Can anyone who understands dates more help?

[ViniVidiVisa]

I had a dating scan a few weeks back, I wasn't sure of the dates at all. I knew I'd had a period though just over 14 weeks ago (and I'm really sure, seared on my brain: visitor at work, handed him a tampon rather than a pen...shame). The month after I had a maybe period, but very light, not sure if real. So based on the fact that you're dated from your last period (aren't you) I said was either about 14.5 weeks or 10.5. The scan said I'm 12 weeks + 4 days which has really thrown me. Can anyone make sense for me? I have a clockwork 28 day cycle.

I think that the scans are dated from the size of the baby not your actual period dates.

I have the same problem with my dates - I'm dated from the day before my period started!!

I just go with the flow with their dates, knowing in the back of my mind that I could be up to 2 weeks earlier or later.

1. In my experience dating scans are not as accurate as they are taken to be. In all 3 of my pregnancies the babies have arrived within a couple of days of the due date I calculated from what I knew. The dating scans had them between a week early and a week late.

2. Just because you normally have a 28 day cycle that doesn't mean you always will and always do. Perhaps just this once you ovulated a bit later than you normally do?

I would say the accuracy of dating is within about a week to 10 days which gives you quite a large window really.

accuracy of dating scans

Thanks, quick replies!!

So not knowing the date at all is really REALLY annoying then

The scans date based on the size of the fetus, not your last period. My scans have all been all over the place, because babies grow differently in utero just as they do on the ground. So my 6 week scan (I am very very old,-41- so I get a lot of scans!) looked a bit small, but my 12 week scan baby measured 13 weeks. I'm sure at the next scan, s/he will measure something different again. My Doc told me that they keep the LMP date if it's only a week or two different. So I'd go with the 14 weeks for now. Most babies aren't even born on their due date, so it's kind of just a general idea anyway!

Also, you're 29 week scan will likely be more revealing as to actual age of baby.

Dating scans are actually pretty accurate and are far more accurate than later scans, because the growth of the embryo is fairly consistent between women and between pregnancies. Later scans are much more difficult to date.

Of course, they are measuring something measuring a few centimetres so a few millimetres each way makes a big difference in terms of dates, so they are probably only accurate to a week either way.

As for which date to keep, I'd go for the scan date. Most first babies (assuming it is your first) are 9-10 days late. If you are down as being 14 weeks, they are going to want to encourage you into induction etc two weeks earlier than you probably need.

Last time my scan date and LMP were the same to a day.

If it is early in some ways I'm pleased, last time I was really pushed into an induction at 41+4, and before 41+3. I think I just naturally go a little longer and neither baby was huge or were there any signs of problems. For both I'd just started slow natural labour when induced.

I'd stick with your scan date then, particularly as you seem to cook babies for a little longer than normal and that this is normal for you. Sounds like your light bleed was an implantation bleed then.

Congratulations, by the way!

Does your scan show the growth chart with both the high & low range, as well as the mean length for the dates?

My scan (taken at 13+2, I ovulated on cd15 when we DTD, so fairly sure this is accurate to a day or two) said my EDD was ok, as the growth was at the high end of the range but still within range for dates. When I look at the growth chart, if you track the fetal length as it is to the mean, I could be 13+6 (the date used for the NT screening) or track back the smallest end of the range to the mean, it could be a fetal length equivalent to about 12+3 I think.

What I'm trying to say is, that the range of lengths considered within the normal range could mean an EDD of up to almost a fortnight difference. Hence, you could just have a small 14wk-er or a big 10wk-er!! Personally, I'd go with the scan dates, assume the 10wk bleed was implantation and assume I may actually go over-due anyway.

Congrats and all the best.