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@mrsjyw: hopefully someone else will have a good explanation for you, but I can tell you my timing. I also had longer cycles and ovulated later. My LMP was 1/12, O date around cycle day 20 or 21, I had an ultra sound at what I thought was 7 weeks, but the babe measured 6w2d, so because I my later ovulation date I couldn't really go by LMP... It was a week later basically --- do you have a dating ultrasound planned? Then they will be able to measure and tell you for sure!

have you used the online calculators using your ovulation date since you know it?

I have long cycle also sometimes with 40+ days. The doctor calculate my due date from sonogram measurements. My 1st one LMP was 2/1 and due date was 11/16. My 2nd one LMP was 3/6 and my due date was 12/17.

mamabolt said it best.

since you were actively trying and know your O date/conception date, go by that (e.g. baby center has two calculators, one based on LMP and one based on O date. use the O date one). if your O date is accurate, a dating ultrasound will probably reflect the same or a very similar due date.

If you have longer cycles going off your ovulation date will be more accurate. Counting from your LMP assumes you ovulated on day 14. Use the baby center calculator and go off date of conception and put your ovulation date in there. From the date you ovulated, you're already considered two weeks pregnant. Count forward 38 weeks from your ovulation date, and you get your due date.

For example, for me my LMP was Jan 9th which gives me a EDD of 10/15. But I know I ovulated around Feb 2nd which gives me a EDD of 10/25. I went off my 10/25 due date for how far along I am. Your doc will go off your LMP until you have a dating U/S. At my first U/S, I measured exactly how far along I thought I was going off my ovulation date. Going by my LMP I should've been 8w2d but baby measured 6w6d. Gosh I hope what I wrote makes sense.

@mrsjyw: I agree with MRSKC. I have longer cycles too, I O'd on CD27 of the cycle where we conceived. So my LMP was January 2nd but my O date was January 28th. I used an online calculator and it calculated me as being 2 weeks pregnant on the 28th and went from there.

This due date calculator has the date you conceived option. I entered my O date there and it calculated my due date based on that (since LMP wouldn't be accurate) http://www.babycenter.com/pregnancy-due-date-calculator.

I have 40-60 day cycles, so until my 12 week ultrasound that dated my pregnancy, I calculated that I was 2 weeks at the time I believe I ovulated. I was only 3 days off