to plan my due date around my LMP and not the scan?

[SadieJey]

I KNOW when me and DH conceives. We only had sex once in that month as I wasn't well. I know it's unlikely that any sperm was still alive for more than 5 days so fertilisation must have happened in this time. My 12 week scan put me two weeks behind this which just isn't possible (he was away and we hadn't had sex for 2 months before this).

AIBU to plan my leave and due date around my calculations? Or are the scans a better indicator?

The dates they have got for me indicate that we conceived two weeks before we even had sex...

You can have ‘your’ date - but the date that all the medical teams and your scans and appointments will be based on will be your dating scan. The date they give you will be the one they work from even if you disagree - sorry! It’s based on measurement as all babies grow at approx the same rate till around 14 weeks.

And I know babies don't always arrive on their due dates but in regards to sorting out my maternity leave at work, I feel like I'm leaving a bit too late at 37 weeks (by my calculations I will be over 39 weeks).

@Since2016 thanks. I'm talking more from a maternity leave point of view etc rather than a medical view. I'm not expecting them to change anything on my notes

pregnancy is dated from the first day of your last period, so technically you are 2 weeks pregnant before you have conceived! 40 weeks is calculated from the first day of your last period which would have been about 2 weeks before you conceived so their dates are actually saying the same as you.

Is this your first baby? Most firsts are late and depending on the trust, most wont induce till you’re over your due date so you may find you’ll be closer to your ‘actual’ date anyway.

@NannyOldElf I've taken that in to account. My dates give me a due date which is 16 days different to the one they have suggested. I've included the two weeks.

@Since2016 it is my first yes

So they are saying you are 14 weeks but you think you are 12 for example?

Sorry this is probably stating the obvious but pregnancy dating does sort of add 2 weeks on because the date goes from LMP (well, assuming 28 day cycle) so eg you are 12 weeks pg 10 weeks after you’ve conceived.

In any case, babies come when they are ready so it’s probably best to think of a month long window anyway and hope baby arrives somewhere in that!

And re leave, just play it by ear. Both times I planned to go fairly late but knew that as mat leave would be triggered if I was off any time from 36 weeks I could always bring it forward if needed. First time I was fine working til 39 weeks, second time I stopped at 35 and could have done with it being a lot earlier! It’s impossible to tell.

You can take maternity leave from 29 weeks so your or 'their' due date doesn't really need to be a factor?

I had IVF (so know the exact day and rough time even of conception) and they still pushed my due date forward 3 days after a scan. It doesn't really make any difference in the end.

My first dating scan was wrong too, like you I knew the date they gave me couldn't be correct. But the second scan at 20 weeks gave them a different date that matched mine, and it was then changed on my medical records

most firsts are late

This is a common misconception that I wish people would stop peddling. MORE first babies are late than seconds etc BUT most of them will still be on time or early.

However I would go from scan dates - I had scans at 6,10 and 12 weeks and it’s very clear the difference in the foetus at those times - they will know from the development.

I don’t know how it works in any great detail but won’t your Mat1b have the due date that they’ve given you on it?
You can take your maternity leave from 29 weeks I believe so it’s up to you how many weeks to allow before the birth, but I don’t think you could actually tell your employer a different due date to the one the medical professionals will use.

I don't understand your OP. You're saying they are saying you're 2 weeks behind, but you conceived 2 weeks before they are saying you did? Pregnancy is always dated from your last period if it's a regular cycle, so say you're 12 weeks pregnant, the foetus is actually only 10 weeks old.

Would you be happy to share the dates OP?
It is a bit confusing with the predating by 2 weeks, going from lmp and theybuou knowing when you actually ovulated and or dtd.

It can leave a bit of a window

LMP
OV (if you know)
DTD
Estimated due Date from scan

My initial scan put me back by 3.5 weeks putting conception after I'd had a bfp and been to the GP for the scan referral. I said that repeatedly and that I'd been away on business on x dates so conception could only have been on Y date. Even got a consultant appointment but the registrar seemed to think I was worried about the baby being conceived while I was on business and not being my DH's. I could not get anyone interested that it just wasn't possible to get a bfp before conception.

So, at every scan and measuring the baby was "big for dates" well no shit sherlock. So I was tested for gestational diabetes - totally unnecessary. Extra growth scans. Of course I was not offered monitoring when I was overdue by my dates as I wasn't by theirs. And at birth there was meconium everywhere which is not unexpected for a 43 weeker but meant extra monitoring. The baby was put in danger and cost the NHS fuck knows how much more that it should have based on 1 very quick measurement at a 9 week scan which couldn't possibly have been wrong. Or was it?