First Trimester Ultrasound Accuracy: What to Expect

Question
thank you so much for your answer to my date of conception. you were right on. my doctor also says my first sonagram is the most accurate way to find out a lot of things such as due date, lmp, and such...that's cool! so the question is...the first sonagram/ultrasound is pretty accurate right? thank you, kristi

Answer
It depends on when you took the first sonogram as to its accuracy. A sonogram that is taken before 9 weeks gestation (from the first day of your last period) has an error of + or - three weeks. A sonogram taken between 12-20 weeks has an error of + or - two weeks. A sonogram that is taken after 25 weeks is completely inaccurate and cannot be used for dating purposes. Usually, we use the sonogram at 16-20 weeks as the most accurate.