I don't remember my lmp. according to ultrasound my due date is oct 10th...around what time did i concieve?

When was scan done?: Accuracy of ultrasound varies from 3 days to 2-3 weeks depending on when it was done. Plus the date of sex can be around not always on the suspected date of conception. The date of pregnancy test and size of the uterus during first pelvic can also help determine the approximate date. So, to avoid misunderstanding, it is best to ask the OB performing the first pelvic and ultrasound if mid-january.