If your ultrasound was done on week 28, with a margin of /- 3 days, you can estimate your due date by counting forward from that point. Assuming a typical pregnancy lasts around 40 weeks, you can add approximately three months (12 weeks) to your ultrasound date. This would give you an estimated due date around week 40 or at the end of your seventh month. However, keep in mind that every pregnancy is unique and individual factors may affect the accuracy of this estimation. It's always best to consult with your healthcare provider for more precise information regarding your due date.