Table 3.

Evaluation of the observations of maternal sleep position against the Bradford Hill criteria

Criteria	Observation
Analogy	There are parallels with the relationship between infant sleeping prone and risk of SIDS.
Biological gradient	These case control studies do not record proportion of the night spent supine, so a biological gradient cannot be determined.
Coherence	Coherence between epidemiological and laboratory findings increases the likelihood of an effect. Maternal supine position results in changes indicative of fetal adaptation to hypoxic stress.
Consistency	In four case‐control studies showing association between supine going‐to‐sleep position and late stillbirth, the effect size appears to be similar in three different populations (Australia, UK, New Zealand)
Experiment	Evidence of a fall in late stillbirth in New Zealand in the last five years that may be due, at least in part, to documented changes in maternal going‐to‐sleep position (Eleventh Annual Report of the Perinatal and Maternal Mortality Review Committee. Wellington, New Zealand: Health Quality & Safety Commission).
Plausibility	Two biologically plausible mechanisms have been proposed, namely sleep disturbed breathing, which is more common in the supine position, and/or inferior vena caval compression and resultant fetal hypoxia.
Specificity	There are no data to determine whether the effect of supine‐going‐to sleep position is specific for late stillbirth.
Strength	The magnitude of the OR is moderate (For comparison the hazard ratio for mortality for smokers (> 10/d) versus non‐smokers is 1.8.33
Temporality	Maternal sleep position occurs before the death of the baby.