If your periods are regular and you’ve been trying to conceive, you likely know when you became pregnant. However, if the pregnancy was a surprise, or you have irregular periods the answer might not be as obvious.
Your baby’s due date is calculated by adding 280 days to the first day of your last menstrual period. So technically, on the day that you conceive (somewhere around the 14th day of an average cycle) you are already two weeks into a 40-week pregnancy! This can be confusing, especially if your doctor refers to you as “6 weeks pregnant” when you know for sure that you only conceived four weeks before. The truth is, you are both right.
If there is real uncertainty about how far along you are, an ultrasound can provide a pretty accurate answer.
I hope that helps, please post any questions you might have!