In a White House ceremony in April, President Clinton vetoed a bill outlawing a technique of abortion done only in the second half of pregnancy. Termed "partial-birth abortion" by the people who decry it, and "intact dilation and evacuation" by the people who perform it, the technique has become the latest lightning rod in the nation's stormy debate about abortion.

Standing next to the president when he announced the veto were five women who had undergone late-term abortions with the controversial technique because their fetuses had severe developmental defects.

The women, Clinton said, "represent a small, but extremely vulnerable group. . . . They all desperately wanted their children. They didn't want abortions. They made agonizing decisions only when it became clear that their babies would not survive, their own lives, their health, and in some cases their capacity to have children in the future were in danger."

Others have sketched similar pictures. The Planned Parenthood Federation of America called this procedure "extremely rare and done only in cases when the woman's life is in danger or in cases of extreme fetal abnormality." The National Abortion Federation, an abortion providers' organization, said that "in the majority of cases" where it is used, there is a "severe fetal anomaly {birth defect}."

But it is not possible to speak with certainty about who undergoes "intact D & E," as the "partial-birth abortion" is known in medicine. The federal government does not collect such information. Physicians do not have to report it to state health departments. Researchers do not study the question or publish their findings in medical journals.

Interviews with doctors who use the procedure and public comments by others show that the situation is much more complex. These doctors say that while a significant number of their patients have late abortions for medical reasons, many others -- perhaps the majority -- do not. Often they are young or poor. Some are victims of rape or incest.

Physicians who perform abortions beyond the first third of pregnancy say that use of intact D&E is quite rare. Just over 1 percent (about 17,000) of all abortions in this country occur after the 20th week of fetal development; it is after that point when the intact D&E procedure is sometimes used. Only a fraction are believed to be intact D&Es, the controversial method in which the fetus is pulled by the feet out of the uterus and the head is punctured so it can also pass through the cervix. What's more, very few doctors perform this surgery; interviews with abortion experts suggest that there are less than 20.

What follows are sketches of the experience of several physicians who perform the intact D&E procedure, as well as the experience of doctors who perform abortions on patients with advanced pregnancies using an alternative technique. Taken as a group, the descriptions and observations by these practitioners paint a more complete picture of who decides to end their pregnancy at an advanced stage, and why. A Question of Safety

One of the better-known practitioners of intact D&E is Martin Haskell, an Ohio physician who in 1992 presented a "how-to" paper on the technique at a medical conference in Texas. The dissemination of this document to antiabortion activists set the stage for the current campaign to ban the technique.

Although Haskell declined to be interviewed for this article, in his 1992 paper he said he had performed "over 700 of these procedures." Three years ago, American Medical News, a weekly publication of the American Medical Association, interviewed Haskell about his technique.

"I'll be quite frank: most of my abortions are elective in that 20-24 week range," Haskell said, according to a transcript of the interview, which has circulated widely during the debate on the "partial-birth abortion" bill. "In my particular case, probably 20 percent {of the abortions} are for genetic reasons. And the other 80 percent are purely elective."

"Elective" is not a medical term generally used with abortion, but it is often used in medicine to denote procedures that are not medically required. In this context, it appears to mean that the fetuses were normal or that the pregnant woman was not seriously ill.

The American Medical News reporter also asked Haskell "whether or not the fetus was dead beforehand." The doctor answered: "No it's not. No it's really not. A percentage are for various numbers of reasons. . . . In my case, I would think probably about a third of those are definitely dead before I actually start to remove the fetus. And probably the other two-thirds are not."

Also performing intact D&E abortions in Ohio is a 45-year-old physician named Martin Ruddock. Interviewed recently, he declined to estimate how many abortions he did each year, but said that only 5 to 10 percent were done in the later stages of pregnancy. Beyond the 18th or 19th week, Ruddock prefers to use the intact D&E technique.

He believes it is safer than its most common alternative, which is called "dismemberment dilation and evacuation." In that procedure, the fetus is removed in pieces, generally limbs first. It requires that the surgeon exert a great deal of force on the fetus inside the uterus, and it often produces sharp, bony fragments that can damage a woman's reproductive organs. On rare occasions, "dismemberment D&E" also exposes a woman to fetal substances (primarily brain tissue) that can cause dangerous reactions.

"To minimize those problems is why the {intact} procedure was developed," Ruddock said.

In practice, however, he employs it only a third of the times he'd like to, he said. Often the position of the fetus, or some other variable, makes intact D&E impossible, and he uses dismemberment instead. However, whenever he uses the intact method, he first cuts the umbilical cord -- a maneuver designed to make sure the fetus is dead before he punctures its skull.

