In 1978, Marybeth and her husband, Joe, made arrangements to adopt a child. That same year, Marybeth became pregnant again. But the Tinnings did not cancel the adoption. Instead, they chose to keep both children. In August 1978, they received a baby boy, Michael, from the adoption agency. Two months later, on October 29, Marybeth gave birth to her sixth offspring, a girl they named Mary Frances. In January 1979, the baby apparently developed some type of seizure, according to Marybeth. She rushed Mary Frances to St. Clare's emergency room, which was directly across the street from her apartment. A capable staff was able to revive her. They saved the baby's life, but only for a time. On February 20, Marybeth came running into the same hospital with Mary Frances cradled in her arms. The baby, just four months old, was brain dead. The explanation was the same as the others. Marybeth said she found the baby unconscious and didn't know what had happened to her.
"There is really nothing to say," she told investigators years later, "than I found her in her crib unresponsive. I believe Joe was there. I can't remember." When an autopsy failed to find a reason for the death, again it was attributed to SIDS.
Once Mary Frances was buried, Marybeth wasted no time in getting pregnant. On November 19, that same year, she gave birth to her seventh baby, Jonathan. In the meantime, the Tinnings still cared for their adopted child, Michael, who was then 13 months old and seemingly in good health. In March 1980, Marybeth showed up at St. Clare's hospital with Jonathan unconscious. Like Mary Frances, he was successfully revived. But because of the family history, he was sent to Boston Hospital where he was thoroughly examined by the best pediatricians and experts available. The doctors could find no valid medical reason why the baby should simply stop breathing. Jonathan was sent home with his mother. A few days later, Marybeth was back at St. Clare's, this time with a brain dead Jonathan. He died on March 24, 1980.
Less than one year later, a pivotal event occurred in the Tinning household. On the morning of March 2, 1981, Marybeth showed up at her pediatrician's office with Michael, then two and a half years old. He was wrapped in a blanket and unconscious. Marybeth told the doctor that she could not wake Michael that morning and had no idea what was wrong. She described what happened next to police, "When I went in, in the morning to get him up and so we could go to the doctors, he was not, I mean he was responsive to a point but he was very limp and so on and so forth and so instead of calling an ambulance, I went from our house...put him in the car, literally threw him in the car and went to St. Clare's or I mean I went to Dr. Mele's office and went in there and...by the time one of the doctors...I guess took me and they said that he died of viral pneumonia" (Tinning).
When the doctor examined the boy, he was already dead. Later, an autopsy found traces of pneumonia but not enough to cause death. Since Michael was adopted, the long-suspected theory that the deaths in the Tinning family had a genetic origin was discarded. Something else was happening, only no one knew exactly what it was. After Michael died, some of the nurses questioned Marybeth's odd behavior. They noticed that when she first realized that Michael was sick that morning, Marybeth could have easily walked across the street to the emergency room to obtain medical care. In fact, she had done just that when the others had died. But instead, she let hours pass until the doctor's office opened for business.
It didn't make sense.