Dating scan due date uncertainty

[bowerpower83]

Hi All,

I am hoping for some thoughts about my 'due date'. I have a regular 26-28 day cycle and my due date as per the date of LMP should be 10th December. Prior to my dating scan I knew this would be slightly off as I ovulated slightly earlier in March. I had a positive OPK on day 12 (Sat 16th) and day 13 (Sat 17th) - from day 6 to day 11 I had negative OPKs. I also had a temperature drop on day 11 (Fri 15th) followed by a rise for 2 days on days 12 and 13. Day 14 was a drop for some reason and I didn't record any temperatures after that unfortunately. I also noticed EWCM on day 10 (Thurs 14th) which I took to mean ovulation was imminent as CM usually dries up following ovulation as I understand.

I expected my 12 week dating scan to say I was due on 7th or 8th December however they say I am due on 3rd December meaning I conceived on day 8 (when I had a negative OPK and low temperatures). I have tried to ask 2 midwifes about this as I was concerned I may be induced early however I keep getting told 'scan dates are more accurate as all babies grow at the same rate in the first trimester'. This makes absolutely no sense to me, particularly when Clearblue digital OPKs are supposed to be 99% accurate. Also, my positive Clearblue digital pregnancy test on day 25 (Fri 29th) said I conceived 1-2 weeks ago. If the scan date was correct it should have read 2-3 weeks since conception.

Has anyone else experienced this?

Thanks!

I find it all very confusing. I had the same thing with DS. I was sure about my dates and when I went for my 12 week scan which was exactly 12 weeks from LMP, they put me at 13.5 weeks. I kept asking if my baby had some sort of time machine but didn't get any answers.

This time round, I went for an early scan at 7 weeks LMP and they have said That the baby measures 6 weeks !?

It freaked me out and I am going again in a week for a rescan in case it means something is wrong. I have heard that babies grow at different rates but perhaps someone who is more knowledgeable can help?

Based on your OPKs, I'd say you ovulated on day 14, as a positive OPK means you'll ovulate in the next 24-48 hours. The temp drop on day 14 confirms that for me.
Now as to the other question; this statement that 'babies all grow at the same rate' in the first 12 weeks is bollocks!
With my first two I charted and used OPKs so I was very sure about dates. With both, I measured ahead at the 12w scan - 5 days/ 7 days. I tried to argue my point but no one would listen to me!
With my second that got me in trouble as I was trying for a VBAC, but according to their dates I went 2.5 weeks overdue (when really it was only 1.5 weeks) and the consultants were on my case all. the. time. Wanting to induce or book c-section. I said again that they had the dates wrong, but again got that stupid statement that the scan is always right! I even asked the consultant if she believed in immaculate conception as according to their date, I had conceived 1 day after my period stopped and before we started having sex that cycle.
Anyway, I got my way but it meant constant fights and hassle.
If you can, see if you can talk to your midwife and explain and see if you can somehow get the official due date changed.

Thanks for replies, it's interesting to hear other thoughts and experiences!

I meant to test opk with FMU on day 14 but forgot so not sure if it was positive or negative. Fertility friend suggested I ovulated somewhere between day 12 and 15. I really didn't think I'd have ovulated on day 14 but that would actually explain the temp drop. I had put it down to a fallback rise but day 12 and 13s temps could have been discrepancies as we were visiting friends, woke up 2-3 hours later than normal, stayed in a warm bedroom and had alcohol the night before.

After asking 2 midwives and the sonographer at the 20 week scan I gave up trying as they basically do not listen and I end up feeling silly asking. My MATB1 form says their date. I asked the sonographer about measurements at the 20 week scan but he just said the measurements fit with gestational age. The only measurement I actually know (only one documented on graph/in my notes) was AC which was on 95th centile for their dates.

Can digital opks be wrong and show you positive after you ovulate? That wouldn't explain low temps or EWCM on day 10 though. I wasn't properly monitoring CM but I definitely noticed it on day 10. Also is CB conception indicator likely to be wrong too? It quotes 97% accuracy too!

