July 1, 2011 -- Nobody can imagine why seemingly loving, devoted mothers would ever harm their own flesh and blood. But more than 200 women a year kill their children in the United States, according to the American Anthropological Association.
The shocking and unimaginable crimes of child-killers have gripped the nation for decades. Here's a look back at the famous cases of mothers accused of murder.
In 1983, Downs, a 27-year-old divorced postal service worker, told police that a "bushy-haired stranger" flagged down her car and shot her three children on a back road near Springfield, Oregon.
Her daughter Cheryl, 7, was dead on arrival at the hospital, and her other children -- Christie, 8, and Danny, 3, were clinging to life.
Downs' story about the stranger did not add up. Reading through her secret diaries, police found a motive: an obsession with a married man who didn't want her children. In February 1984, nine months after the shootings, they arrested her and charged her with one count of murder and two counts of attempted murder.
Downs' trial was a national spectacle that was later depicted in the TV movie "Small Sacrifices," starring Farrah Fawcett as Downs.
She was sentenced to life in prison plus 50 years. She was denied parole in December 2010 and will not be considered for parole again for ten years, when she will be 65.
Watch the full story on "20/20" tonight at 10 p.m. ET.
Casey Anthony, 25, is on trial in Orlando, Fla. on first-degree murder charges in the death of her 2-year-old daughter, Caylee. She has pleaded not guilty and has been held in a Florida prison since the summer of 2008.
Caylee lived with her grandparents and her single mother, Casey. Caylee disappeared in June 2008, but was not reported missing until a month later. Anthony told police she left Caylee at the apartment of a babysitter named Zenaida Fernandez Gonzalez, and that both were missing when she returned. Police found, among other discrepancies in Anthony's story, that the apartment had been vacant for more than 140 days.
Investigators said Anthony repeatedly misled and lied to police in the course of their work. She was named a suspect in her daughter's disappearance and charged with the Caylee's murder in October 2008.
After a six-month-long search, remains of Caylee's body were found in Orlando, Florida, in December 2008.
Casey Anthony's defense team has argued that the toddler accidentally drowned in the family pool and that George Anthony, Casey Anthony's father, helped dispose of the body.
The defense has accused George Anthony of sexually abusing Casey Anthony. They have argued that Casey Anthony hid her daughter Caylee's death in the same manner that she hid the molestation.
In his testimony before the court this week, George Anthony denied the defense's claims.
In October 1994, Susan Smith drowned her two young sons, buckling them into their car seats and pushing the car into a South Carolina lake. The car sank with the sleeping children in the back.
Smith initially told police that a black man had hijacked her car and abducted the children. Americans desperately searched for the boys for nine days until Smith confessed that she killed her sons, Michael, 3, and 14-month-old Alex.
She was convicted of the two murders in July 1995 and sentenced to life in prison. A judge rejected Smith's appeal in March 2010. She is eligible for parole in November 2024.
In June 2001, Andrea Yates methodically drowned her five children in the bathtub in their Houston home. The case shocked the American public.
Yates told police and psychiatrists after the crime that Satan had ordered her to kill sons Noah, 7; John, 5; Paul, 3; Luke, 2; and 6-month-old daughter Mary, to save them from eternal damnation.
Yates, a former nurse and high school valedictorian, suffered from mental illness for years -- depression with bouts of psychosis, suicide attempts and hospitalizations.
In tapes of Yates' psychological evaluation, released exclusively to ABC's "Primetime" in 2006, she recalled details of the morning she murdered her kids, describing how she waited until her husband left the house to start filling the tub. "Drowning them" was "all I thought about," she said.
Yates was convicted of capital murder in March 2002, but an appeals court later overturned the verdict. In 2006, Yates was retried and found not guilty by reason of insanity. At the time, she was committed to a state mental hospital.
June 20 marked the 10-year anniversary of the Yates murders. Yates is now being treated in a minimum-security mental hospital in Kerrville, Texas. Her longtime lawyer, George Parnham, said he's "highly optimistic" she will be released after her recommitment hearing in November.
Encouraged by the case, Parnham formed the Yates Children Memorial Fund in June 2002 to educate women, their families and their doctors about postpartum psychosis and similar illnesses.
China Arnold was accused of killing her 3-week-old daughter in a microwave oven in Dayton, Ohio, on Aug. 30, 2005. Investigators said the baby, Paris Talley, was burned to death in the oven after Arnold and her boyfriend had an argument over who the child's biological father was. Arnold was sentenced to life in prison without parole Sept. 8, 2008. Judge Mary Wiseman told Arnold during the trial, "No adjectives exist to adequately describe this heinous atrocity. This act is shocking and utterly abhorrent for a civilized society."
Michelle Kehoe, 36, was sentenced to life in prison without the possibility of parole on charges of first-degree murder in the death of her 2-year-old son Seth, and an attempted-murder charge related to her 7-year-old son, Sean.
Kehoe and her sons were reported missing by her husband, Eugene Kehoe, after the boys and their mother reportedly left to go visit relatives in Sumner, Iowa, on Oct. 26, 2008. The next morning, Kehoe walked into the home of residents in Littleton, Iowa, and told them that her sons were in danger.
Seth was found dead outside the family's van. Autopsy results revealed the boy had died from severe cuts to his neck. His older brother, Sean, was found struggling for his life inside the vehicle and had suffered similar cuts.
According to the arrest warrant affidavit, Kehoe falsely claimed that her children had been abducted. "She stated that she couldn't explain why she had done it. She stated that she couldn't face anyone. She stated that she wanted to die or be locked up where she couldn't hurt anyone else," the affidivit read.
At the trial in October 2009, Kehoe pleaded not guilty. Her attorneys argued that she suffered from extreme mental illness. An Iowa jury heard a tape of her surviving son telling police how his mother covered his eyes, nose and mouth with duct tape, slashed his throat, and then did the same to his younger brother. On Nov. 5, 2009, she was found guilty of first-degree murder, attempted murder and child endangerment causing serious injury. The following month she was sentenced to life in prison and an additional 25 years.
The Associated Press contributed to this report.