I have a really stupid question ...

[Newbabyenroute]

How can the sonographer/Dr "date" the pregnancy if you haven't been tracking your cycle?

I understand they measure the baby but what if your baby is small or large for their gestation? And on the other hand, how can they tell you the baby is big or small if you aren't sure on your dates?

For example, if you haven't had a period in 18 months due to cycle issues, I assume they'd offer you a scan if you got a positive test but how would they know how far along it was vs it being small/large?

Follow up - all the apps/info on NHS website etc all talk about weeks pregnant. Does this mean from your last period date (•assuming you know it) or is it based on the actual conception? e.g is the baby the size of a plum (as an example) at 6 weeks since your last period or 6 weeks since you conceived?

Even if you're sure on dates, the 'X number of weeks pregnant' is still an estimate, going on both your cycle (if you know the info) and size of baby

I knew my dates exactly. At booking appointment with midwife she told me I was X number of weeks. At my dating scan they put me forward by a week due to the size of baby

I had a friend who knew the exact date (IVF) but was given a different date at scanning. She was perplexed.

I had two ‘false’ periods so my first scan was at ‘about 20 weeks’ and I was told it was plus/minus a week. DS2 I managed to get an ‘early’ scan because of the above and was dated as ‘exactly 12 weeks’ so I think they are more confident at that point than later (when I guess the margin from tiny to huge is larger).

See this is what confuses me.

I understand your ovulation date isn't necessarily your conception date. And that sperm can live inside and therefore you could potentially conceive a day or two after you had sex.

But I still don't understand how they decide whether its a 'we think you're a week ahead of where we originally thought because the baby is bigger than expected' or a 'your baby is measuring big for their gestation'

Initially the midwife etc go off the date of your last period. This gets adjusted after you scan based on how big they measure baby. They're not really telling you how long since you got pregnant, i.e. how many actual weeks you are. They're telling you based on how big the baby is how many weeks you have left to go (roughly). They try to do it at this particular point of pregnancy because it's when the prediction based on size is at its most accurate.

I knew the exact day and hour my daughter was conceived. It was the only time I had sex! But they kept saying my dates were wrong. I knew the date but they wouldn’t accept it… 🙄

I was confidently told the 6th was my due date they moved it to the 26th on the scan measurements she arrived on the 6th

Ouch! Doesn't that mess with your notification for work and stuff too?

It's worked out using a norm range so most babies with a certain measurement of X are 12 weeks etc etc. of course they get it wrong, that's why your due date is only really an estimate with a plus or minus of around 2 weeks either way but they need some form of date to know roughly where your up to other wise no one would know how many weeks you are, when to do certain appointments etc. also i wonder by 12 weeks there isn't that much variability in the size of a baby, the baby is mainly been developing vital organs rather than growing big/tall etc.

Babies pay no attention to work timelines 😂

Got my notice in on time, she arrived 6 weeks early lol

Time to go is a good way of thinking about it, thank you.

My sister has diabetes and they were really worried all along about the baby being big but there were some question marks around the date. When the baby arrived, he was 7lb. I wonder if she was just further along than they thought?

The dating scan isnt like 100 per cent accurate as you say. It's accepted by medical professionals that there is a margin of error either side. I believe it's roughly up to one week either side.

In my two previous pregnancies my due date from my dating scan has been around one day different from the one I came up with from my last period, as I have regular cycles, so makes sense. However this time I was moved a week behind. I'm confident about my dates so was a bit concerned that baby is on the small side. Have then found out a few weeks later that I have low papp A which can also cause babies to be small. So nervous about my 20 week scan which won't be for a few more weeks and I guess I just have to see how it plays out really.

Very good question :-)
In the first trimester, babies usually develop at more or less the same rate. The gestation is calculated from the crown - rump length, but the development of the organs etc helps to confirm it. Studies of dating scan accuracy (compared to known IVF dates for example) show that first-trimester ultrasound has an accuracy of +/- 5 to 7 days.
As the pregnancy progresses, you see more difference in growth rates, so scan dating becomes less and less accurate. Some babies are smaller because they're constitutionally small, ie small and healthy. Others may be smaller because they aren't growing to their full potential, eg IUGR. The sonographers compare the head size, abdominal circumference and femur length, as the relative sizes of these give a good indication of how well the baby is growing. eg If the baby's head looks average sized but its tummy is skinny and femur length is well below average then that may suggest growth is less than ideal, for instance. But a constitutionally small, healthy baby should have had, abdominal and femur measurements all roughly proportionate with each other.

The whole dating thing is absolutely bizarre - in particular the bit where they add two weeks at the start (so you could hypothetically "have conceived" days before you first met the father, if you went straight to unprotected sex).

My sister also had low papp A (separate sister!) My neice was born healthy but small back in November. Good luck!

Thank you for taking the time to explain this. Working on development rather than size makes a bit more sense to me but I understand there's a margin of error.

Seems crazy doesn't it? I don't know how women who don't have regular cycles manage to work it all out ...

They date it from the first day of your last period, not the day you conceived so they likely added 2 weeks on. That's just the way that pregnancy has always been calculated.

Nope they added more on and no matter what I said they wouldn’t listen. I knew to the min when dd was conceived ( long funny story, on holiday etc) It meant my dates were consistently wrong, I got one day off on maternity leave before I went into labour. So I effectively worked up to 40weeks, was so heavily pregnant I couldn’t drive car or put my own shoes on.
And if she had been born one day earlier she would have been classed as prem as according to them I was 36+5…

I had IVF so know the date the embryo went in but my due date is actually 6 days ahead of the IVF calculator date. This was predicted at my 6 week viability scan and continued to track the same through all scans. However I'm now 32 weeks and she is measuring on the small side, because it's only 6 days I'm not too concerned. As others have said it's only an estimate babies are very rarely born on their due date.

Dating scans are done between 11+0 and 13+6 because at this time there is very little "natural" variation in size of the embryo and size more or less corresponds to gestation. Once you get past this point then size becomes increasingly inaccurate as a means of judging gestation. But they need one universally applicable method for estimating gestation and this is the best available.

Dating the pregnancy in weeks from your period date is simply a hangover from when there was no such thing as dating scans and the only thing you had to work with was the woman's last period date. Nobody actually thinks you were pregnant during weeks 1-3; it's acknowledged that the minimum weeks pregnant you can be is 4. Once you do have dating scans, well, you can rewrite the entire obstetrics corpus to have a new dating convention, or you can just stick to the existing one, which is in any case still useful before the dating scan.

This always annoyed me too. I have longer slightly irregular cycles. It really annoyed me that I was dated from the first day of my last period which was 3 weeks from when I actually conceived. I always felt pressure then about going over your due date was misplaced!

As regards dating scan, my consultant explained it as being like an inverse funnel up to 12 weeks…regardless of height of parents etc, all babies generally develop at the same rate until they’re 12 weeks. So a 6 week old foetus of two short people will measure the same as a 6 week old foetus of two bigger people.