In considering risk factors, the distinction between non-causal associations
and causation is frequently (and unfortunately) blurred in the
medical literature on AFE. For the life-threatening manifestations of
AFE that occur between 1 in 20–50,000 pregnancies, determining risk
factors among an array of related variables is a daunting (impossible?)
task. Due to their well-known propensity for ascertainment bias, very
little can be learned regarding risk from case series. Population-based
studies are an improvement but some of the methodology and interpretation
problems can be seen in the previously cited, two separate
studies of 3,000,000 women apiece in North America.