Due date

[jessi1983]

I got confused with last mentrual period dates so my midwife never actually took the date of my last period and our due date went on what they saw on the 12 week scan,
Do you think I should go by last period?
If so my baby would be due Aug 21st as opposed to August 29th when midwife has said

Scan dates are more accurate, although not perfect.

It's a small difference in the context of things. From my LMP I was meant to be due 30th jan, but my scan said a week ahead so actually due 24 jan officially. The scan is a better judgement as it measures development and development rate therefore will predict more accurately. I wouldn't worry about it, I'm not! :) plus it just means your maternity exemption card lasts a week longer too ;)

The rate of development of the foetus in the first trimester is very similar for most pregnancies, therefore dating a pregnancy at 12 week scan is much more accurate than using LMP. That assumes that every women ovulated on day 14 and has a 28 day cycle, which is not the case.

Due dates are usually changed after your 12 week scan anyway as LMP is not accurate, it’s just a rough guide so they know when to do your dating scan.

No, you go by what the scan says.

I'm always waaaah further along than I think when I have my scans.

FWIW for me, by now (pregnancy 4) I've figured out it's because I ovulate early.
For this pregnancy we had sex once that month due to a painful ovarian cyst, and that was on day 7 of my cycle and the last day of my period. So to go from the first day of my last period to get my due date would have been automatically inaccurate.

Same goes for those who ovulate later than they think. Or those who have long cycles or those who have irregular cycles.

For all those reasons we need to get our due dates from the scan.

To be honest it makes very little difference anyway, the baby will come when it comes!

I'm not sure I agree, sometimes it can be the difference of a few weeks.

That means the difference of when you have tests, when you can have an amniocentesis if one is needed, the size of the baby (if there's a concern) not to mention if the baby is over due and the working of the placenta.
It's pretty important.

^ of course, however the OP hasn’t indicated that any of those issues are relevant in this case.
Regardless, the 12 week scan is often called the ‘dating scan’ for a reason... it is more accurate at dating the pregnancy than LMP. I ovulate late so LMP is useless for me.

And no dating method is 100% accurate, plus ‘full term’ is anything from 37-42 weeks.

You go by the first day of your last period until the scan date, and then they use that for the rest of your pregnancy.

So my EDD based on last period was 21/12 but then when I went for my scan they adjusted that to 17/12 (even though I was only dated at 1 day earlier than I thought I was)

Hope that makes sense!

The scan date is more accurate, LMP doesn’t really mean anything.

In addition to the reasons absolute has given above, it’s relevant when you’re full term. If you’re overdue the Drs will start discussing an induction in where case you’ll be glad of the extra week.

I have PCOS and going by LMP I would have been “due” a full week earlier than I actually am. EDD was adjusted by 7 days at the 12w Scan which actually lined up with my own calculations.