"The fundamental argument {of the technique's opponents} is that the fetus is alive. And what I am saying is that in my practice that never happens," he said.

In 45 percent of the cases done beyond beyond 20 weeks of gestation, he said, the fetuses have obvious developmental abnormalities or the women carrying them have illnesses that are being made worse by the pregnancy. In the other 55 percent, however, the fetuses are normal.

Another practitioner, who did not want to be identified, is a physician in the New York area who is affiliated with several teaching institutions. He does about 750 in the second trimester of pregnancy. He uses intact D&E in "well under a quarter" of those, he said. About one-third are his private patients, and the rest are ones he sees at the teaching hospitals, where he instructs physicians in training.

This doctor said that the "great majority" of the private patients have medical reasons for their abortions: Either the fetus is abnormal or the pregnant woman's health is threatened by the pregnancy.

The nonprivate patients, however, are different. They tend to have lower incomes, and the fraction of them who have medical reasons for abortion "is not nearly as high, {but} I can't quantify it," he said. In the cases in which there is no medical indication, the fetuses are usually normal. A California Doctor's Experience

The notion that intact D&E is done only in the third trimester -- very late in the pregnancy, generally after 24 weeks -- and only when the fetus has catastrophic defects, appears to have arisen from widespread publicity about the practice of a doctor in Los Angeles named James T. McMahon, who died last year. His specialty was the very late abortion of fetuses with severe developmental defects.

Patients came to him from across the United States and sometimes even from outside the country. All of the women who appeared with Clinton at the veto ceremony had their abortions done by him.

McMahon used intact D&E extensively because after about the 26th week of gestation dismemberment of fetuses is extremely difficult, if not impossible.

In a letter written in 1993 to doctors who referred patients to him, he said that in 1991 he'd done 65 third-trimester abortions. All of these cases, he said, were "nonelective." Of all the abortions done beyond 20 weeks, 80 percent were for what he termed "therapeutic indications" -- that is, medical reasons.

In documents submitted to the House subcommittee on the Constitution, McMahon provided a list of some of these reasons. He categorized 1,358 abortions he'd performed over the years, all of them done (his testimony suggested) on women at least 24 weeks pregnant.

Most of them were for extremely rare genetic defects. The list contained a few slightly more common conditions including anencephaly (lack of a brain) in 29 cases, spina bifida (open spinal column) in 28 cases and congenital heart disease in 31 cases. A few of the conditions on the list, however, are rarely fatal. Cleft lip, cited as the "indication" in 9 cases, is surgically correctable after birth, sometimes with permanent disability and sometimes without.

The maternal indications in McMahon's list were similarly varied. The severity of the illnesses can't be inferred, although many of the problems he gave are not commonly life-threatening. These included breathlessness on exertion, one case; electrolyte disturbance, one case; diabetes, five cases; and hyperemesis gravidarum (intractable vomiting during pregnancy), six cases. The two most common maternal indications were depression (39 cases) and sexual assault (19 cases).

Although the few other doctors who are known to use the intact D&E method refused to be interviewed, one overseas practitioner would. He is David Grundmann, a 49-year-old physician from Brisbane, Australia, who learned the technique from McMahon about five years ago during a visit to the United States.

Grundmann performs abortion up to 22 weeks of gestation and, like McMahon, treats patients who travel great distances for his services. He and his two partners do 60 to 100 intact D&E cases a year.

In an interview last week, he said that in about 15 percent of those cases, there is a severe defect of the fetus. In about 2 percent, the pregnant woman is severely ill, and in about 10 percent there are "serious concerns about suicide." In the rest, the reasons for abortion are rape, incest, a previous physician who failed to detect that the woman was pregnant or denial on the part of the woman that she was pregnant until it was too late for her to get an early-stage abortion. The Women Affected

It's difficult to say how representative these five doctors are of the rest of the small fraternity of practitioners who perform intact D&E in the United States. Interviews with physicians who use other abortion techniques -- generally dismemberment -- may help indirectly illuminate why most late-term abortions, including intact D&E abortions, are done.

Warren Hern, a 57-year-old physician who practices in Boulder, Colo., has a master's degree in public health and a doctorate in anthropology. He is one of the few providers of late-stage abortions who publishes research on the topic in medical journals.

Hern performs between 1,500 and 2,000 abortions a year. About 500 are on women 20 to 25 weeks pregnant. Of those, about one-quarter involve abnormal fetuses. He does between 10 and 25 abortions each year on women more than 26 weeks pregnant, and all of them involve fetal abnormalities or serious maternal disease, he said.

