Your estimated due date is:
How is your pregnancy due date calculated?
To calculate your pregnancy due date, your health care provider will use the date your last period started, add seven days and count back three months. He or she could also simply count ahead 40 weeks from the start of your last period. When your baby arrives, it will have been about 38 weeks since he or she was conceived — but your pregnancy will have officially lasted 40 weeks.
Estimating your pregnancy due date is an important part of your prenatal care, but there are no guarantees. It's just as normal to deliver your baby a week or two before — or after — your due date.