Hello doc, my last period started august 13th and my cycle is 40 days long. i had protected sex on the 15th of september.ive had 4 blood pregnancy tests on 7 days, 14 days, 21 days and 27 days after said protected sex, all negative.what do you think?

Would repeat the blood HCG test this week and consider an ultrasound next week if negative. May be stress but have to rule out pregnancy.

It at 4 weeks after the episode of sex in question the blood test is negative for pregnancy, it is unlikely that you are pregnant. You may do a home pregnancy test in a week or so, unless you get a period by then. Wish you good health!