>>… why so narrow a class ? (almost a "sub-group" !?) <<
It’s not a subgroup. Surgery or childbirth are the times when individuals with hereditary antithrombin deficiency must have their level of AT brought up to the normal range. Not to do so risks a fatal blood clot during the procedure.
In everyday life, individuals with HD can manage with an oral anticoagulant such as Warfarin, but an oral anticoagulant cannot be taken during surgery/childbirth due to the risk of bleeding.
“The efficient-market hypothesis may be the foremost piece of B.S. ever promulgated in any area of human knowledge!”