If an ultrasound at 6 weeks shows a yolk sac but no heartbeat, it can be concerning. However, it is important to remember that every pregnancy is different and not all heartbeats are detectable at this stage. There could be several reasons for the absence of a heartbeat, such as incorrect dating, slow development, or even a miscarriage. It is advisable to follow up with your healthcare provider for further evaluation and possibly repeat ultrasounds to monitor the situation closely. They may also recommend additional tests like blood work or another type of imaging to gather more information. While it is natural to worry in such situations, try to stay positive and seek support from your loved ones during this time.