Children who live in large troubled families may be more at risk of coming to harm, a review by the care watchdog has concluded.

The study, covering 20 cases in which a vulnerable child, or another person connected to them, had been seriously hurt or had died, found that six incidents involved families with four or more children.

It said this and previous findings indicated that "the increased stress of parenting four or more children meant that risk of harm was greater."

The cases surveyed were all subject to significant case reviews (SCRs) - which are carried out on behalf of local councils and child protection committees when a child dies, or is significantly harmed amid abuse or neglect.

Of the 20 commissioned in Scotland between April 2012 and March 2015, six involved families with four or more children. Three families had five children and one had six. The Care Inspectorate's report says the size of a family can increase existing risks, adding: "Some parental behaviours such as domestic abuse, substance misuse and poor supervision of the children added to the family's difficulties. Multiple births bring additional demands, which place considerable strain on even a highly functioning family."

Overall the watchdog says lessons are not always being learned when SCRs are carried out and the reviews themselves vary in their approach and their quality.

It highlights other factors which also increase the risk to children, including substance misuse - which was a factor in more 11 of the reviews (55 per cent), domestic abuse, which was involved in 13 of the cases (65 per cent) and parental mental health problems, which were also present in 13 cases.

The cases surveyed are not identified by name but cover 23 children. They include one in which a child had caused the death of another individual. This is almost certain to be that of a 13 year old boy jailed in 2012 for killing his foster carer, 34 year old Dawn McKenzie in 2011.

They also include 11 cases resulting in the death of a child or young person, including a baby who drowned when left unattended in the bath by drug-using parents who were "distracted".

Two infants died while sleeping with their drug or alcohol-dependent parents, and one pre-school child died as a result of a physical assault. SCRs carried out in this period include the case of toddler Mikaeel Kular, who was killed by his mother.

Six teenagers aged 15-17 died, five of them female, all as a result of their own risk-taking or self-harming behaviour, the report says, coupled with alcohol and drug misuse.

The Care Inspectorate said that even when social work, health, education, police and other agencies work tirelessly in challenging circumstances, they cannot always prevent terrible things happening. But the report says that policies which allege that agencies share responsibility for the keeping children safe are not always followed through in practice with social workers usually left to make decisions about increasing intervention.

The inspectorate says staff shortages and workload may have been a factor in seven of the cases, with examples of cases unallocated for lengthy periods and inexperienced staff handed responsibility for complex child protection cases because more experienced staff were not available.

The report also warns social workers can be over-optimistic about the abilities of parents to cope or change, particularly where domestic violence is a feature of a child's life.

Karen Reid, chief executive of the Care Inspectorate said: "We owe it to these children to understand what happened and to find out what must be done differently to prevent harm in the future.

"Learning from tragedy and mistakes requires candour and frankness. They are essential ingredients in preventing the same things happening again.

"Strong local leadership and a clear focus on working together are essential if we are to prevent harm, keep children safe and reduce health and social inequalities.

Matt Forde, national head of service for NSPCC Scotland said: "The report from the Care Inspectorate highlights significant weaknesses in how SCRs are currently carried out. When a child dies or is seriously harmed by abuse it is absolutely crucial that the SCRs that follow enable all relevant lessons to be learned.

"We need a more evidence-based and systematic process to examine what went wrong in these cases but also, crucially, to examine what needs to change.”