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?
Determination of uterine size on pelvic examination Quantitative serum HCG level Crown-rump length on abdominal or vaginal ultrasound Determination of progesterone level along with serum HCG level