Heart activity in a fetus can be seen easily around 14 weeks of gestation (also in experienced hands a little earlier). You have not written about gestation and who performed ultrasound
for cardiac activity and how much was gestation. Your wife's obstetrician can detect any problem with the fetus competently and I would urge you to consult your obstetrician. You will get the best advise.
With Best Wishes
Dr Anil Grover