Sterilization has become the most popular form of birth control among married couples in the United States. By the time their youngest child is 15, an estimated two thirds of white couples will opt for sterilization.

The popularity of this "permanent contraception" is related to growing disenchantment with other methods of contraception, says Dr. Susan G. Philliber of Columbia University, who with her husband, William, has studied the worldwide literature on attitudes toward sterilization. This is true, she says, "particularly among upper socioeconomic groups, who view it as a matter of convenience."

Consider the alternatives:

*The birth control pill is inadvisable for women past 35, and carries an increased risk of circulatory problems and liver disease.

*The intrauterine device, IUD, has been associated with an increased risk of pelvic infection and possibly pelvic inflammatory disease. The devices are increasingly being taken off the market.

*"Barrier methods" -- condoms, diaphragms, sponges and foam -- have a relatively high failure rate and, many couples believe, interfere with the spontaneity of lovemaking.

"When it came to contraception, we thought our only real choices were sterilization or a diaphragm," says a 38-year-old Washington man who had a vasectomy last July. "My wife didn't feel comfortable using the pill, the IUD had developed a horrible reputation by that time, and condoms were beyond the pale. Then when my wife started developing an allergy to the spermicidal jelly used with the diaphragm, we really didn't have any options."

Sterilization hasn't always been the first choice for couples who have finished their childbearing. In 1965, only 18 percent of such couples chose to be sterilized. That number doubled to 36 percent in 1973 and climbed to 62 percent in 1982.

Nearly 10 percent of the nation's adult population -- more than 16 million men and women -- had been sterilized as of 1983, the last year for which figures are available, according to the Association for Voluntary Surgical Contraception (AVSC). Another 500,000 men and 650,000 women undergo sterilization procedures each year.

Couples thinking about "permanent contraception" typically want to learn the pros and cons of the two procedures -- tubal ligation for women, vasectomy for men. And they often have trouble finding out how the two procedures compare, in terms of surgical risk and possible long-term complications.

All other things being equal, vasectomy is probably safer and easier than tubal ligation. "The data we have indicates that vasectomy is relatively risk-free," says Dr. D.J. Patenelli, a reproductive physiologist at the Center for Population Research at the National Institute of Child Health and Human Development (NICHD). "The initial risk of the procedure itself is clearly larger for women."

Vasectomy is a quick procedure, involving only local anesthesia and taking about 20 minutes. Tubal ligation, especially sophisticated "belly button" surgery -- performed through an incision in the navel -- usually requires general anesthesia and takes about 45 minutes.

With vasectomy, recovery is quicker, complications less common. No one has ever died during a vasectomy, whereas tubal ligation has a death rate of about three to four per 100,000, or an estimated 250 deaths since 1971. The risk of bleeding, hemorrhage, inadvertant burning of the bowel, or infection is higher with tubal ligation.

In addition, vasectomy tends to be slightly more reversible.

But all other things aren't equal. Women, perhaps more accustomed than men to bearing the burden of contraception, seem more willing than men to undergo the surgery involved in voluntary sterilization. As a result, female sterilization is currently leading male sterilization -- 60 percent to 40 percent -- among couples who choose permanent contraception.

In black couples, the figures are even more skewed toward women. Vasectomy is quite rare among black men, as is indicated by a 1982 survey of couples who used contraception. Among blacks, 28 percent relied on female sterilization, while only 2 percent relied on male sterilization. Among whites, 21 percent used female sterilization, 12 percent male.

"My husband was slightly uncomfortable about the whole idea. He had some feelings about a vasectomy being a threat to his masculinity," says Marcy Gustafson, 39, of Silver Spring. "And at the time we were considering the surgery, in 1982, there were enough mumblings about long-term problems for men that we decided I should be the one to get sterilized."

Possible long-term complications are the great unknown in the decision about sterilization. In the late 1970s and early 1980s, reports were published that vasectomized lab rats developed atherosclerosis at an alarming rate. But every study that has tried to reproduce these findings in humans, most recently a four-city study coordinated by Dr. Frank J. Massey Jr. of the University of California at Los Angeles, has found no connection between vasectomy and hardening of the arteries.

"The incidence of diseases," wrote Massey and his colleagues in the Journal of the American Medical Association, "is similiar for vasectomized men and their paired controls, or higher for the controls."

Possible long-term complications of female sterilization are now being uncovered. Some doctors have identified what they call a "post-tubal ligation syndrome," characterized by menstrual irregularities such as heavy bleeding, severe cramping and spotting between periods. There is some controversy as to whether these symptoms result from the tubal ligation or from such coincidental changes as going off the birth control pill. Because the pill tends to reduce bleeding during a woman's periods, she may find after surgery that her flow increases.

