Is the "of" supposed to be "if" (after the 1000.2%)?
I think that the calculation goes like this:
For each given gestational age, the median AFP level for the population is calculated. For example (with very theoretical numbers) lets say that the population median AFP for a fetus at gestational age of 10 weeks is 10 units.
If a fetus at 10 weeks gestation has an AFP of 25 units, this is a converted into 2.5 "multiples of median".
So, the prevalence of an open neural tube defect when testing shows an AFP of 2.5 MOM is 1 in 1000.2%.
At least this is how I read this. Interestingly, there are only 512 google hits for "multiples of median", and virtually all of them are obstetrics! I think this MOM is being used as a substitute for standard deviation. Of course standard deviations are based on means. This is probably because most labs report AFP with a "<" for the lower range. Therefore no mean can be calculated. Without a mean you cannot have a standard deviation. This use of MOM probably represents a simple calculation to show dispersion (deviation) from the median.
These are all thoughts off the top of my head, so folks who know better feel free to contradict me!
Mindy
