Yes. Early or 1st trimester ultrasound is generally an accurate way to date pregnancy. However, the further along the pregnancy is, the less accurate the dates.
Yes. Sonograms (ultrasounds) are very accurate, to within a few days, in the first trimester. They are accurate to within about two weeks for the second trimester. They are accurate to within about 3 weeks in the third trimester. Note that your due date will not change once established. By accurate here, I mean "able to accurately determine gestational age (how far along you are) without knowing lmp.
Yes. Early ultrasound dating by crown-rump length is one of the most reliable pregnancy dating techniques that we have. That being said- it is not perfect and there is still a margin of error of +/- several days. Pregnancy dating is not an exact science; we frequently refer to pregnancy dating as the "best obstetrical estimate." ultrasounds after the first trimester are less reliable dating tools.
Yes. An ultrasound in the first three months of a pregnancy gives the most accurate due date. As you get further from the first trimester the accuracy decreases. In the third trimester ultrasounds can be as much as three weeks less accurate than a reliable last menstrual period.
Yes. Serial sonograms of about 3 weeks apart are extremely accurate for dating especially in the early pregnancy.
Yes. Sonograms very accurately measure gestational dates early in the pregnancy. 7 week babies are all roughly the same size so the measurements predicts age well. Later in pregnancy babies growth starts to individualize. It becomes less accurate to predict the gestational age based on their measurements. As a general rule, the earlier the sonogram the more accurate the predicted due date.
Yes. As a general rule, ultrasounds done in the first trimester are accurate to within a week. Ultrasounds done in the second trimester are accurate to within two weeks. Ultrasounds done in the third trimester can be off by three weeks. So ultrasounds done in the first trimester are very accurate.