A 4-week ultrasound is not visible when you are 4 weeks pregnant. The 4-week period is the 4th week from the first day of the last menstrual period. At this time, the fertilized egg has just completed the process of implantation and the gestational sac has not yet formed, so it is not visible during the ultrasound examination. In the 5th week of pregnancy, the gestational sac will be visible, and a small gestational sac will be visible on ultrasound, but the gestational bud and heartbeat will not be visible at this time. After the 6th week of pregnancy, the development of the gestational sac and the pulsation of the heart can be seen, which is a key indicator of normal embryonic development in the early stages of pregnancy.