"It is true that a significant proportion of the community is offended by any abortion after 26 weeks that is not medically indicated," he said. "We practice medicine in a social context. So that is why I will not perform an abortion after 26 weeks just because a woman has decided she does not want to carry the pregnancy to term."

Women seeking an abortion late in pregnancy "are often young, frequently not married, and many have a child already, or more," said Steve Lichtenberg, a obstetrician-gynecologist in Chicago who does abortions up to 22 weeks of development. Many are poor, have not completed school or established themselves in the work force, he said, and are in excellent health.

"It's not uncommon for us to see several patients a week who give a history of rape or incest. The number who volunteer that information is substantially smaller than the number who've actually been subjected to social or sexual violence."

Herbert Wiskind is the administrator of the 19-bed Midtown Hospital in Atlanta, whose four doctors perform about 25 abortions a week on women at least 18 weeks pregnant. In his experience many of the late procedures occur simply because of denial.

"You have a young girl who becomes pregnant, someone 15 or 16 years old," he said. "She doesn't know how to tell her parents or her boyfriend. So she puts herself on a diet and tries to deny she's pregnant."

However, Wiskind said, some fetal defects aren't diagnosed until late in pregnancy for unavoidable reasons. Amniocentesis, one technique of fetal genetic screening, is done between weeks 15 and 17 of pregnancy. Several weeks can then pass before test results are known, and when they indicate a problem it often takes a woman several more weeks to decide about abortion, he said. In addition, many deformities can only be diagnosed through sonograms and aren't apparent until the midpoint of pregnancy or later.

Thomas J. Mullin does abortions through the 24th week of gestation, as calculated by sonographic measurement of the fetus's head. He practices in the New York area.

Of the procedures Mullin does in weeks 20 through 24, about one-third are for fetal abnormalities, he said. In about 10 percent of cases, the woman has an illness, such as severe diabetes or painful uterine fibroids, that is not necessarily life-threatening but is clearly made worse by pregnancy.

"The remainder of them are just errors," he said. "Many are young patients -- 12 to 20 years old -- who are not in touch with their reproductive system as well as they should be, so they get stuck later than they want in pregnancy. They get surprised, basically."

Jaroslav Hulka, a professor of obstetrics and gynecology at the University of North Carolina, supervises a teaching program whose physicians do 250 to 300 abortions a year on women carrying fetuses between 13 and 22 weeks old.

"Ninety-five percent of those are normal -- that's fair to say," he said. Occasionally, fetuses up to 24 weeks old are aborted if they have a condition incompatible with life. The physicians use the dismemberment technique -- an arduous and potentially risky procedure.

"The technique that the Congress is concerned about {intact D&E} is a level of skill above this," Hulka said. "They are doing what we're all supposed to do -- namely, minimize the risk to the patient."

Practitioners of the intact procedure argue that their method is the least traumatic among the many variants of dilation and evacuation abortions used and is not -- as their critics claim -- the most barbarous. In testimony submitted last year to a congressional subcommittee, the late James McMahon wrote:

"In a desired pregnancy, when the baby is damaged or the mother is at risk, the decision to abort may be intellectually obvious, but emotionally it is always a personal anguish of enormous proportions. . . . For the physician who is willing to help the patient in this dilemma, choices are few. Intact D&E can often be the best among a short list of difficult options. . . . Dealing with the tragic situations that I confront daily makes me constantly aware that I can only limit the hurt by doing gentle surgery and giving sympathetic counsel." Is the Fetus Capable of Feeling Pain During Abortion?

Does a fetus aborted late in the second trimester, or early in the third trimester, feel pain? It's a question that reverberates in the debate over so-called "partial-birth abortion."

"If you want to get at what I think the heart of this issue is . . . it's the sense that you're killing someone who knows what you're doing to them," said Steve Lichtenberg, a Chicago physician who performs late-term abortions. "That is one of the things that gives this particular controversy its heat."

When asked, virtually all physicians providing late-term abortions say they believe fetuses cannot feel pain at the most advanced stage of gestation at which they will do the procedure. In all but a few cases, that is no later than the 24th week out of a 38-week gestation.

The current knowledge of the anatomy and physiology of the fetal nervous system suggests this belief is probably -- but not definitely -- correct.

Scientists must deduce pain's presence (or absence) by looking for the physiological signs of the sensation. Those include hormones and other biochemicals that appear in the bloodstream when pain is produced, as well as more subjective signs, such as facial grimaces or the movement of limbs. Nobody can say for certain, however, whether these things denote pain in a developing human being.

