It is not uncommon for an ultrasound to not show signs of pregnancy at 6 weeks. At this early stage, the embryo may be too small to detect or the positioning of the uterus may make it difficult to visualize. Other factors such as incorrect timing or a low-quality ultrasound machine can also contribute to the lack of visibility. If you are concerned, it is best to consult with your healthcare provider who can provide further guidance and possibly schedule a follow-up ultrasound if necessary.