It can be concerning if a sac is not found in the uterus during a 6th-week scan. However, it's important to remember that every pregnancy is different, and there could be several reasons for not detecting a sac at this stage. It could be due to an early pregnancy, an ectopic pregnancy (where the fertilized egg implants outside of the uterus), or even an error during the scan itself. To get a clearer picture, your doctor may recommend additional tests such as blood work or another ultrasound after a few weeks. These tests will help determine if you are indeed pregnant and provide more information about the situation. It's essential to stay in close contact with your healthcare provider for guidance and support throughout this process.