Due dates - how are they calculated exactly?

[Piglet's Mama]

My due date is 16 Aug which works out to be 38w from conception (I know when I ov'd the month I got my BFP) but is it that it could be anytime between 37w (10 Aug) and 42w (14 Sept)? 40w takes me to 31 Aug & I would have thought baby is more likely to come at this point? Any ideas?

Xx

 

[flopsybunny]

I asked a similar question the other day in the thread shauna put up about going over due. I know I ovulated on the 15th of September and DH and I only dtd on the 15th and 16th of that month. Now I read in one of my books by Miriam Stoppard that the "average pregnancy is 266 days from conception or 280 days from your LMP. 280 days from LMP iss average and depends on you having a regular 28 day cycle - all doctors are prepared to say is anything from 38 to 42 weeks may be considered normal".

That confused me slightly because my LMP was 8th september and I ovulated only a week later, so, I could (if calcualted by conception date be due 38 weeks from 15th September, which is...8th June! Only a week before my due date from LMP.

I have really confused myself with this now! Lol! Sorry if I have confused you too.

Xx

 

[nmf1987]

sorry can't be any help but am interested to know myself. im sure i saw somewhere that they take it as 2 weeks after the 1st day of your last cycle or something. xxx

 

[spice]

as far as i understand it they are worked out from the first day of your last period! well that's what it says at the top of all my scan sheets . . . the date of the first day of my last period and my cycle length! so not from conception, so from your ovulation date it would be about 2 weeks less than from the first day of your last period ie the 38 weeks you said! so 40 weeks would be from your first day of your last period!

hope that helps and makes sense!!!!

 

[flopsybunny]

This confusion I have got myself into is bugging me! The cycle I fell pregnant would have been a 21 day cycle, not a 28 day cycle. How would that affect things? Because 8th of June, which is 266 days after I ovulated isn't my 38th week, its my 39th as my LMP was 8th September. I have had the calender out and got into even more of a muddle!

Gah!! She'll arrive when she wants to!

Xx

 

[Piglet's Mama]

Ahhhh - too much! Lol!

My periods were 50+ days so I'm reluctant to take them into consideration, esp as I know when my egg popped! Hmmm... Might have to get on trusty google, will update if I find any answers x

 

[spice]

i found this to be quite useful at the beginning

http://www.bbc.co.uk/health/tools/pregnancy_calendar/

your cycle length is supposed to be an average, my midwife said to add up as many as i could remember and divided by the number to get the average length! which was quite easy as i was marking my periods in my calender! i'd say the last 3 or 4 cycles is a good amount to give a good average cycle length!

 

[flopsybunny]

I'm not going to think about it anymore. It hurts my brain! Lol! I have been told I could go at anytime now anyway due to having a show and pains so I am just taking it a day at a time.

Xx

 

[snr1982]

i've no idea, and when i tried to work it out it didn't make much sense to me.. so gave up. i'm interested to see what happens though as my origninal due date they gave me was the 3rd of June, which would make more sense to me than the one i have now.

 

[Piglet's Mama]

I just found this:

http://pregnancy.about.com/od/gettingpregnant/p/duedate.htm

266 days from Conception - 38 weeks. My due date is 38 weeks exactly - sorted!

Flopsy - in theory, your due date should be 266 days from 15 September x

 

[Piglet's Mama]

i found this to be quite useful at the beginning

http://www.bbc.co.uk/health/tools/pregnancy_calendar/

your cycle length is supposed to be an average, my midwife said to add up as many as i could remember and divided by the number to get the average length! which was quite easy as i was marking my periods in my calender! i'd say the last 3 or 4 cycles is a good amount to give a good average cycle length!

This gives me the same due date too - it must work it out on 266 days x

 

[hopefulmummy]

It's 40 weeks taken from the first day of your last period as everyone has different cycle lengths and they can't know when you ovulated or conceived. However, often if you have an early scan they can figure that out and get the date more accurate. That's why due dates sometimes change. Mine went from the 17th based on my lmp to the 24th to the 22nd based on early scans. Xx

 

[Constantstar]

It's confusing cause if I went by my lmp I would be due a week earlier but when they scanned me Ive been moved so now my due date is a week later . I guess just coz u ovulated on a certain day doesn't account for when the swimmers reached the egg or when the egg settled into place etc x