Mitral valve scleorosis is a condition where the leaflets that close over the valve opening may not close tightly enough to prevent blood to flow back into the upper chamber. It can be a medical problem or it may just be insignificant leakage.
I don't know any reason to not have any sweets for a valve disorder, but there may be a concomitant disorder that has not been clearly stated to you. A severe mitral valve disorder can effect normal functioning, and no one without further information can give you an insight to what to expect...pregnancy or otherwise.