Empty embryo sac

Hello, my name is Mili. I just finished 8 weeks of my pregnancy today from first day of last LMP(26th july). However, my doctor couldn't find any embryo in the sac after 6 weeks ultrasound, 7 weeks ultrasound, and 7 weeks and 5 days ultrasound. However, my beta hcg was 256 after 4 weeks, 2886 after 5 weeks, and 16000 after 7 weeks and 4 days. He had told me to start progesterone suppositories from 10th August and I had continued that till friday, 18th september. He says that i should go ahead and take cytotec to abort the pregnancy, however, I haven't had any bleeding after stopping the progesterone for almost three days. Should I wait another week and get another beta hcg test done meanwhile, or go ahead and take the cytotec? Also, I don't understand why they count the pregnancy period from first day of last LMP. Doesn't pregancy start when conception occurs, i.e. when egg and sperm meet? So, technically I just finished 6 weeks of pregnancy.