WHEN IT CAME to being a mother, Stacey Newkirk-Smalls was a supermom, say those who know her best.

The licensed-practical nurse taught her 18-month-old twins - Adam and Eve - a form of sign language so she could communicate with them before they learned to talk, recalled Newkirk-Smalls' mother, Yvonne Newkirk.

Newkirk-Smalls, 41, of Tacony, bought a membership at Philadelphia's Please Touch Museum to expose the twins regularly to enrichment activities, Newkirk said. She enrolled another daughter, Stacey, now 5, in ballet classes.

"The kids were her life," gushed Newkirk, 57, seated at the dining-room table in her West Philadelphia home, scrolling through family photos. "If she went out to dinner with friends, she would take them with her. I mean, they just were her life."

Their lives ended violently on May 24, less than two weeks after Mother's Day.

City police and prosecutors contend that Newkirk-Smalls murdered the twins by poisoning both, then drowning one and suffocating the other.

Authorities say she also tried to poison Stacey, who survived and was unharmed.

Unless exonerated at trial, Newkirk-Smalls will join the nation's growing rogues' gallery of murderous parents who include Andrea Yates, the Houston mother who drowned her five children in the family bathtub in 2001; Susan Smith, the South Carolina mother who intentionally drove her car into a lake, drowning her two sons, in 1994; and Marie Noe, the Philadelphia mother who confessed to killing eight of her children over a 19-year period.

Just last month, Chanthy Mao, of South Philadelphia, accepted a negotiated guilty plea for fatally stabbing her daughter, 12, and son, 8, on Aug. 31, 2011.

Mao, 28, a mentally ill Cambodian immigrant, was sentenced to 30 to 60 years in state prison.

About 250 to 300 children are slain by their parents each year in the U.S., crime experts estimate.

The U.S. Department of Justice reports that 63 percent of all children under age 5 who were slain from 1980 through 2008 were killed by a parent.

Although every case is unique, experts say that most parents who kill their children are driven by a group of defined motives.

Phillip Resnick, an internationally known forensic psychiatrist and leading expert on parents who kill their children, identified five leading causes for these killings.

* Altruism: Committed by a parent who believes the child would be better off dead.

* Acute psychosis: Committed by a mentally ill parent.

* Unwanted child: Committed by a parent who perceives the child as a hindrance or who would benefit from the child's death.

* Accidental: When a parent unintentionally kills a child during abuse.

* Spousal revenge: Committed by a parent seeking to hurt a spouse due to infidelity or some other grievance.

Assistant District Attorney Mark Cipolletti, who is handling Newkirk-Smalls' case, said such killings strike all segments of the community.

"These things have no social or economic boundaries," he said. "In this case, it's really a very sad set of circumstances that led to the deaths of these children."

The authorities said Newkirk-Smalls confessed to the crimes when police arrived at her Tacony home on Ditman Street after being summoned by her husband, Ron Smalls, who was employed as a city correctional officer at the time.

Newkirk-Smalls' public defense attorneys, Daniel Stevenson and Fred Goodman, declined to comment or permit her to be interviewed. She has pleaded not guilty.

Cipolletti said his office has given notice to Common Pleas Court that it reserves the right to seek a death sentence against Newkirk-Smalls, whom he said is the only suspect in the slayings.

Newkirk-Smalls' mother insists that she was not suicidal, depressed or mentally ill.

But two weeks before the slayings, Newkirk said, Newkirk-Smalls was hit by a bombshell: She learned that her husband was having an affair with a family member.

"I want the truth to come out, that's all that I want," Newkirk said. "I suspect my daughter didn't do it."

Ron Smalls, in an interview, said he was in therapy and therefore would not discuss Newkirk-Smalls' claim that he was having an affair with his wife's daughter from another relationship.

He has custody of the couple's surviving daughter, Stacey, and said he plans to divorce his wife.

Cipolletti declined to speculate on the motive that led Newkirk-Smalls to allegedly kill her twins and try to poison her 5-year-old daughter.

"I just think it was a well-thought-out, well-planned-out, premeditated act," he said.

Newkirk-Smalls, herself a former correctional officer before becoming a nurse, is locked up at Riverside Correctional Facility on State Road.