Recent reports have suggested a physiological explanation for post-ligation problems: levels of the female hormone progesterone are slightly reduced after sterilization, which could account for a change in bleeding patterns.

"Part of the controversy," notes Dr. Marvin C. Rulin of the University of Pittsburgh, "rests on the absence of a large prospective longitudinal study that analyzes a single sterilization technique, quantifies the alleged symptoms, takes into account preexisting pelvic pathologic conditions, and controls for the prior use . . . of oral contraceptives and IUDs."

The Center for Population Research at NICHD is coordinating studies of the long-term hazards of sterilization. Scientists are measuring changes in hormone levels in men and women who have been sterilized; measuring blood flow in sterilized women; studying whether sterilization increases a man's risk of becoming impotent or dying of prostate cancer; and examining whether sterilization increases a woman's risk of developing breast or cervical cancer or heart disease.

No problems have been confirmed by this research, which will not be completed for several years. "Right now," says center director Florence Haseltine, "we can't say that either procedure is particularly unsafe." The reproductive systems of men and women are similar in one respect: Each requires healthy tubes to carry the sperm or the egg from the site where it is produced to where the two will meet for fertilization. Surgical sterilization in either sex involves severing these important tubes.

In the male, sperm are manufactured in the two testes and carried up two tubes to the seminal vesicles, located just beneath the bladder. Each of these tubes is called a vas deferens, from the Latin for "vessel that carries away." Semen is produced in the seminal vesicles, and released, along with the sperm that the vas deferens helped deliver, when a male ejaculates. About 98 percent of a man's ejaculate is semen; just 2 percent is sperm.

A vasectomy involves cutting each vas deferens just above the testis and then sealing off each cut end by tying, clamping or cauterizing it. After the procedure, all the reproductive organs work as before. The testes continue to manufacture sperm; the seminal vesicles continue to manufacture semen; the man continues to achieve orgasms. The difference is that the semen is missing the 2 percent that had been sperm, because the sperm are never delivered to the seminal vesicles. Instead, the sperm stay in the lower stump of the vas deferens and eventually are absorbed by the body.

A man is still capable of impregnating a woman immediately after vasectomy, because sperm may still remain in the vas deferens above the cut. After a vasectomy, a couple is told to use another method of birth control for about 10 ejaculations, or until the man has brought in two successive semen samples that are sperm-free.

In a woman, eggs are released once a month from one of the two ovaries that lie above and on either side of the uterus. The egg travels down the fallopian tube, where it may meet with sperm from the male ejaculate. In this tube fertilization occurs. The tube opens into the uterus, into which the fertilized egg descends to be implanted.

A tubal ligation severs each fallopian tube about midway between the uterus and the ovaries. The surgeon reaches the tubes through one of several methods: either through a probe inserted through the rim of the navel, a procedure called a laparoscopy; through an incision in the abdomen, in the form of a laparotomy (usually performed during a Cesarean section) or a mini-laparotomy; or through the vagina, a technique called colpotomy.

Common jargon calls a tubal ligation "tying the tubes," but what really happens involves cutting and sealing. As in a vasectomy, the surgeon removes a few centimeters of tubing and seals the cut ends either with a clasp, with a clip, or through cauterization.

Despite its popularity, sterilization is still not the answer for everyone whose family is complete. Some people avoid sterilization because they want the option of changing their minds.

"It's just so permanent, so final," says Jeanette Brenner, 54, of Great Neck, N.Y. "After my third cesarean in 1962, my doctor was standing over me with my belly open saying, 'Jeanette, no more children.' And even though I couldn't have any more, do you know what I said? 'Don't you touch anything!' "

Others shun sterilization for religious or medical reasons. Women who have had other abdominal surgery, including cesarean sections, or who are extremely overweight may be advised against tubal ligation.

One critical factor in the decision, says sterilization expert Philliber, is whether the couple has a son. Education, occupation and income do not seem to make a difference; nor does the number of children a couple has had.

"Probaby more important than the number of children," she writes in her literature review, "is the presence of a son. Not a single study we reviewed found more than a few sterilizations among couples without a son."

Melissa and Steven Rose go against this trend. The Silver Spring couple, who asked that their real names not be used, plan to end their childbearing even though both their children are girls.

"I might have thought I'd prefer a boy," says Steven, 34, "but that was before my first daughter was born. Now I really don't care that I'll never have a son." Melissa, 32, says her ideal family has always been two girls.

The Roses say they cannot imagine wanting another child no matter what the circumstance, and they are not satisfied with other forms of contraception.

Now the Roses must decide which partner will be sterilized.

