Dr. Frederick Mandell, Harvard pediatrician, listened recently to a woman recount her own experience with sudden infant death syndrome (SIDS). "She told me how she and her husband were sitting in the emergency room waiting to hear what happened to their child, and she told me how the nurse kept looking at them, slyly, with this innuendo of blame that they had done something wrong," Mandell said. "You could feel the pain.

"And you know what? It happened 25 years ago."

The still mysterious nature of the syndrome, which takes more than 7,000 tiny lives each year in this country, means that the suggestion of blame still exists -- often adding an agonizing burden to a couple whose lives have just been shattered.

"As recently as October, a couple whose child died of SIDS in Montana was initially charged with murder and exonerated only after a full investigation," Carrie Sheehan, Western Regional Director of the National SIDS Foundation, testified at a congressional hearing last week.

"This happens over and over again," says Mandell. "I remember very well a father who woke up early to go to work in the morning and went in to check his young son and found him dead. He called police to come help resuscitate the child and when they came in, the first thing they did was read him his rights."

Then there is the self-blame.

"There isn't a family who isn't left with an 'if only,' " says Mandell, who is vice chairman of the National SIDS Foundation. " 'If only I hadn't gone out for that walk. If only I hadn't left him with the sitter. If only we'd stayed home.' As if anything they might have done or not done could have averted the tragedy.

"SIDS kills families. These are often very young families who have never experienced death before, and now they experience the harshest kind of grief reaction. It never ends. It doesn't go away."

Once known as "crib death," SIDS was seen historically as someone's fault. As Dr. Charlotte S. Catz of the National Institute of Child Health and Human Development (NICHD) said at last week's hearings, "So did the Bible 2,000 years ago in the passage: '. . . and this woman's child died in the night because she overlaid it.' "

SIDS specialists and researchers, and the National SIDS Foundation, formed in 1962 by grieving SIDS parents, now know that "overlaying" has almost nothing to do with SIDS.

Accelerated research over the past decade has teased out some problems that may cause a few cases of SIDS -- problems like infant botulism or sleep apnea -- but there is still no list of symptoms for the clinician to check-off to determine if a baby might simply die one night in the crib. Most SIDS babies apppear healthy and only in retrospective studies of large numbers do seemingly minor variations from normal -- rapid heartbeat and respiration at birth, for example -- take on significance.

However, new research, combined with emerging results from the largest epidemiological study ever undertaken of SIDS babies, their families and matched sets of control babies, is identifying risk factors and some physiological differences between SIDS babies and other infants.

The NICHD Cooperative Epidemiological Study of SIDS Risk Factors was begun in 1979 and involved pathology studies of more than 800 SIDS infants as well as interviews with their families and with the families of about 1,600 other babies. The final results will not be published until next year, but so far experts have been able to identify as high risk babies who are male, black, of subnormal birth weight, in low socio-economic groups, and siblings of SIDS babies. Women at risk for having SIDS babies were teen-agers or smokers.

Ninety percent of the SIDS deaths occurred before the child was 6 months old, and 98 percent before their first birthday. The peak incidence was between the second and fourth months. More SIDS deaths occurred during the cold weather months, and the peak time was between midnight and 8 a.m.

Studies cited by SIDS Foundation president, Dr. Marie A. Valdes-Dapena, are beginning to detect brain stem and liver abnormalities in SIDS infants, but, she notes, "it is too early to tell if they are cause or effect."

Other studies, along with the NICHD survey, have already ruled out some popular hypotheses about what causes SIDS -- among them use of immunizations. The survey showed that many more of the children who did not die of SIDS had received DPT shots than had the SIDS infants.

The increasingly widespread use of electronic monitors designed to alert parents if the baby stops breathing is of unproven worth, the congressional panel was told, because, said Mandell, "we're not sure exactly who to monitor. We're not sure exactly who's at risk and we're not sure they actually help the ones who are at risk."

According to all the witnesses at last week's hearings, including parents of SIDS babies whose stories were uniformly wrenching, the tragedy they have already suffered is being compounded. Just as research is beginning to hint at answers, just as family counseling centers were beginning to offer specialized support to bewildered and hurting young families, funds for SIDS research and support have been slashed -- from a peak of about $3 million in 1982, to only $600,000 last year. Witnesses testified that the switch to block grant financing saw the decimation of support programs barely a year after a report from NICHD declared:

"No systematic studies have yet been undertaken concerning the impact of social, cultural and sexual factors on the grieving process or on coping with the loss of a family member who dies suddenly and unexpectedly from no apparent cause. Cross-cultural studies might sugget ways of making SIDS a less lonely and traumatic experience through more supportive community attitudes and interventions."

"Can't you help me," pleaded one SIDS mother, "just tell me why my baby had to die."