It is generally expected to see a visible heartbeat on ultrasound by 6 weeks of pregnancy. However, there can be various reasons why no cardiac activity is seen at this stage. It could be due to an early pregnancy, inaccurate dating, or potential issues with the embryo's development. In some cases, it may indicate a miscarriage or ectopic pregnancy. To get a clearer understanding, your healthcare provider may recommend follow-up ultrasounds and blood tests. It is important to consult with your doctor for further evaluation and guidance based on your specific situation.