"We believe that whichever member of the couple is more firmly convinced that he or she doesn't ever want to have another pregnancy or cause another pregnancy is the one who should have the procedure done," says Miriam Ruben, public information manager of AVSC.

Ten or 15 years ago, more often than not it was the man who was sterilized. In 1971, for example, 72 percent of the sterilization procedures performed were on men. But that changed in the 1970s, when two events occurred simultaneously: tubal ligation became easier, and vasectomy came under new suspicion.

By 1982, the technique for tubal ligation had become so simple that most women could be sterilized and sent home the same day. And by then, concern had been voiced about the clinical significance of anti-sperm antibodies that could be detected in the bloodstreams of more than half of vasectomized men. The body forms these antibodies to fight off the circulating sperm cells, which are seen as foreign organisms. That year, the male-female figures for sterilization flip-flopped: 69 percent of the procedures done in 1982 were done on women.

The latest trend is toward a more even split between male and female sterilizations. In the past several years, the male-female ratio has been about 40:60. This has occurred largely because the scare about anti-sperm antibodies has passed. A series of large-scale studies of vasectomized men, some of them involving as many as 10,000 subjects, have turned up no adverse health effects even in the men in whom the antibodies are present.

The only documented health impact of anti-sperm antibodies, according to physiologist Patenelli, is that they could impair a man's fertility if he should decide to have his vasectomy reversed.

The NICHD, which has funded much of the post-vasectomy research, summarized its most recent findings in 1984: "The men with vasectomies had no significantly higher rates of any health problems than other men; the rates were either similar for both groups or lower for the men with vasectomies. This held true for several conditions that had been cited as possible complications of vasectomy, including hardening of the arteries, rheumatoid arthritis, blood clotting disorders, and gout."

It could be argued that from a psychological standpoint, it is more difficult for a woman than a man to terminate fertility. But for a woman who is sure she never wants another child, tubal ligation need not have any emotional impact.

"I never mourned the loss of my fertility," says Marcy Gustafson four years after her tubal ligation. "I was sure this was a good decision and the right step for us to take when we did.

"But I'm glad we waited to have our third child. When we had just two children, I had a lot of ambivalence about whether to have a third. If we had had the sterilization done at that time, I'm sure my ambivalence would have come out in other ways. But once we had our third child, the ambivalence cleared up completely."

Just as some women might mourn the loss of fertility, some men might be prone to fearing the loss of masculinity as a result of sterilization.

"The people who have a problem with vasectomies are those who associate masculinity with their ability to impregnate a woman," says Patenelli.

But not every man faces this issue. "I can honest to God say I never worried about any threat to my manhood," says a Washington father who had a vasectomy eight months ago, when his sons were 8 and 5. "I'm as vain as the next person, I take pride in my lower middle-class background, my credentials are well in order for being someone who worries about his masculinity. But it just didn't occur to me. Yes, someone is cutting into your testicles -- but he's not taking them off."

"We first started talking about sterilization when my youngest son was about 3," he says, "but that wasn't quite the time to do it. We waited until balancing the chances of ever wanting to have more children versus the very real costs of not getting the vasectomy tipped in favor of the operation."

Gustafson had a tubal ligation when her youngest was 6 months old. "We wanted to wait a while to make sure everything was all right with the baby," she says. "But I was very unhappy with the diaphragm, and I was unwilling to use the pill or IUD because of the risks involved. I was keen on getting rid of that whole birth control business."

This type of wait is apparently a good idea. Studies show that women who are sterilized immediately after giving birth are most likely to regret the decision.

Melissa and Steven Rose have waited two years after their second daughter's birth to begin talking about sterilization.

"This sounds terrible," Melissa says, "but we wanted to wait until they were both distinct enough personalities to us that we would never try to replace them if anything happened to them."

For his part, Steven wanted to see that both girls were developing normally and had passed what he considered the vulnerable years of infancy.

Not only does this delay make psychological sense; it actually makes medical sense.

"We know now that the failure rate is much higher if you do a tubal ligation at the time of delivery," says Haseltine of NICHD. "The blood supply is so much greater at delivery that the tubes have a greater chance of healing, of growing back together."

A generation ago, many physicians adhered to the so-called "rule of parity" in deciding whether they would perform a tubal ligation. The American College of Obstetricians and Gynecologists recommended an equation to determine whether a woman could be sterilized: if she was 25 years old, she needed to have five living children; if 30 years old, four children; if 35 years old, three. This formula was widely used as late as 1969.

But today, anyone can have the operation, even those who are single, young or childless. While some studies show that the younger sterilization patients are the ones most likely to change their minds and regret their decision, this may not always be the case.