If I ovulated on day 14 that's put me 6 days ahead of their date which is more than the 5 day margin of error quoted (I checked this on that BMUS measurement guide)? I'm so confused by it all. I have a 32 week scan to check the position of the placenta as it was low so not sure how that will go!

I hope all goes well with your next scan totesamazeballs!

Greatjoanumber, that sounds quite stressing fending off doctors and nurses. How did you manage that?

Thanks again!

I have no experience with digital OPKs as I used the strip type ones; but you wouldn't get false positives unless in an anovulatory cycle (where they pick up the LH surge but no ovulation actually happens).
It was tough fending off the consultants; they tried to scare me with all sorts of horror stories that can happen after 42 weeks gestation. I just wanted to scream at them that I wasn't even 41 weeks yet!! Once I left the hospital in tears as the head consultant had bullied me so badly.
It helped that I was absolutely certain of my dates, and that I knew what I wanted. I had informed myself best as I could and I knew my rights. I suppose it also helped that it was my second - you don't feel quite as helpless second time around! Figure out what YOU want and stick to it. If you don't want to be induced, no one can force you. You just come in for regular monitoring instead. I suppose Drs aren't used to women who speak up for themselves and it annoys them, but stick to your guns!

At my dating scan in my first pregnancy the date we were given would have had us conceiving whilst dh and I were several hundred miles apart! When we told the sonographer she measured again, and got a date two weeks later. This was also impossible, so she measured a third time and got something vaguely in the middle. I was charting and using opk's so knew my ovulation date, and we happened to have had very limited shagging opportunities that month. The date we ended up with was about 4 days out.

In the event, ds was born at 37+5 (or 38+2 according to the scan) so it didn't matter in practical terms. However, I am now pregnant again and this time we chose not to have a dating scan at all. My midwife was most put out I really have no confidence in the accuracy of scans, and our major concern was that it would give us a date later than the actual conception and I would end up being told I was in "preterm" labour when I actually wasn't (assuming I may go early again), which would scupper our plans for a home birth. The same would obviously apply in the other direction with regards to being put under pressure for induction.

If you're sure of your dates, stick to your guns. It might not matter at all, but it could make a huge difference if you go into labour at one end or the other of the "term" window.

Ok so if I'd had an lh surge around day 8 I probably wouldn't have ovulated then but would have at the second surge on day 12 which was picked up? I can only think that there was no lh surge on day 8. The previous month I had a positive opk on day 15 so 3 days later than the month I conceived. My cycle that month was 28 days indicating a 12-13 day luteal phase. If I correlated the LP to my March cycle (am I right in saying LP doesn't change?) it would have been a maximum of a 21 day cycle which is incredibly short and has never happened before!!

Ok thanks for your advice! I guess particularly being my first child it's quite intimidating being told you're wrong. I may need a CS at 39 weeks if the placenta hasn't moved up so it may never happen. How were your babies in terms of being 'Overcooked' at birth? Did anyone comment based upon how they looked?

Thanks for your comment ontherun...

A the 12 week scan they measured two CRL at 2 weeks apart? Wow, that seems incredible! How did your first baby measure at the 20 week scan?

Will you have any scan at all this pregnancy? Do the hospital take your word for dates if you don't have a scan? I can imagine the response from your midwife but I can totally understand your reasons.

He measured perfectly average at 20 weeks, and was 6lb 3oz at birth, which is also quite average (on the small side of average, but then so am I) for gestational age.

I will be having a scan at 20 weeks, but they can't give a date at that point as there is too much variability in foetal size by then. They will be able to tell if I am way out - e.g. If I conceived a month before I thought I did, but they can't date more precisely. So yes, they are taking my word for it

I had the same midwife last time, so I think she is used to me by now

In terms of being 'overcooked', my second son who was allegedly 2.5 weeks late was still covered in vernix no one commented that he looked like a late baby!

