A woman who allegedly dumped her baby in a drain within hours of giving birth followed NSW Health's best practice protocols in her decision to leave hospital soon after delivery.

A trend towards shorter postnatal stays in hospital has been growing for decades and is among the aims of NSW Health's "Towards Normal Birth" directive to simplify childbirth.

Hospitals are encouraged to discharge women from hospital four to 48 hours after the birth, provided that they had normal pregnancies, uncomplicated vaginal births, healthy babies and family support at home.

NSW Kids and Families chief executive Joanna Holt said postnatal care plans were developed with women and their partners during pregnancy and included discussions about what family and social support was available after the baby was born.

"The length of time a woman stays in hospital after the birth of her baby will vary and be decided in partnership with the woman and her healthcare professional and take into account the health and wellbeing of the woman, her baby and the level of support available following discharge," Ms Holt said.

Depending on their circumstances, the midwife might link a woman to support services.

In 2011, 17 per cent of NSW mothers were discharged less than two days after birth, up from 11 per cent in 2002 and 4 per cent in 1994.

Just over 3 per cent of NSW mothers were discharged after less than one day.

Police allege the 30-year-old woman whose baby was discovered down a stormwater drain on Sunday had left Blacktown Hospital on Tuesday afternoon after delivering him in the early hours of that morning.

Australasian College of Midwives spokeswoman Hannah Dahlen said it was best practice for women to leave hospital as soon as possible after childbirth, provided they had been screened for mental health issues and had ongoing support from their midwife at home.

"If everything is normal there's a move to increasingly discharge women earlier and earlier but with the understanding that you have postnatal services for the first week," Dr Dahlen said.

NSW had excellent antenatal psycho-social screening processes to ensure women were well supported on discharge, she said.

But there was no way of ensuring that they always told the truth about their circumstances. Nor could staff stop women from leaving hospital if they were determined to do so.

"Of course, there's autonomy to do that," Dr Dahlen said.

"We can do a lot more to support women [after childbirth] but staying in hospital longer is not going to make a difference and, in fact, sitting in hospital is going to lead to more infections, poorer sleep, conflicting advice, so that's not the answer."

A Sydney mother admitted to police that she dumped her newborn baby boy down a drain knowing it would kill him, court papers showed.

The case against the 30-year-old woman, who cannot be named for legal reasons, was heard briefly in Blacktown Local Court on Monday after she was charged with the attempted murder of the boy.

Police will allege the boy was born in the early hours of November 18 and pushed down a stormwater drain near the M7 motorway at Quakers Hill only hours later.