This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Diagnosis in obstetrics, Subject name: Gynaecology & Obstetrics
A patient present for her first antenatal 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? 
Select and explain. 
A. Determination of uterine size on pelvic examination, B. Quantitative serum hCG levels, C. Crown rump length on transabdominal or transvaginal ultrasonography, D. Determination of progesterone level along with serum hCG level
C: Crown rump length on transabdominal or transvaginal ultrasonography
Answer: C: Crown rump length on transabdominal or transvaginal ultrasonography 
Explanation: Dating of pregnancy crown rump length is measured from the top of the head to the bottom of the buttocks excluding limbs CRL between 9 and 11 weeks is the most accurate method of dating CRL in cm + 6.5 gives a rough estimate of gestational age in weeks refer pgno:476 Sheila textbook of obstetrics 2 nd edition