Hi, Thanks for the query. Usually transvaginal ultrasound can detect the gestational sac at 5 completed weeks and fetal heart beat at 6 completed weeks. Sometimes gestational sac can develop without embryo it is called blighted ovum. In some pregnancies after developing for few weeks the heart beat of the fetus can get stopped. But to come to one conclusion repeat ultrasound after one to two weeks is recommended. Sometimes because of decreased production of hormones embryonic growth can get affected. In that case supplementing beta hcg and progesterone hormones from outside can regularize the baby development. So, you can take the medicines recommended by your doctor and repeat ultrasound after one week. Take care.