The only thing I can say about that is, is that the Diastolic reading is the one that measures the pressure when the heart takes a momentary rest. The upper number the Systolic one, measures the pressure whne the heart is pumping.
If the heart is slowing down for some reason, I.E. it's possibly not pumping as well as it should, then it could possibly be related to restricted electrical impulses reaching it, or some kind of stiffening of the left ventricle, known as Ventricular stiffening, or something like a Valve prolapse causing regurgitation, where the blood tries to go back the wrong way.
If there was an electrical stimulus issue, it might be related to an electrolyte issue like some problem perhaps with Sodium, Calcium or both, that being either too little or too much, too much if they've overcompensated a for what they think baby might get, though it could then be a a too little scenario, if some of the normal amounts are being siphened off for the in vivo babies needs, leaving Mother short of certain nutrients, required to keep her functions on top of their game.
I cannot tell you exactly what is going on with the individuals blood pressure, because I am in no position to diagnose people and I can't and won't, but if the individual needs to know more and are worried, they MUST seek an appropriate medical proffessional.