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