Dates, baby size - confused! can anyone help?

[Burmama]

Hi ladies, this is a bit complicated but can anyone please help me to figure this out...? Massively grateful as I am a quite confused FTM Before my 12 week scan, doc calculated EDD at feb 2 due to LMP date (which I am fairly certain is no more than 24 hrs out). Then at 12 wk scan baby was measuring big (12+4 instead of what would have been 11+6 based on LMP) so he said based on that larger size my EDD would be 28 Jan (quite a big change from 2 feb). But then at my 20 week scan the same original gestational age based on LMP (and the EDD of 2 feb) was back on the screen from the start! And during that scan baby is measuring even bigger, more than a week ahead on average! WTF? So now I have several questions that have all only occurred to me now the day after the scan (not helpful). Basically, shouldn't baby's gestational age now be based on size from 12 wk scan rather than my LMP? If so, wouldn't that mean baby isn't in fact a week ahead but only a few days? If not, and baby's age wasn't adjusted forward at 12 wk scan, is it normal that baby is so consistently bigger than he should be? Arms, legs, tummy and head width all at least a week or more bigger than whatever is normal. But then what is normal? According to EDD based on LMP or EDD based on size at 12 wk scan?? Also just to be clear doc said all looks great and nothing to worry about. I just can't get my head around the dating/ size questions! Thoughts please, I'm going mad.

Dating should be off 12week scan. Its probably not been altered on system properly after 12week scan, I'd question it with hospital.

Thanks WhyOWhy! Is that pretty standard to take the age from 12 wk scan rather than LMP? So angry with myself for not asking about it yesterday - doctor is an hour flight away in Bangkok. I have emailed them already.

Yes should be off 12 week scan, mine was changed at 12 weeks scan, only 3 days earlier xx

Thank you both! That must be why baby is measuring big then really really grateful, feel so relieved! xx

Have nothing useful to add, but just wanted to add that (freakily) I am in exactly the same position, including identical dates. Due date was originally Feb 2 calculated from LMP (and husband was away, so know exactly when we conceived not early in cycle but bang in middle!). At 12 week scan it was changed to Jan 28. Also noticed original date put on 20 week scan (which I had at 18 + 6, by new dates, because we had a holiday booked), and that baby was measuring very big for original dates, but fairly normal (big head, other measurements all spot on) for new date. Don't really know what explains baby being ahead - I had a scan at 6 weeks because of bleeding, and it was exactly on course with original dates then; had gained a few days by 10 weeks (when we did the Harmony test, being paranoid first time parents) and then nearly a week by 12 weeks. Jan seems a lot sooner than Feb, so hoping all on track now for both of us! Wishing you well.

Thread title made think you were after some really small dates, or perhaps stumbled across them in the supermarket and they threw you.

PS, NHS is keeping date of Jan 28 for me -- Feb 2 date just on scan photos.

In the same position but I need my prenatal abroad where they fix the EDD at 20 weeks. EDD was moved forward a whopping 11 days due to babies size! Now potentially having my EDD move back to the original which was based on LMP and 12 week scan. Bit of a nightmare as had arranged things around my 20 week scan date but hey ho.

oops 'did' not 'need"

LMP is an ok calculation to use however it assumes all women ovulate on day 14 of a 28 day cycle, so if you ovulated later, have longer between periods etc then your date will be different at 12 week scan (although there's still a margin of error of a few days).

Wow, missed all these responses and especially omg hopsalong that is so weird!!! Same dates?? I am in Feb antenatal group and haven't seen anyone with same date, are you in the Jan group? I was torn which way to go

While they do use the dating scan, 12 weekers don't all measure the same.

If you look at appendix 1 in this paper you can see that a crown-rump length of 54mm is taken as 12+0 days but the 5%-95% centile range is 11+2 to 12+5. So it really isn't accurate to the day.