Many neuroscientists (and nearly all abortion doctors) believe the muscular movements fetuses make when they're touched exist purely as reflexes. These events, they argue, are devoid of the brain activity and cognition necessary to register them as either pleasant or noxious. Some even claim that human beings must be capable of storing memories and having emotions in order to feel pain.

Nerve fibers must be "wired" in order to carry impulses. Without specific circuits, impulses cannot get to the cortex of the brain, which most scientists agree is the place where pain is perceived.

Sensory nerve endings have spread throughout the skin of the fetus by 20 weeks' gestation. A key part of the circuit -- connections between a structure in the base of the brain called the thalamus and the brain cortex -- doesn't start developing until the 22nd to 34th weeks. The definite "arrival" of sensory impulses in the cortex can't be detected by electrophysiological tests until about the 29th week of gestation.

Fetal nervous system development, however, is a process that involves quadrillions of cells growing, laying down new connections, or dying. It's simply not possible to know in any individual fetus when the capacity to feel pain begins.

A study published two years ago in the Lancet looked for signs of pain in 15 fetuses undergoing blood transfusion while still in the womb. Six had the blood delivered through a needle placed in the umbilical cord, which has no sensation. In nine the transfusion was given into a blood vessel in the abdomen -- an area that, by the time of birth at least, is well endowed with sensation.

Nicholas M. Fisk and his colleagues at Queen Charlotte's and Chelsea Hospital, in London, measured two substances in the fetal bloodstream just after the needles went in and then later, at the end of the transfusion. One was cortisol, a hormone produced when a person is in pain or under physiological stress. The other was beta-endorphin, the morphine-like substance the body makes as a built-in painkiller.

Bloodstream concentrations of both substances rose in the fetuses that had the needles through their abdomens, but not in those that got transfusions through the umbilical cord. Although the average gestational age of the first group was 30 weeks, one was only 23 weeks old, and it also had a significant rise in cortisol and beta-endorphin.

Still, doctors increasingly acknowledge that even very premature infants probably feel pain.

"I use anesthesia or analgesia for every premature infant I treat," said K.J.S. Anand, a pediatrician at Atlanta's Emory University School of Medicine and Eggleston Children's Hospital and a prominent researcher on the question of fetal pain. "I have treated many 26- to 28-weekers. They react to pain and I do believe that they feel pain. This is a viewpoint that may not be shared by other people."

Some doctors who perform late-term abortions believe that the pain issue is irrelevant because the anesthesia given the woman will also anesthetize the infant. In most cases these patients are given intravenous sedatives, barbiturates or potent opioid pain killers.

Anesthesiologists, however, dispute this claim.

"If you are using those drugs appropriately, then it has little or no effect on the fetus," said David Birnbach, president of the Society for Obstetric Anesthesia and Perinatology. "From a clinical point of view you can't depend on the fetus being asleep."

Many anesthetic drugs get across the "placental barrier" and into the fetus but only after a delay and usually with less dramatic effects than they produce in the pregnant woman.

Mark Rockoff, vice-chairman of anesthesiology at Children's Hospital, in Boston, said he "would have grave concerns that if the operations are very fast that, indeed, the infants may not be adequately anesthetized."

As physicians perform more procedures on unborn infants, the subject of fetal pain sensation is certain to become more a practical, and less a philosophical, issue.

The authors of the Lancet article noted this, and suggested that doctors consider giving "adequate analgesia {pain medication}" to fetuses just as they would to newborns. They concluded their article with a stunning, though inevitable, observation:

"This applies not just to diagnostic and therapeutic procedures on the fetus, but possibly also to termination of pregnancy, especially by surgical techniques involving dismemberment." Viability and the Law

The normal length of human gestation is 266 days, or 38 weeks. This is roughly 40 weeks from a woman's last menstrual period.

Pregnancy is often divided into three parts, or "trimesters." Both legally and medically, however, this division has little meaning. For one thing, there is little precise agreement about when one trimester ends and another begins. Some authorities describe the first trimester as going through the end of the 12th week of gestation. Others say the 13th week. Often the third trimester is defined as beginning after 24 weeks of fetal development.

Nevertheless, the trimester concept -- and particularly the division between the second and third ones -- commonly arises in discussion of late-stage abortion.

Contrary to a widely held public impression, third-trimester abortion is not outlawed in the United States. The landmark Supreme Court decisions Roe v. Wade and Doe v. Bolton, decided together in 1973, permit abortion on demand up until the time of fetal "viability." After that point, states can limit a woman's access to abortion. The court did not specify when viability begins.