Very interesting to read your experience OnTheRun! Very good way to outfox them

I was lucky with my sonographer this time. At the 12w scan, baby measured 8 days ahead. I told her about all my previous trouble with the 'amended' due date; and she just didn't change the due date for me!! Put me down as "12w 4d" and left the EDD as it was; even though the measurements were off the charts for that gestational age. And here's the thing: no one ever questioned it! Not my midwife, not the consultant. They just look at the date and take that at face value. The only person who was a bit surprised was the sonographer at my 20w scan - kept asking how far along I was and then still went to check my notes, as obviously baby is still measuring a bit big!

I think my babies just grow a bit bigger a bit sooner. I don't see why the NHS think all babies must be plotted on the 50th centile line! After all there are short and tall adults (and children); that must start at some point.

I admire the determination you both have.

That's brilliant that you managed to convince the sonographer this time. Are you certain of your dates from OPK and temping?

Yes I agree. By my dates the AC at 20 weeks scan was slightly above 95th centile so that got me doubting myself. I am certain of my charting though. Have any of your sonographers ever clarified the margin of error at 12 and 20 week scans? Do they ever change dates at 20 week scan if dating has been carried out at 12 weeks? Apologies for all the questions, just trying to get my head around the whole thing!

This thread's really reassuring, thank you! My dating scan put my conception before we'd had sex, but when I tried to discuss this with the midwife she just said 'dating scans are the most accurate' and that's that.

I'm worried about work because I want to work until 38 weeks, but that would be 39 weeks by NHS due date. I'm telling all friends and family what I believe is the correct due date, could I do the same with work, or is there some kind of legal obligation to use the 'official' due date?

That's really interesting. If you know you didn't have sex for a week before their due date it seems crazy that they can dispute that!

I would think that the MATB1 form will be what your employer will use which the midwives will record from your dating scan date, unless you can convince them otherwise!!

I'm afraid with work you will have to use your 'official' due date which will be on your MAT B1 form. But there's nothing to say you can't work until 39w; with my second I worked until 39.5 weeks (which was 4 days past my official due date!).
bowerpower, the due date determining scan is the 12w one. They don't to my knowledge change it after that (like at the 20w scan) as they do acknowledge babies grow at different rates after that.
Another word on scans though, if they get you in for a growth scan later on if you're measuring big for their dates: just ignore them! They are notoriously inaccurate and a waste of time really. Don't believe them if they say stuff like "Baby is 8lbs already, we'll have to induce at 38w". Those scans can be out by 2lbs either way! The measurements only have to be out by a few millimetres to change the weight estimate by a lot.
So, have confidence in your body and what you know and don't let anyone scare you!

I find it really silly that some midwives and drs are so inflexible about scan dates. They can be out. I've seen folks told that they must have conceived on X date when they had IVF so the time of conception is known within a maximum window of 24 hours (they take the eggs and sperm on one day and call you the next morning to say if they have fertilised). Conception at any other time would have been impossible!

If a baby is IUI or IVF I don't understand how they could change dates at a dating scan! Makes no sense at all!

It's all very odd. I got moved back by 1 day at my 12 week scan this time round. What is the point of that I wonder?!

My dating scan actually put me back to where I thought I should have been as have irregular cycles. I was due 25th September from dates but tested on day 28 of my cycle BFN, then day 31 also BFN. I then had an implantation bleed and then tested about day 35 BFP. A clear blue digital said 1-2 weeks. Went for my dating scan 12 weeks according to dates and measured as 11 weeks but that fits with when I got my BFP and when I think I ovulated due to ovulation pain.
Due dates are arbitrary anyway as there was a recent study that pregnancy lengths varied by 5 weeks naturally.

I had ivf so know exactly when I conceived, yet measured a week ahead at 12 weeks. They didn't change my edd. It's ridiculous they are so inflexible.

Babies are expected to come anywhere between 37 and 42 weeks do tbh the due date is quite inaccurate. If you don't want to be induced you don't have to be. You may be asked to come in for extra monitoring

Keepitgoing, thanks, that's really interesting. What did the midwife/sonographer say about? How disturbing baby measure at 20 weeks? I've read before that they sometimes try to change dates with IVF babies!

*did not disturbing!!

bower they just said, oh, the accuracy is within 7 days, so all is fine. At 20 weeks it was measuring closer to dates. It's all within a mm or two at that point.