Many studies have shown that sleeping with baby on the same sleeping surface, usually a mattress, increases the risk of sudden unexpected death in infancy (SUDI). Indeed over 50% of SUDI cases now occur in a bed sharing situation. There are certain factors that increase the risk further. These include the first 3-4 months of life, whether or not the mother smoked in pregnancy, whether she took drugs or alcohol before sleeping with the baby or was excessively tired and whether the baby was vulnerable, such as low birthweight or preterm. Even without these additional risks bed sharing increases the risk 3-fold, so our advice is “For the first six months, the safest place for baby to sleep is in a cot in the parent’s bedroom.”

We believe the mechanism is accidental suffocation. The nose of the baby is soft and can easily be compressed. Also the jaw can be pushed back and occlude the airway. A healthy baby will arouse (wake up) and wriggle, but if mother has taken drugs or alcohol she won’t be responsive to this. Also if the baby has been exposed to smoking when in the womb, the arousal mechanisms are blunted, and the baby may not wake up.

Only a few studies have examined this. One of the best is the Chicago Infant Mortality Study. This case-control study found that bed sharing with siblings (with or without the parents) raised the risk fivefold.

Although observational studies do not tell us the mechanism, all parents would have observed how deeply preschool children sleep, and it would not be surprising if the sibling did not respond to the struggles (arousal) of the baby if baby was overlaid.  
However, by 6 months of age 85% of SUDI cases have occurred and after 12 months SUDI is very rare. So for the safety of your baby do not let baby sleep with an older sibling until at least 12 months of age.

What about co-bedding twins? Certainly there are SUDI deaths that have occurred in a twin when sleeping together (co-bedding), however, this practice is quite common in New Zealand, so it is difficult to estimate whether co-bedding is a risk or not, but given the increased risk of SUDI associated with sleeping with older siblings, I would recommend caution.

Yes it has been made confusing by an anthropologist. He postulated that co-sleeping (that is sharing the same sleeping surface) would reduce the risk of SIDS. As you know that is not the case, indeed it increases the risk. We showed that being in the same room as the parent but not in the same bed is associated with a reduced risk. He then redefined co-sleeping as including sleeping in the same room, and stated he was right all along!!

I believe we should avoid the word co-sleeping, and use bed sharing and room sharing, because we’re trying to keep the terminology succinct and clear to professionals and families when we discuss SUDI. The meaning of bed sharing and room sharing is self-evident and thus preferred.

One of the aims of the Nationwide SUDI case-control study was to see if there were ways of bed sharing that were safe. The answer is yes, providing infants are older than 3 months, mothers did not smoke or smoke in pregnancy, no alcohol, no drugs, infant not born preterm or small for gestational age, not sharing with another child. However, if any of those factors are present there is an increased risk, sometimes markedly (32 fold increased risk where mother is a smoker).

For infants less than 3 months who have none of these risk factors then the risk is very small – that is the parents are doing everything right. If they also bed share the risk increases 3 fold but the risk in absolute terms remains small (see Carpenter).

There is no physiological studies on infant bed sharing when other risk factors are present. All the studies that have been done are with extremely low risk families. It is unethical to do such studies when you know the baby is at increased risk of death.

I am not aware of any official advice from the Ministry of Health relating to co-sleepers (also referred to as bedside sleeper or "co-sleeping bassinet”). As far as I am aware there are no regulations relating to co-sleepers in New Zealand, although there are some in the US.

As you point out there are potential advantages of a co-sleeper. They achieve room sharing without direct bed sharing. In my opinion, co-sleepers are not as safe as bassinets (simply because more things can go wrong) but are nowhere near as dangerous as sharing a bed. They probably have about the same risk as wahakura or Pēpi-Pod.

The main risks to avoid are:

1. Baby getting stuck between the two beds (entrapment). A co-sleeper should be securely fixed to adult bed to avoid a gap.
2. Loose bedding from the adult bed
3. Bed height not aligned perfectly. If the top rail of the co-sleeper is higher than the adult bed surface, there’s a risk of the baby’s neck getting caught onto it.

On the other hand, a lower sleeping surface increases the risk of some of the bedding sliding over and down to their bed.

A study from the US found a total of 26 incidents (6 deaths and 20 injuries) were reported to the Consumer Product Safety Commission. Of these, 5 deaths were caused by asphyxia, and 1 was attributed to SIDS. Almost half of the injuries occurred after the co-sleeper was improperly assembled. This suggests death and injuries are infrequent.

Note. My only other concern is the cost. A quick search online suggests prices around $300+ in New Zealand, which is only safe to use for a few months (US regulations state up to a maximum of 5 months). For many families, the cost might be better spent on a normal cot which will last many more months.