Is it normal to see only a gestational sac in early pregnancy on ultrasound?
Very early in pregnancy (5 to 6 weeks gestational age, around two and a half weeks after conception) you can see a gestational sac on ultrasound but you can't necessarily see the embryo or a heartbeat. If you are further along in pregnancy though, seeing the sac without an embryo can be a sign that the pregnancy is destined to miscarry. This is especially true if the sac is big enough that an embryo ought to be seen. If the situation isn't clear it may be helpful to check the blood hCG level, to correlate it with what is expected on ultrasound. Sometimes you just need to repeat the scan around a week later to see if the pregnancy is growing appropriately.
A week can feel like forever, while you wait to find out the destiny of your pregnancy. Hopefully everything will be fine for you and your baby-to-be.