How early can you get a pregnancy test after ovulation?

I'm pretty sure I know when I ovulated and my husband and I had sex the day before and the day of. My ob/gyn prescribed me Provera because I haven't had a period in 3 months. Taken blood and urine tests, all negative. I will take the Provera to help me shed the old uterine lining and to keep things "clean" down there so to say, but don't want to take it until I'm sure I'm not pregnant. I've heard horror stories about women being pregnant and taking it, only to find out later that they lost the baby because the meds made them miscarry. Anyone with any information? Thanks.