According to http://www.marchofdimes.com/Pregnancy/complications_umbilical.html 5% of the 1% of babies with a true knot in their cord die.
Also according to http://depts.washington.edu/hswork/multnomah/ten/mom_baby/mchd02/005a.html there are about 133,000,000 babies born world wide each year.
If this is true, then 66,500 babies die each year by miscarriage or stillbirth due to a true knot in their cord.
That is .05% or 1 in 2000.
In your lifetime, your average chance of dying in a car crash is 1 in 83.