"Highly motivated, young nonparents may be at less risk of later regret than an older parent, who is more in touch with what parenthood means emotionally," notes Betty Lanting Gonzales, deputy director of national programs at AVSC. Medicaid requires that individuals wait 30 days between the time they sign a consent form for sterilization and the time the procedure is done. This is in part to ensure that the patient won't regret the decision later. But private patients have no required waiting period, and even their spouse's consent is not required. Each year, an estimated 10,000 to 20,000 men and women attempt to reverse their "permanent" contraception. Sterilization reversal procedures, called tubal reanastamosis in women and vasovasostomy in men, are risky, expensive, and don't always work. Only about 30 to 50 percent of women and 50 to 60 percent of men who undergo reversal operations will actually be able to parent more children.

Most insurance companies will not pay for sterilization reversal procedures. The man's reversal can cost $5,000, the woman's $10,000 or more, and each requires major abdominal surgery under general anesthesia, plus a hospital stay of up to one week.

The desire for a reversal usually cannot be foreseen. The most common reason for requesting one is a change in marital status.

Jean Saxon, 40, of Glen Burnie had a tubal ligation in 1973, when she was 28 years old and the mother of four children, ages 7, 8, 9 and 10. Soon after the operation Saxon was divorced, and when she remarried in 1979 she wanted to have a family with her new husband.

"It didn't concern my husband at all that my tubes had been tied," Saxon says. "I told him before the wedding that we couldn't have children together, and he said that was fine. He already loved my four children. But once I got married, all those feelings about wanting a baby came back."

With her husband's encouragement, Saxon sought out Dr. Richard Falk, a Washington gynecologist who at the time performed just four or five tubal reanastamoses a year (his rate is now up to about 15 a year, with a pregnancy rate in these carefully screened women of about 70 percent). After doing a preliminary laparsoscopy to examine her tubes, Falk told Saxon she had a 50-50 chance of becoming pregnant again with the operation. The ligation reversal took five hours, and two months later Danielle Saxon was conceived. Danielle, who is 5, now has a 2-year-old sister, Lindsay.

"Once my babies were born, everything fell into place," Saxon says. Immediately after Lindsay's birth, Saxon had her tubes severed again on the delivery table. "Six children is definitely enough for me," she says.

Other situations also may lead to a change of heart about sterilization: the death of a child; an improvement in financial status; an unsuspected yearning to enlarge the family.

Vasectomies seem to be -- technically, at least -- more easily reversed than tubal ligations, says Patenelli of NICHD. It is a fairly simple procedure, she says, to rejoin the split ends of the vas deferens; in about 90 percent of reversal procedures, the tubes are successfully reconnected. But only about 35 to 45 percent of the men whose vasa are successfully rejoined are able subsequently to father a child.

The reason for this failure may be a change in the cells in the testes. In a study last year, Dr. Jonathan Jarow and his colleagues at the Johns Hopkins School of Medicine in Baltimore found that nearly one quarter of vasectomized men developed an abnormality called interstitial fibroses -- a change common in elderly men, but rare in the non-sterilized middle-aged. The majority also had other microscopic changes in the testes. These changes, which ordinarily cause no problems, could contribute to the low fertility rate in men after vasovasostomy.

In women, the reversibility of the sterilization procedure depends on how much of the fallopian tube has survived intact, according to Haseltine. If there are at least four centimeters of tube to work with, she says, pregnancy rates can approach 70 to 80 percent. When clips or bands are used to ligate the tubes, four centimeters usually remains; but when the ends have been burned, the remaining tube measures only two to three centimeters on average.

A woman who has had a tubal reanastamosis runs a risk of an ectopic pregnancy about 16 times the normal risk -- one in about 12 pregnancies will implant in the fallopian tubes and fail to descend into the uterus. Ectopic pregnancy is a life-threatening condition that can be treated only by removing the tube involved.

Success rates of 60 to 70 percent have been frequently cited for tubal reanastamosis, but these figures can be misleading. Miriam Ruben of AVSC points out that the rates are often calculated after the least promising surgical candidates already have been eliminated.

Considering all the pros and cons of each procedure, including the operation itself, the risks of complications, the failure rates, and the possibilities for reversal should the unforeseeable occur, Melissa and Steven Rose have put their sterilization on hold. Since Melissa is a nonsmoker, her gynecologist told her she may safely stay on the birth control pill at least until she is 35.

"I guess we're relieved not to have to take such a permanent step just yet," Melissa says. But in about a year, the Roses decided, it will be Steven's turn to play a role in the couple's contraceptive efforts. He plans to have a vasectomy next spring.