MAT1B - Change of due date? Is due date changed when scanned?

[LavenderInTheField]

Hi all
So I understand that we calculate our due date based on first day of last period and cycle length, 3 questions on this:

1. At the 12 week or 20 week scan, if you are measuring ahead or behind, does the midwife change your due date?

2. Does this then become the new official date to calculate the 15 weeks before birth to tell your employer for maternity leave?

3. If you have a longer cycle, 33 days instead of 28 days, is it best to tell the booking midwife this? As the cycle length does change the due date on the NHS due date calculator. The midwife didn't seem bothered and just took it based on 28 days.

Thanks!

1. 12 week scan yes I think so, 20’week I think not.
2. I think so
3. yes probably. My first booking apt they asked me, the second one they didn’t. I suspect my due date will be about a week out at my 12 week scan as a result but we will see.

Yes, yes and up to you.

The MATB1 isn’t usually issued until about 24-28 weeks so will go off the due date given at the scan.

In respect of question 1 - my pregnancy is an IVF one so I know the exact date of implantation, and I know exactly what my due date is based on that, but at my 12w scan I was measuring 3 days ahead and so they changed my due date 🤷🏻‍♀️ Because I need a C section this has really annoyed me, because they’ll take me in a tad too early and so I’m trying to push for them to start listening to my “actual” due date

It'll be set by gestation at your 12-week scan; even if the date they predict doesn't line up at all with your cycle/possible ovulation dates; and it won't change from that date.

Your period is used to inform when your 12-week scan is booked.

Your 12-week scan overrules the cycle dates; and will be used everywhere, for your MATB1, your 20 week scan date, etc.

They won't.

DS had a bizarre 12-week scan. We knew exactly when he was conceived. His 12-week scan made him younger. He'd caught up by 20 weeks, and we had to endure weekly and then daily scans to check on his growth and he'd grown so big. The sonographers all agreed that it was purely that he was back to measuring his original date... But there is no moving it from the 12-week scan. That is gospel, even if it's wrong.

Yep, 12-week-scan is what they will use and your cycle dates become obsolete from then on.

Yes im also wondering about this. Im waiting for my booking appointment but i think i ovulated on day 22 which would put me a whole week behind LMP dates. Dd1 was born at 38w so dont want this ones dates to be wrong from the start! 🥴 If using the 12w scan dates, i hope it should correct things?

@VillageFete i wouldn’t get too set on the dates. Some women would prefer to think about due months rather than specific dates.

That aside, baby’s growth rate is not universal. Baby’s grow and develop at different rates. If you think about it, spontaneous vaginal births don’t all happen bang on 40 weeks. Anything from 37 weeks is considered term.

A planned section will usually take place at 39 weeks unless there’s a strong reason to do it earlier.

As PP have said, you're given an Estimated Due date at your '12 week' scan and that doesn't change.

Thanks all!