Ahhhhh

I had my last baby in December. I’m very sure I don’t want any more seeing that was baby #3. My husband and I have been very careful, tracking my ovulation/fertile days and using condoms. I’m suppose to get a IUD put it but we’re currently waiting on the prescription to get to the doctors office. Well in the heat of the moment, we had unprotected sex, on what was suppose to be the 1st day of my period. I’m currently 2 days late, which is weird, but I know we was careful except that one time. Can you or has anyone ever gotten pregnant having sex on the day of your expected period?