Your Patient Comes for Her First Prenatal Visit on March

Your patient comes for her first prenatal visit on March 4. She is sure of her last menstrual period (LMP) which was January 1, giving her an estimated gestational age of 9 weeks today. She is worried because she was taking oral contraceptives when she conceived. When reviewing her medical record you note that she was seen in the Emergency Department for nausea and vomiting and had an ultrasound done at that time where the crown-rump length (CRL) gave her a gestational age of 8 weeks (3 days different from her LMP date) . On her bimanual exam at today's visit her uterine size is 10 weeks. Which of the following would you use to establish a best estimate of the delivery date (EDD) .

A) Last menstrual period
B) Ultrasound
C) Uterine size