''Prior to the age of two years, overnight time away from the primary care-giver should be avoided, unless necessary,'' according to the Australian Association for Infant Mental Health's ''guidelines for protecting the very young child's sense of comfort and security''.

The guidelines recommend non-custodial parents, nine out of 10 of whom are fathers, should instead see children under two during the day, up to three times a week, gradually phasing in overnight visits after the second birthday. Families fighting custody battles in the Family Court should not share custody until the child is three.

''The infant's voice can be very soft in the often highly conflictual family court arena and we hope that these guidelines will give it some strength and volume,'' the association's Victorian chapter president, Nichola Coombs, said.

These warnings follow several other studies critical of the 2006 Family Court reforms introduced by the Howard government, which singled out shared care as the option judges must consider. Three government-commissioned reports released last year highlighted widespread confusion over shared care, as well as the dangers faced by children from violent families.

The infant custody guidelines are based on a 2010 study of shared custody by clinical child psychologist Dr Jennifer McIntosh, which found ''babies under two years who lived one or more overnights a week with both parents were significantly stressed''. Using reports from the separated parents of 258 children under two, as part of the Australian Institute of Family Studies' divorce research, Dr McIntosh concluded: ''In their general day-to-day behaviour, these babies were more irritable and worked much harder to monitor the presence and to stay close to their primary parent than babies who had less or no overnight time away from their primary caregiver.

''Decision makers need to think about what a very young child can cope with compared to an older child. There aren't black-and-white answers but there are very good reasons to be cautious about frequent, regular overnight schedules.''

While the guidelines have no legal weight, the executive secretary of the Shared Parenting Council of Australia, Wayne Butler, fears they will influence judges' rulings. ''When parents are together, they care for the babies on a shared basis,'' Mr Butler said. ''There's no reason why there couldn't be reasonable overnight contacts when the parents are co-operative.''

Law Institute of Victoria president Caroline Counsel said it's rare for Family Court judges to award shared custody of babies. But, she warned, ''there are some children that manage well and some who really struggle given their age and stage of development''.

Co-author of frank new parenting book S--t On My Hands, Bunny Banyai, said her daughter Clementine was only 18 months old when Ms Banyai and her partner separated. They decided their daughter was too little for overnight stays with her father but Ms Banyai now regrets it. ''I was almost psychotic with tiredness, so my relationship with my daughter suffered,'' she said.