Regina Twigg, 46, sat in a laboratory office at Johns Hopkins Hospital several weeks ago, dazed, listening to a woman in a white coat. There was no mistake, the genetics specialist said. The 9-year-old daughter whom Regina and her husband, Ernest, had nursed and ached for and who now was dying, was not their own.

There were diagrams on the lab blackboard showing 25 protein-matching comparisons that proved beyond doubt that the child, Arlena, could not be the natural offspring of Regina or Ernest, 44. Breathless, close to fainting, Regina found herself flashing back to a conversation that had taken place shortly after her little girl's birth.

The baby, robust and fair-skinned when first brought to her mother, seemed weak and sickly, with a dusky blue complexion a day or two later.

"Gee, this doesn't look like the same baby," she told the nurse.

"Oh, well, why wouldn't it be the same baby?" she recalls the nurse replying. "The wristbands are the same, aren't they?"

For nine years the Twiggs cherished and nurtured the child they brought home from Hardee Memorial Hospital in Wauchula, Fla. Born with a congenital heart defect, Arlena required continual medication and tests, and the bills for her treatment mounted into the tens of thousands of dollars. "She was very fragile, very delicate," Regina says. "I would put my hand on her heart, and I would ask, 'Please God, let her live.' " Ernest, a railroad reservations clerk for Amtrak, worked so many hours of overtime and worried so much about supporting his wife and eight children that he nearly had to be hospitalized himself.

This past August, Arlena went from the family's Langhorne, Pa., home into a New Jersey hospital for open-heart surgery, which is sometimes required by children born with one heart ventricle instead of two. During preoperative tests, the Twiggs learned that the little girl had blood type B, whereas they are both type O; it was a virtual certainty, they were told, that Arlena could not be their natural daughter. Stunned, the Twiggs sought additional proof. It was at the Johns Hopkins Immunogenetic Laboratory in Baltimore that they reached the inevitable, shocking conclusion that the infant they had brought home had been born to another woman.

They never told Arlena of these doubts, nor did their feelings for her waver. "Love isn't something you feel one day, throw down the garbage can the next," Regina says. On Aug. 4, the day after the operation, the frail child died.

"We will never regret one iota of the years with Arlena," says Regina. "But at the same time you know that somewhere out there is another child—your own child. We loved her at birth, and she was ours. And now it's scary to us. Where is she? Who has her?"

At first the Twiggs believed the hospital in Wauchula had made the almost inconceivable error of accidentally giving them the wrong baby. Then they concluded that the hospital had done something far worse—that it had deliberately given them a seriously ill child and kept their healthy child for someone else.

"She was stolen," Regina charges.

"We want her back," declares Ernest.

In their grief over the daughter they lost, the Twiggs are pursuing a bizarre goal; if they find the missing child, they want her taken from her family and brought to live with them. They have also filed a $100 million lawsuit against Hardee Memorial, three attending physicians and one registered nurse, charging that they fraudulently switched babies and baby bracelets. They claim that records at the hospital show that Baby Girl Twigg's birth weight was changed from an initial reading of 8 lbs., 8 oz. to 6 lbs., 8 oz. In the space where congenital malformations are recorded, the word "none" was crossed out, and the words "congenital heart disease" were written in. The Twiggs' attorney, medical malpractice specialist Marvin Ellin of Baltimore, says it is "impossible...to believe that this was an accident."

In addition to Hardee Memorial, the four defendants in the Twiggs' suit are:

•Dr. Ernest Palmer, a family physician still practicing in Wauchula, who examined the newborn baby. He denies any knowledge of a switch.

•Dr. William Black, an obstetrician now practicing in Ocean Springs, Miss., who was the attending physician during the birth. He recalls delivering a healthy baby. "To this day I still don't know what happened," he says.

•Dr. Adly Sedaros, a pediatrician now practicing in Melbourne, Fla., who is accused in the Twiggs' lawsuit of arranging the switch in advance. Sedaros' attorney says his client saw the Twigg baby only once, when he was called in to listen to her heartbeat.

•Dena Spieth, the nurse who handed Regina the child that Regina at first suspected wasn't her own. She too denies any knowledge of a switch.

In the six days before and after the birth of the Twiggs' daughter, only two other children were reported born at Hardee Memorial. One was the daughter of an 18-year-old woman of Mexican extraction. The other was the daughter of a 30-year old Florida woman, and it is this child who, the Twiggs suspect, was secretly switched for theirs. The child, whose mother died of cancer in March 1981, is now living in Sarasota, Fla., with her father, a salesman.

"Intuitively, I feel there's a very big possibility that she could be ours," says Regina Twigg.

It seems at least equally possible that the 9-year-old in Sarasota is not the Twiggs' missing daughter. The father of the child remarried shortly after his wife's death, and that marriage ended in divorce last December; his ex-wife, who lived with father and child for six years, says she is "99.9 percent sure" the girl is not the Twiggs' daughter. The woman claims that the girl looks very much like her deceased mother and that in personality and physique she takes after her father.

As a first step toward gaining control of the child, the Twiggs have requested that the little girl be genetically tested, which the father initially refused to permit. He would not reveal his daughter's blood type and told a reporter, "My sympathy goes out to those people. They have every right to pursue their real child. [My daughter] is not their child.... She never will be." Lawyer Ellin says he will seek a court order if the father does not voluntarily allow genetic testing.

Even more perplexing than the search for the Twiggs' missing child may be the search for a reason for the alleged baby switch. "I mean, I honest to God don't know what the motive was," Ellin says. He has speculated, however, that a possible one was money—the lucrative fee that could have been earned by putting a healthy baby up for adoption.

Therefore, a deliberate switch could only have made sense if someone giving birth in the hospital was planning to hand over her child for adoption. Regina recalls nurse Spieth mentioning that a baby in the nursery was up for adoption, but there is no record in the Florida Office of Vital Statistics of an official adoption taking place at Hardee Memorial at that time.

This suggests either that Regina Twigg's memory is faulty or that an even darker scenario was unfolding around her—that there was an unrecorded birth at Hardee, followed by an unofficial adoption.

Although the Twiggs have no doubt that a child was taken from them, and though medical tests appear to confirm their suspicion, nothing else in the case can be clearly established. When they are not speaking in anger, the Twiggs seem to understand that. They say that even if the child in Sarasota turns out to be theirs, they might not insist on immediate custody after all—not if the girl is happy and has a good home. "We don't want to uproot the child, or frighten her or cause her any pain," says Regina.

For nine long years, the Twiggs endured a sorrow beyond measure, caring for and ultimately losing a critically sick child. They loved her, and when they learned she was not biologically their own, they loved her still. The question now is whether parents so schooled in grief could bear to inflict the pain of parting on another innocent family.

—By Alan Richman, with Meg Grant in Wauchula and Tom Nugent in Baltimore