When you find out your pregnant how many weeks do go back to when got pregnant?

When calculating pregnancy, doctors refer to gestational age which starts from the beginning of the last menstrual period (LMP). This means that by the time woman finds out she is pregnant she may count around four weeks, but in reality be only two weeks pregnant according to medical calculations of gestation age since conception. Pregnancy generally last up to around 40 weeks although many are born between 37 and 42 weeks of gestation age after conception .