4th pregnancy, 3 children. gestational sac measuring a week ahead of embryo. no heartbeat detected. embryo measures 6 weeks?

A 6 week embryo usually does have a detectable heartbeat on ultrasound, so the absence of that could mean something abnormal about this pregnancy. But if you are not having pain or bleeding it is usually recommended to repeat the ultrasound in about a week to look for heartbeat again, as it may just be a bit too early. The waiting & uncertainty in between is difficult!