Hello, I was prescribed bactrim several days ago for a bad sinus infection. My husband and I are trying to conceive, and the prescribing doctor told me to call her if my period did not come so that she could prescribe a different medication. I am now about 2-3 days late, but I have taken 2 at-home pregnancy tests, both of which came up negative. I normally have an extremely regular 26-29 day cycle, and this would be my third pregnancy. I m 35. So I guess I am wondering if 1) the bactrim could be giving me a false negative somehow, and 2) if my age or # of pregnancies could play into getting a false negative? I need to kick this sinus infection, but should I call the doctor to have them prescribe something else and get a blood test?