If an ultrasound gives you a date of how far along you are is it possible you could have been pregnant 5 weeks ago and still had your period and the ultrasound be off?

The ultrasound doesnt give you the date. It is estimated by the days you give to the doctors to help, narrow down the date. The first day of your last period and how long you've missed help them in determining your conception date. The picture shows them an image of a fetus and they use that picture to say at what developmental stage the baby is in. Anything is possible.

AnswerContrary to the opinion above, the ultaasound does give you an approximate date of conception. By comparing the results of millions of ultrasounds physicians can calculate the age of the fetus based on head diameter, bone length, and other measurements on the fetus. They are usually accurate to within two weeks of when you are due to deliver. Since some women have irregular periods, it is not always possible to calculate the date based on the first day of the last period. Trust the ultrasound!

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I just wanted to add to the above answer. A patient's Last mentrual period is accurate if the patient has a regular mestrual cycle, but this is not the case for many women. Therefore, it is not the most accurate source for fetal age. Ultrasound is the most accurate dating tool for fetal age. The measurement of the embryo (crown rump length) up to 12 weeks gestation is the most accurate. The rate of growth of a fetus will be based on that early measurement for the remainder of the pregnancy. It allows physicians to monitor the growth of the fetus which in turn gives them an estimated due date as well as help them diagnose growth abnormalities such as IUGR (Intrauteriune growth restriction).