Women who test positive for antibodies to the virus that causes AIDS appear to be about eight times as likely to have twins as women who have not been infected, scientists have found. And often when twins are born to AIDS-infected mothers, one turns out to have the disease and one does not.

Neither of two AIDS experts who reported on the twinning phenomenon at a recent medical conference in Crystal City had an explanation for it. Nor could a third scientist explain how one twin could elude the virus.

The conference on sexually transmitted diseases and their effects on fetuses, held the week of Sept. 28, was arranged by the New York Academy of Sciences.

Dr. Louis Z. Cooper of the College of Physicians and Surgeons at Columbia University cited a study showing that about 8 percent of pregnant women in New York City who test positive for AIDS give birth to twins.

According to the American College of Obstetricians and Gynecologists, twins normally occur about once in every 100 deliveries among whites and about once in 79 among blacks -- about 1 percent nationwide.

Dr. Theresa A. Calvelli of the Albert Einstein College of Medicine said this same unexpected increase in twinning had been noted among AIDS-infected mothers in three areas of the Bronx, where the residents are predominantly black, with a large Hispanic minority and a small white one. Race did not seem to make any difference in the twinning rate if the woman was positive for AIDS, Calvelli said.

Women who shoot heroin and other intravenous drugs with shared needles are at extremely high risk of getting AIDS. So are women who are not drug users themselves but have sexual partners who are.

In the north and east Bronx, about 35 percent of women in those two risk groups carry evidence of AIDS infection, while in the South Bronx, a heavily Hispanic section of the city where shooting galleries are common and communal use of needles is widespread, screening at one methadone clinic revealed 80 percent of non-pregnant women to be infected with AIDS. Most showed no obvious evidence of the disease.

"This is a totally asymptomatic group, an invisible population," Calvelli said. "These were women who were perfectly healthy to all intents and purposes, who wouldn't have been seen by anybody if we hadn't gone in with a screening program."

The fact that many women of child-bearing age are unwittingly carrying human immunodeficiency virus (HIV), the AIDS virus, means that the present case load of children with AIDS is "just the tip of the iceberg," Cooper said.

Concern over AIDS transmission from mother to child is not restricted to doctors who practice in high-risk areas, Cooper indicated. He cited an AIDS-positive rate of about 5 percent in all maternity patients at St. Luke's-Roosevelt medical center in New York, compared with an estimated 1 percent in the U.S. population at large.

Since a child born to an AIDS-positive mother has about an even chance of being born with AIDS, the unexpectedly high rate of positives among pregnant women in New York "gives us every reason to have major anxiety for the future," Cooper said.

As of Sept. 28, the latest date for which figures were available last week, there had been 584 diagnosed AIDS cases in American children under 13 since 1981, according to the federal Centers for Disease Control in Atlanta. This is roughly 1.4 percent of the 42,354 cases diagnosed to date in people of all ages.

Put another way, there has been about one pediatric AIDS case for every 72 cases in teen-agers and adults nationwide. But in two pockets of heavy intravenous drug use -- the New York metropolitan area and southern Florida -- the ratio of children to adults is much higher. In New Jersey it is 1 to 31, Florida 1 to 42 and New York 1 to 56. By contrast, in California, where more AIDS victims are homosexual and bisexual males, the ratio is 1 child for 247 adults, and in the District of Columbia, 1 to 97.

In the District, there have been eight children under age 13 diagnosed with AIDS, and 843 other AIDS cases.

CDC figures show that of 584 children diagnosed with AIDS in the last six years, 381 are dead. This 65.2 percent death rate compares with 57.5 percent among older persons. Most of those diagnosed with AIDS die before reaching school age.

Although boys suffering from the inherited bleeding disease hemophilia have received a lot of notice in AIDS news coverage, this source of infection has never been a big one. CDC statistics show that only 32 out of 584 children (less than 5.5 percent) were hemophiliacs who became infected from blood products used to control their disease.

The most common category of pediatric AIDS -- almost four fifths of all cases diagnosed to date -- has been children who got it from their mothers, either in the womb or during birth. But some acquire the virus later, Calvelli said. One child in her practice was sexually abused by an infected adult; another became infected when its drug-abusing mother injected the child, the Bronx physician said.

Although the incubation period for AIDS is shorter in children than in adults, as is average time from diagnosis to death, pediatric AIDS should not be considered just a speeded-up version of adult AIDS, Calvelli said. Pneumocystis carinii pneumonia, the most common cause of AIDS deaths in adults, is also common in children, but Kaposi's sarcoma, the second most common adult involvement, is rare. Another pulmonary disease, lymphoid interstitial pneumonia (LIP), is very rare in adults but common in children.

Dr. Robert Parrott of Children's Hospital-National Medical Center here described LIP as a disorder that is not caused by a virus or germ but results when the patient's own lymphatic cells invade the chest cavity, causing what amounts to pneumonia.

Because of their weakened immune systems, children born with AIDS may not receive any vaccine made from live virus, making them easy targets for childhood diseases. Calvelli said four of her young patients have died of chicken pox.

In contrast to the progress of AIDS in adults, which may last for several years, the downhill course in children is rapid. Cooper said that if a child falls ill before its first birthday, its average life expectancy is four months; if the onset of illness is after 12 months of age, life expectancy is 19 months. Almost all the children known to have AIDS who are now alive were diagnosed in 1985 or later, whereas 20 percent of the adult AIDS patients diagnosed in 1984 are still alive.William Hines is a writer in Washington.