Am confused. Please help me understand...

[breadandhoney]

...how due dates are calculated.

First day of my last period was 11th March. GP and MN due date calculator have given me due date of 16th December. This is 40 weeks from first day of last period, but isn't pregnancy itself 40 weeks? Shouldn't the 40 weeks start from the date of conception (roughly 25th March)?

When I was pregnant with DD my dating scan put me two weeks behind where GP had me. Is this happening again or am I being thick?

According to NHS direct pregnancy last anywhere between 37 - 42 weeks. Your due date is roughly in the middle of this (40 weeks). That's where they get the date from.

I'm told that in France they use 42 weeks to calculate due date.

Always known it as 40 weeks from date of last period - ie 38 weeks from conception or thereabouts!

Pregnancy starts on the first day of your last period. Thats how the MW explained it to me when I had DS1.

So your due date is roughly 40 weeks from the first day of your last period.

It is 40 weeks from the first day of your last period, so roughly 38 weeks from ovulation. The egg that is released at ovulation which then becomes the embryo starts developing from day one of your cycle and is released at roughly 14 days and so your pregnancy is technically dated from two weeks before the sperm meets this developing egg

Your pregnancy isn't dated from conception because not all women ovulate at the exact same point after thier period, it would be too confusing to give all women a custom calculated due date so all new pregnancies are dated from the first day of a new cycle. Your offical due date will be calculated from the dating scan which can put you forward or back a few days/weeks.

Thank you all for your replies. I had always known that due dates were approximate but didn't realise there was so much leeway. I'll see what date i am given at the dating scan and take it from there. I'm not hung up on having a definite due date ( which is good I suppose since that's kind of impossible!) I was just wondering how it all worked. So pregnancy starts when egg is released but no way to tell exactly when that is so they calculate it from first day of last period?

I'm not keen on induction either puddlejumper. As frustrating as it is for baby to be 'late', it will come when it is fully cooked ready.

So, if 2 mummies-to-be conceived on the 21st of the month, but for one their cycle day 1 fell on the 1st of the month and so they ovulated on day 21, and the other started their cycle on day 7 and so ovulated on day 14, would one be 2 weeks pregnant and the other three weeks pregnant? And would their due dates then be a week apart despite conceiving on the same day?

Sorry for the confusing scenario, just trying to wrap my head around how it's worked!
XxX

crumm yes, that's correct, but when you go for 12 week scan they adjust due date if necessary based on the size of baby & in your theoretical example if both babies grew at the same rate they would both end up with the same due date after scan. I had very long & irregular cycles & was originally date according to first day of last missed period with dd of 8th May, after scan was changed to 15th May. Only 5% of babies are born on due date so really not worth thinking about too much when you have 5 weeks of being full term at the end :o

With dd I was originally given due date of 19th June and it was changed at dating scan to 3rd July. She arrived on 7th. I am taking this to mean I possibly tend to ovulate late in my cycle and perhaps my due date will be changed this time to later in december. Just hope I don't end up in hospital over Christmas. For dd's sake. But only time will tell. And what a Christmas present!

Wow! How do you (and he) find him having a Christmas birthday? If you don't mind me asking.

Hi breadandhoney. Looks like we're have had the same cycle. My first day of my last period was 11th march so like u have been Told edd 16th December. Will be interesting to see at dating scan what date I get told and if it's similar or totally different to urs!! Have to let me know.

Seems like ages away- on we wait
X

Bread and honey, I had the same situation, I knew exactly ehn I ovulated (very late in long cycle) and midwife dated me on LMP. Due date was pushed back by 10 days after my 12 week scan which matched my calculations.

If you have a long cycle and know when you ovulated, count 38 weeks from that date and you will have a better answer.

joy dd arrived on Christmas day 2 years ago. So far we have done birthday treats and tea the weekend before to split up the presents, she's too young to notice.

mytime congratulations! how exciting that we have the same due date (for now). Have you got a date for your scan yet? I haven't heard from hospital yet but last time they were scanning early, around 9 weeks. They put me at 7 weeks at that scan and I had to go back again.

blacktea and joyful a christmas birthday would definitely be interesting, and perhaps a bit of a challenge to come with creative ways to celebrate both. DH and I both have December birthdays too so, all being well, it will be a crowded month but in a lovely way!

breadandhoney not had my date for scan yet! I've only got my date for booking in which is 9th may. Should I have had my scan date booked by now? My drs don't seem to know what they are doing!
I thought that perhaps they book it when I am there or after my booking in appointment?

I think it depends on what part of the country you're in. My booking in appointment and dating scan were done together with dd. Will this be your first baby?

Yeah this will be my first. I need to find out then I guess as my oh can't make the booking in appointment but will definitely need to be if it's the scan so may have to change it.
[calling drs surgery]

My MW phoned up for my scan whilst I was at my booking in appointment. Booking in done at 8 weeks, scan at 12 weeks.

Thanks black.
I just called and she said the me will book it when I'm there for 12 week mark!

On another note me trying not to get carried away as its still early days, hadn't worked out how far along I'll be when we go away! I thought I'd be about 4 months then having counted the weeks I'll be about 20 weeks. I didn't think Id have much I a bump when we go but I think i will now!!!
( those wedges I bought for the hol will have to go back in wardrobe until next year lol)

Where are you going? I went to thailand at 20 weeks and it was fine. I was in my normal bikini but with a bump!

That's good to hear then
We're going to Croatia :-)