A patient presents for her first initial OB visit after performing a home pregnancy test and gives a last menstrual period of about 8 weeks ago. She says she is not entirely sure of her dates, however, because she has a long history of irregular menses. Which of the following is the most accurate way of dating the pregnancy?

Measurement of the fetal crown-rump length is the most accurate means of estimating gestational age. In the first trimester, this ultrasound measurement is accurate to within 3 to 5 days. Estimating the uterine size on physical exam can result in an error of 1 to 2 weeks in the first trimester. Quantification of serum HCG cannot be used to determine gestational age because at any gestational age the HCG number can vary widely in normal pregnancies. A single serum progesterone level cannot be used to date a pregnancy; however, it can be used to establish that an early pregnancy is developing normally. Serum progesterone levels less than 5 ng/mL usually indicate a nonviable pregnancy, while levels greater than 25 ng/mL indicate a normal intrauterine pregnancy. Progesterone levels in conjunction with quantitative HCG levels are often used to determine the presence of an ectopic pregnancy.