Before coming to any conclusion I would advise you to wait until 10-11 weeks and re-scan. At 7 weeks, often an arterial pulse is misdiagnosed as fetal heartbeat. If at 11 weeks also there is no heart beat, then it could be a blighted ovum if you do not miscarry and HCG levels rise. Please discuss with your gynecologist.
For your information: An anembryonic gestation is characterized by a normal-appearing gestational sac, but the absence of an embryo. Normally a gestational sac with a yolk sac appears at about 5 weeks of pregnancy and can be well visualized by 6 weeks on a transvaginal ultrasound. A pregnancy is anembryonic if a transvaginal ultrasound reveals a sac with a mean gestational sac diameter (MGD) greater than 13 mm and no yolk sac, or an MGD >18 mm with no embryo.
Hope this helps. Take care!