Could I have gotten pregnant before my ultrasound LMP? I had a one night stand on February first, then had my period February 5, and it ended on the eleventh. I have normal periods. I got back with my boyfriend and we were together on February twelfth and eighteenth. On my ultrasound, it puts my LMP as the fifth, and that makes my conception date roughly the nineteenth. However, the one night stand has come out of the woodwork and started demanding a paternity test. If my child was his, would the LMP go back to January 18? How worried should I be?

While it is possible, it is unlikely that the baby would be from the one night stand for a couple of reasons. You had a regular period afterwards, and it would be very unlikely for you to have ovulated just a few days before a period.

The ultrasound, while not an exact science, is the best tool for dating a pregnancy early on (determined by what is seen on the scan and measurements taken of baby). Towards the end of pregnancy, they are not as accurate for dating or sizing purposes.

To know for certain, a paternity test would be necessary.

However, Doula, food for thought here.

She had the one night stand on Feb 1 and her period on Feb 5. That is roughly 4 days for the sperm to reach the egg then travel back down to implant in the uterus. That meaning that she still could have had the regular period prior to implantation. While uncommon, it is known to happen.