An Australian woman who gave birth to a girl fathered by a boy, 12, has been sent to jail for six years.

An Australian mother of three who gave birth to a child fathered by a 12-year-old boy she had been sexually abusing for more than two years has been jailed for six years.

A Victorian County Court judge, who cannot be named, said on Monday the woman had behaved in an abhorrent and shocking way when abusing the vulnerable boy for her own emotional and physical needs.

The judge said the woman's moral culpability was very high given she was supposed to have been looking after the boy for his mother when she had to work early, and had been in a position of trust and friendship.

"The protection of children is an extremely important matter," the judge told the woman when jailing her for six years with a non-parole period of three years and six months and placing her on the sex offenders register for life.

The Melbourne woman, 40, gave birth to a daughter in May last year when her victim was 14.

The judge said the woman had taken the boy's adolescence away from him and while he clearly loved his daughter, he was having difficulties adjusting to fatherhood and what had happened to him.

She said the boy was angry and confused, his relationship with his mother had deteriorated and he should not have to deal with these issues at such a young age.

The baby girl is now being cared for by the boy and his mother but the Department of Human Services is expected to take her for weekly prison visits to see the woman.

The judge said the victim's mother was doing her best to support her son and his baby daughter despite feeling betrayed.

The woman had been married and living in suburban Melbourne with her three children when her daughter became friends with the victim when they were both 11.

The woman became jealous of her daughter's relationship with the victim and began taking him to and from school when she was having marital problems.

The woman, who separated from her husband in January 2013, first had unprotected sex with the boy when he was 12 and in Year 7 and she was 36.

It was the boy's first sexual experience and he later told police they had unprotected sex more than 20 times over the next two years, including in her car and when camping in the backyard.

The woman, who pleaded guilty to one count of persistent sexual abuse of a child under 16 between April 2012 and May 2014, claimed she was ashamed, embarrassed and guilt-ridden at what she had done.

She told psychologist Professor James Ogloff she was sexually naive and did not realise the boy was old enough to father a child.

The woman said she was attracted to the boy because he was sexually maturing but denied being attracted to children.

Professor Ogloff found the woman had not been acting in a predatory way or been driven by a deviant sexual desire, but instead had allowed her feelings of care and nurturing for the boy to develop into a sexual relationship.

The psychologist said the woman had been a supportive and caring parent with no prior convictions who failed to recognise her feelings for the boy were completely inappropriate.

Professor Ogloff did not believe the woman suffered from a paedophilic disorder and was unlikely to reoffend.