The body is an amazing thing--science and .Dr's think they have all the answers but they do not. There are no set facts in science. Every individual--is just that individual.
The reason they time the pregnancy from the last known menstrual cycle--is because it has happened at different times, for different plple, and in different pregnancy's. So they say we will count from menstrual cycle--cuz we dont exactly know when you ovulated or when you conceived, for everyone is different. We might be on a 28 day cycle and our eggs live for exactly 48 hours after it escapes the ovary. Then we would know within days of our ovulation and our conception day. But we may also never ovulate at the same time, we may expell numerous eggs thru out the month, we may have just come off of b/c and ovulated within days of that--and may not be anywhere near our regular cycle, we may have just miscarried or had a baby--and my ovulate within days of that also.
This is why they measure from our last know menstual cycle--because they do not know for sure. They say it is this way or that--but it is not true. "no set facts in science" especially with the .God factor--and that amazing human body.