General Discussion
In reply to the discussion: Four babies hemorrhage after parents refuse vitamin K shot, a practice on the rise [View all]jeff47
(26,549 posts)Factor 5 is a genetic disease, and the "disease" version of the gene is dominant. Thus one of the parents has to have it in order for the baby to have it. It's not like hemophilia where a parent can be an asymptomatic carrier.
Even if it isn't diagnosed in the parents, being dominant means it would show signs in the family history. If the OB is doing their job, they should be asking the parents about family tree, and be very interested if there's parts of the tree that experienced blood clots.
And even if that isn't enough, genetic testing can easily find it.
So the probability isn't the incidence rate of the disease. It's the probability of an undetected instance of the disease. Which is much more remote than the incidence rate of the disease.