In Doe v. Bolton the court ruled that abortion could be performed after fetal viability if the operating physician judged the procedure necessary to protect the life or health of the woman. "Health" was broadly defined.

"Medical judgment may be exercised in the light of all factors -- physical, emotional, psychological, familial and the woman's age -- relevant to the well-being of the patient," the court wrote. "All these factors may relate to health. This allows the attending physician the room he needs to make his best medical judgment."

Because of this definition, life-threatening conditions need not exist in order for a woman to get a third-trimester abortion.

For most of the century, however, viability was confined to the third trimester because neonatal intensive-care medicine was unable to keep fetuses younger than that alive. This is no longer the case.

In an article published in the journal Pediatrics in 1991, physicians reported the experience of 1,765 infants born with a very low birth weight at seven hospitals. About 20 percent of those babies were considered to be at 25 weeks' gestation or less. Of those that had completed 23 weeks' development, 23 percent survived. At 24 weeks, 34 percent survived. None of those infants was yet in the third trimester. Methods of Abortion

About 90 percent of the 1.3 million abortions performed in the United States each year occur in the first 12 weeks of fetal gestation. At that stage, the procedure is extremely safe and quite simple. A small suction catheter is put into the vagina and then inserted into the opening of the uterus, called the cervix. The embryo or fetus, which is generally less than three inches long, is "aspirated" out.

Abortions done after the 20th week -- roughly the midpoint of pregnancy -- are another matter. They are riskier and more demanding, both technically and emotionally.

At this relatively advanced stage, there are two general strategies for terminating pregnancy. One is to induce labor, often by injecting a toxic substance into the fetus with a needle inserted through the woman's abdomen. This method has fallen out of favor in the past 15 years because it requires the patient to undergo labor over several days -- often alone, in pain on an obstetrical ward -- before she delivers a dead fetus.

More common now are techniques in which a physician removes the fetus -- which at week 20 is about nine inches long -- using surgical instruments while the woman is heavily sedated or anesthetized.

Although the actual procedure often takes less than a half hour, several days are required to prepare a woman's reproductive tract for it. Specifically, small sticks of highly absorbent material are put into the cervix. As they pick up fluid and swell, they slowly open the cervix until it is large enough for the physician to insert instruments. However, the opening is never as large as it is at the time of delivery, and not even large enough for the half developed fetus to get through.

Most abortion doctors circumvent this problem by dismembering the fetus and removing it in pieces small enough to pass through the cervix. This is obviously a fatal procedure, although in practice the fetus is often dead by the time it occurs.

The physician generally injects the fetus with one or more toxic substances a day before surgery, a maneuver that softens the tissue and makes dismemberment easier. It also eliminates any possibility a live birth will occur. Alternatively, some doctors cut the umbilical cord, which kills the fetus, 15 or 20 minutes before the procedure.

A few providers of late-stage abortions, however, use a different strategy. They remove the fetus intact. This requires less physical force than dismemberment and overall may be less risky to the patient, although this has not been proved because no studies have compared the intact method to the dismemberment one.

Practitioners of the intact technique make efforts to open the cervix wider than usual, but it's never wide enough to accommodate the fetal head. So the doctors deliver the fetus feet-first until only the head remains inside the uterus. The doctor then makes a hole in the base of the fetal skull and removes the brain and spinal fluid with a suction hose. Because the bones of the skull have not yet fused to form a solid structure, the empty skull can then be flattened and brought through the cervix.

Usually the fetus is dead when the doctor does this "decompression." If not, this maneuver obviously kills it. In the latter case, the precise moment and cause of fetal death is known -- something that's not true with almost every other method of ending pregnancy and one reason that the method has raised such unusually strong emotions. STAGES OF FETAL DEVELOPMENT 1 week Sperm fertilizes egg in the Fallopian tube 2 weeks The fertilized egg passes into the uterus and burrows into the lining 4 weeks Contractions of heart begin, early brain chambers form: most malformations begin during this critical period 8 weeks Nearly all body organs have begun to develop. 12 weeks Ears are now on sides of head; eyes move from sides to front of head 14 weeks Fetus has fine hair on head; umbilical cord is attached to abdomen; fingernails are well formed 28 weeks Fetus can survive outside the womb; weighs about 3 lbs. SOURCE: KNIGHT RIDDER GRAPHICS CAPTION: Pregnancy usually lasts 38 weeks. In the first 28 weeks the embryo grows from a cluster of microscopic cells into a three-pound fetus.