On February 6, I had unprotected sex with ejaculation outside. On February 19, I had my period (my period is relatively regular). On March 13, I had unprotected sex again but couldn't pull out in time, so I thought there was some semen left inside. On March 19, I saw a little blood, thought it was menstrual blood, but then it disappeared. I bought a pregnancy test and saw a faint second line. Doctor, can you tell me how to calculate the age of the fetus? Because I didn't think that from March 13 to March 19, the test would show two lines. If it was due to the sex on February 6, will it affect me if I take antibiotics or have sex for a month afterwards?