What Does Lack Of Fetal Heartbeat At 5th Week Of Pregnancy Indicate?

Hi, I went to the doctor for my first prenatal visit. She said that my baby is already 7 to 8 weeks old, she claculated it based on the first day of my last menstration. When we had an ultrasound, there was no heartbeat yet and i was told that i am only 4 weeks pregnant. The doctor prescribed me progesterone and asked me to go on a bedrest for 2 weeks. I will have another ultrasound after 2 weeks. Is my baby safe?