It depends. Calculating by your first day of your last period is not too reliable, since women's cycles can vary. But if your cycle is always exactly the same length, it might work. Using the measurement of the embryo from an ultrasound works well, IF IF IF you do it early in the pregnancy. In other words, fifth week, sixth week or seventh week. But if you say "On the one hand I've got my LMP calculation and on the other an ultrasound in my 18th week," well, either would be about as good a guess as the other. How early was your ultrasound?
My ultrasound was at 12 weeks, and my cycle is about 30 to 31 days apart. I started my period in May on the 31st and I started my period in June on the 30th. I was reading on *****************.org ultrasounds can be 5-7 days off but on other websites I was reading they said they were accurate. I'm just really concerned with the fact which one my LMP or ultrasound would give me an better way to determine conception. However, since my cycles are so long and the average is day 14th to ovulate that would mean I ovulated around the 15th, 16th, which would conclude I couldnt have gotten pregnant throughout the week of July 7-12th?
One surprising thing I have learned from MedHelp's information pages is that no matter how long a woman's cycle is, her ovulation is always two weeks before the first day of her next period. Not 14 days after the first day of the last period. That means a woman with a 30- or 31-day cycle would ovulate on day 16 or 17, not day 14. I don't know if this helps or further muddies the waters.
That helps a lot because that explains the fact that I couldn't have ovulated the week of the 7-12th. Thank you for all your help and suggestions!