[Kiwi]

Disclaimer: Not pregnant! Just a question I was going to ask when I was pregnant last year, but by the time I remembered to post it, I was suffering from morning sickness so bad that I didn't care any more!!

So... for an example... if I was pregnant this cycle, and used the JM calculator to work out my due date, with the following information:
First day of last period: 21st January 2010
Cycle length (can vary, but for this example we'll use): 26 days
Luteal Phase (again, can vary, but for this example we'll use): 11 days

...then clicked 'calculate from period', I get a due date of 29th October,

...but if I use 'calculate from conception' I get a due date of 14th October...

So, my question - why are they so different? Is there something really obvious I'm missing here?

[Chelnae6]

The difference in your Cycle length (26) and Luteal Phase (11) is 15 days. Meaning that you O'ed on CD15.

If they calculate from the 1st day of your last period being on 1/21/10, then you'll be due on 10/29/10. But if you put in your date of conception of 1/21/10, then you'll be due 10/14/10 because the start of your period wouldve been 15 days earlier.

Most docs go by your last period, some docs go by your date of conception.

dont forget that the 21st Jan 2010 date is either your date of last menstrual period OR conception date depending on which calculation option you choose.

[LisanAndy]

I have 35 days cycles on average. I have a 14 day LP. I go by conception calculators because of this. For example, this cycle my period started Jan 25th and I'm Oing today or tomorrow, so CD 21 or 22. I put in my conception date as CD 21 and it tells me I'd be due November 7th

[*krista*]

The way I see it, there should really only be 1 day difference. If you go by LMP, they calculate a conception date at CD 14, and yours would have been around CD 15.

I've never heard of starting the count from the day of conception. I go by conception date, but I start the count two weeks before conception date. In my case I conceived around CD 19. Doc wants to go off of LMP, so she conciders my due date May 28th instead of what I concider my due date going off of conception date, June 2nd. She is basically going off a conception date of the "typical" CD 14.

[~*sierra*~]

i think chelle got it right when she said that when you switch the date type from LMP to conception you are essentially saying either you started your period on that date you put in OR thats the day that sperm met egg, which would be around 2 weeks difference. So, like as lisa said she puts in her conception date since hers is just wayyy off from the normal Oing on CD14 and she knows it, which would give her a way more accurate EDD than using the LMP method. But for someone like me, who has 28 day cycles pretty much all the time and O's on around CD15-16, my dates would only be a few days difference no matter if i went by LMP or conception date, so for me i could use either method to calculate it.

disclaimer: i have no idea if the above made any sense! still recovering from the buck and doe last night!

[Mars]

I think Chelle got it.