The controversial foetal rights bill Zoe's Law is unnecessary and will lead to unjust sentencing results, with its definition of an unborn child likely to spill into murder and manslaughter cases, the Law Society of NSW says.

Weighing into the bitter debate over the bill, the Law Society has written to NSW politicians outlining its opposition to any change in the law, ahead of a conscience vote in the State Parliament.

Backing the bill: Energy Minister Chris Hartcher. Credit:AFR

Zoe's Law seeks to define a 20-week-old foetus as a living person, so that charges of grievous bodily harm can be laid if a pregnant woman loses her unborn child in a motor vehicle accident or assault. The law at present recognises grievous bodily harm to the mother, with a maximum penalty of 20 years jail, if her foetus dies in these circumstances.

The new definition of an unborn child in Zoe's Law is ''arbitrary'', says the Law Society, which argues the present law is adequate.

Changing the law so an unborn child is a ''living person'' under the Crimes Act would have broader consequences, the society's criminal law and juvenile justice committees have argued in the letter.

''This would be a very significant change to the law in NSW and it would be very difficult to resist the application of this new concept to other areas of the law such as murder and manslaughter,'' the submission says.

The Law Society has also backed the position of the NSW Bar Association, which came under attack in Parliament last week for opposing Zoe's Law. Energy Minister Chris Hartcher had attacked the Bar Association's position as ''unconscionable''.