The due date for a pregnancy can be calculated using either the last menstrual period (LMP) or the conception date. Most healthcare providers use the LMP method because it's easier to track and more reliable for dating purposes. According to this method, your due date would be approximately 40 weeks from your LMP.
However, if you have accurate information about your conception date through an ultrasound, it can also be used to calculate your due date. In your case, if the ultrasound determined that your conception occurred on January 20th, then your estimated due date would be approximately 38 weeks from that day.
It's important to note that these are just estimates and not exact dates. Only about 5% of babies are born on their actual due dates. Your healthcare provider will monitor the progress of your pregnancy through regular check-ups and ultrasounds to ensure the health and well-being of both you and your baby.
If there are any concerns or discrepancies between the two methods of calculating the due date, it's best to discuss them with your healthcare provider who can provide personalized guidance based on your specific situation.