperm are put directly into the uterine cavity. As the final resort for the parents to have their own baby, there is the use, where it is available, of in vitro fertilization--surgically removing an egg at the time of ovulation, fertilizing it with the husband's sperm in a laboratory and re-implanting it into the mother's uterus.

Young, a board-certified obstetrician-gynecologist and his partner, Mark R. Geier, a physician with a Ph.D. in genetics, and ob-gyn training, have just begun to accept patients in their suburban in vitro fertilization program, which they call MAP, for "medically assisted pregnancy." Sophisticated fertility centers--without IVF programs--exist at George Washington Medical Center, Columbia Hospital for Women (in conjunction with Georgetown University) and at Johns Hopkins in Baltimore. Although they and several other facilities are planning IVF programs, they are still several months away. The MAP program in Bethesda is the only one currently doing in vitro fertilizations in the area and is believed to be the only non-hospital private program operating anywhere. According to Falk, Columbia's unit has just had a $500,000 budget approved for its forthcoming IVF unit. There already are 2,000 active patients at the Columbia center and there is a wait of about 4 months for a consultation. Falk believes that the first candidates for IVF there will be drawn from their current patient population.

Couples eligible for Young and Geier's MAP program for in vitro fertilization must be healthy, stable and married. They should have tried, and not responded to treatment for any known obstacles to their fertility. So far, Young and Geier have implanted fertilized eggs in two patients and currently are awaiting results. It usually takes several tries before there is a pregnancy.

The experience in other IVF programs has taught the experts that sperm may be handled more safely and easily than had been feared at first. And new techniques already can make some distinctions between sperm with a male's Y chromosome or a female's X. (The extra leg on the X makes the female sperm minutely heavier and slower.)

Says Young, "The technology of choosing the sex is nearly here. Already, there are some little things you can do about timing . . . We're interested in it, not because you want to let people choose necessarily, but because of the X-linked disorders (like hemophilia or Duchenne muscular dystrophy). These are carried by the female, but only affect the male. No one would want to abort all boy babies, of course, because half would be normal, but if you could guarantee a female, some might choose to do so."

For free brochures on the MAP program or on genetic counseling, phone 530-6900 or write Genetic Consultants, 5616 Shields Dr. Bethesda 20817.