TORONTO - Mothers charged with murdering their newborn babies should not be allowed to use infanticide as a defence and receive a drastically reduced sentence, Ontario's top court was told Thursday.

The Court of Appeal for Ontario heard arguments in the case of a woman who was convicted of infanticide for killing two of her young children.

The Crown is appealing, arguing that since the judge found the Crown had proven two counts of first-degree murder, the woman should not have instead been found guilty of infanticide.

Infanticide is defined as a woman wilfully killing her newly born child when her mind is disturbed as a result of childbirth or lactation.

The penalty for first-degree murder is 25 years with no eligibility for parole, and the maximum sentence for infanticide is five years.

Convicting the woman, L.B., of infanticide instead of murder even though the case met the requirement for first-degree murder cheapened the babies' lives, the Crown said.

"In effect, what the trial judge found was ... the respondent did not have to shoulder responsibility for those murders because giving birth had produced some effect, however negligible, on her mind," Crown attorney Jennifer Woollcombe writes in court documents.

This is the first time an appeal court in Canada has considered whether infanticide is a legally available defence to murder, the Crown says.

Woollcombe argues that wording in the Criminal Code allows police to charge a woman with infanticide. It also allows for a woman charged with first-degree murder in the death of her baby to be found guilty of the lesser offence of infanticide if the test for murder is not met, she said.

But, she says, it does not allow a woman to raise infanticide as a defence to murder.

The defence for L.B. disagrees, saying Parliament did intend the Criminal Code to allow for infanticide as a partial defence to murder.

"Since every infanticide is by definition a murder, infanticide could never be an available verdict on a charge of murder where the prosecution failed to prove an essential element of murder," he writes in court documents.

The woman in this case, known as L.B., was 16 when she decided to get pregnant with her boyfriend, the Crown said. She smothered the baby at 10 months old and four years later she smothered another one of her babies at about two months old.

The deaths were attributed at the time to Sudden Infant Death Syndrome and L.B. only confessed years later.

"This case aptly demonstrates why an accused charged with murder should not be able to escape culpability for murder on the basis of a defence of infanticide," Woollcombe says in court documents.

The trial judge found the killings were planned and deliberate and two psychiatrists who examined L.B. said she was capable of rational thought at the time of the killings and was not suffering from post-partum psychosis, the Crown said.

"Both doctors agreed that the respondent my have killed her babies because she did not want them and was jealous that they were taking from her attention that she felt she deserved," Woollcombe wrote.

However, both doctors agreed there was some connection between her mental state and giving birth. One psychiatrist said she had a borderline personality disorder that may have made her more susceptible to post-partum depression.

The offence of infanticide only requires a mental disturbance be present, and doesn't require any causal link between the mental state and the killing of the baby, Woollcombe says.

"Any statutorily required connection between the disturbance and the accused's wilful act is a further reason not to allow the offence section to create a 'defence of infanticide' to a charge of murder," she writes in court documents.

If a woman is truly disturbed, the defence of not criminally responsible is available, Woollcombe said.