LMP or Ultrasound due date

[jemae]

My doctor gave me an due date of April 6th based on my LMP but my ultrasound gave me an due date of April 1st. In order to get an more accurate ovulation date which due date should I go by? Are ultrasounds always accurate to calculate ovulation or are ultrasounds more based on the babys size? My phone apps give me an due date the same as my doctor which would give me an ovulation date of July 14th but if I calculate my ovulation date from ultrasound due date that would give me an ovulation date of July 9th. Which would be an more accurate way to determine ovulation?  

[jemae]

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!

[AnnieBrooke]

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.

[jemae]

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?

[AnnieBrooke]

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?