The 39-year-old Swiss citizen of Austrian origin broke down in tears at the opening of her trial in the town of Horgen confessing that she smothered her twins, a girl and a boy, to death with a pillow on December 23th, 2007, the ATS news agency reported.
The woman, identified only as Bianca B., also confessed to previously smothering her first child, a seven-month-old baby believed until now to have died from sudden infant death syndrome.
"I wanted him to be quiet for a while," she explained.
The defendant had until Wednesday maintained her innocence and blamed the twins' father for their deaths.
She was nonetheless sentenced in March 2010 to life in prison for killing the twins, but she was granted a retrial after the court determined her defence had been flawed.
"I can no longer live with the guilt," she told the judges on Wednesday, according to ATS.
Bianca B. described in detail how on the evening of December 23, 2007, she had gone into her son's room after placing the children's Christmas gifts under the tree and had suddenly, without thinking, picked up a pillow and pressed it against his face "until he stopped moving."
She had then gone into her daughter's room and done the same to her, before realizing what she had done.
"I didn't want this," she told the court, in tears.
Bianca B., who has been jailed since the end of 2007, is set to receive her new verdict on December 19th.