Talk to Gynecologist on Difference Between Lmp Date And Ultrasound Date And CRL

my last periods date is 15 September i do beta hcg at 27 October which is 348.42 .i go for scan but nothing show in utras .i repeted beta hcg at 31 October which is 1670.11. i again do transabdominal ultrasound which show only gestational sac 7.7mm to 4 _5 weeks pregnancy. no fetal pol show. doctor prescribed me folic acid 5mg ,duphaston twice a day and asprin. after 3 days i face slight bloody brown discharge .i go to the hospital doctor said stop asprin and she prescribed me biogest injection 100mg twice a week for 12 weeks of pregnancy. after this i again repeated my beta hcg at 9 November its 20278.21 . i repeated my scan at 11 November which show 6 weeks pregnancy with 23.5 mm gestational sac .a tiny small fetal pol and pulsation 125bmp .Crl is 2.7 mm corresponding to (5 weeks & 3 days )+_ 2 days . My question is that why these are different from each other like lmp is 15 September according this its 8 week start but according ultrasound 6 week but crl show 5 weeks and 3 day . i