I know i'm not the one you asked this to, but when I was pregnant, I asked the Dr. How acurate the ultrasound is and they said it could be a week eaithr way, so more likely it was the 31st, because I don't think you ovulate that quickly after a period, especially if your periods are regular, so when they do the ultrasound it might say the same thing or be a little different, but I would think it was more likely to be the 31st. Hope I helped