Can your pregnancy scan give the wrong due date?
Had a pregnancy ultrasound that doesn't match your Estimated Due Date (EDD)? Mums in our Community share their stories plus expert sonographers explain why it happens and what to do if you're told your due dates are wrong
We know, from listening to mums-to-be in our MadeForMums Community that, for a fair number of pregnant women, when they're told their estimated due date at their 12-week dating scan, it just doesn’t seem quite right.
That was certainly the case for Sarah from our MadeForMums Community. "My due date got moved 2 whole weeks at the dating scan," she says. "My GP had estimated December 18th but at the scan it was moved to Dec 31st. But that just doesn't quite add up by my reckoning of when I think I ovulated and when I know we did the deed!”
So what's going on here? How is your due date calculated at the 12-week scan? And how accurate is it?
Here, with the expert help of Gill Harrison, professional officer for ultrasound at the Society and College of Radiographers and Jan Steward, Director and Co-Founder of the private ultrasound provider Ultrasound Direct, we explain all...
When do you normally have a dating scan?
Unless you have an early pregnancy scan most of you will be given your estimated due date at the 12-week scan. It’s at this time that you will be told how many weeks pregnant you are.
Of course you have probably already used an online due date calculator. This is where you discover that your due date isn't based on the date you conceived, it's actually calculated from when your last menstrual period started (your LMP). Weird huh? It's because it gives a definite date for health professionals to work with. There would be so many variables if it was calculated from when you think you conceived, especially as everyone's cycles are different and the fact that you don't necessarily conceive on the day you have sex!
Pregnancy normally lasts from 37 weeks to 42 weeks from the first day of your last period, explains NHS Choices. But this will only give you a rough idea. This is especially true if you have an irregular cycle or have been on the Pill for example.
“Scans are an accurate way to give you a due date,” confirms Gill from the Society and College of Radiographers. “If the scan date differs from your LMP dates, your scan dates will be used as they are generally more accurate. If you are concerned, though, do discuss this with your sonographer and midwife. The best time to establish the due date is at the scan you are offered between 11 weeks and 2 days and 14 weeks plus 1 day,”
Just how accurate (or not) can the dating scan be?
Experts say that the scan date can sometimes come out as less pregnant than suggested by your LMP. This was the case for Leanne.
“My 1st scan with this baby put me 1 week earlier than I thought I was. But then again with my first baby I was spot on with dates, he always measured big and he was a week overdue... these babies like to be tricky! Babies are ready when they're ready - try not to worry (easier said than done).”
But Gill also says that the estimated (and that is all it is) date is normally accurate plus or minus about a week.
“The due date is estimated by measuring the ‘crown rump length’ of your baby which is essentially their length from head to bottom. When this is done at the 12-week dating scan, the due date is the one that is usually used for the rest of the pregnancy and gives a baseline for growth throughout pregnancy."
So can my due date change during my pregnancy?
While the professionals like to try and stick with the date that you’re given at your dating scan, it can and does change.
This was certainly Lou's experience. “Trust me your due date might change. Your baby's growth can slow down and then have spurts.
"I'm 16+5 and have already been given 3 due dates and its changed for a 4th time back to the 2nd estimated due date! I've not even had my 20 week scan yet. I have scans every few weeks so it will probably keep on changing. My baby will come when it's ready anyway, maybe weeks before or maybe weeks after my due date."
What about early scans?
Like Lou above, some of these first EDD are derived from early pregnancy scans (pic above). These can be carried out from week 5 or 6 of pregnancy, which is a notoriously tricky time to be accurate on dates. Understandably, when it is such early days it’s easy to worry when the scan dates are different to what you were expecting.
As Lara explains, it's really common for these to change when it comes to the dating scan at around 12 weeks.
“I was measured at 6+6 (6 weeks and 6 days) when I should have been 8 weeks. Measurements at such an early stage aren't accurate which is why they wait for 12 weeks to date you.
"At my 12 week scan I was back to bang on dates for my last period and was told to ignore the previous two scans, which they said were to check for 'viability' as opposed to dates. I know it’s hard, but please try not to worry.”
Catherine also found the date that she was given at an early dating scan was changed when she got to her dating scan.
“All the dates I was given were wrong up until my 12 week scan - which gave my EDD as one day before the date I had calculated myself!”
Expert Jan Steward, who is director of Ultrasound Direct and Babybond, agrees that early scans carry a much greater risk of inaccuracy. “At our clinics we scan from 7 weeks but at this time due dates are not totally accurate as it is very early stages. We always recommend mums get a definitive dating scan closer to 12 weeks as this will be much more accurate.”
Sian confirms that it’s worth the wait for the 12 week scan.
“Definitely agree that dates from early scans are not accurate. Just half a millimetre at such an early stage can change your date by a week. Your 12-week scan will be far more accurate.
"I fell over at ‘7’ weeks by my last menstrual period (EDD 18 March) and was sent for an early scan. The sonographer then said that I was only about 4.5 weeks [EDD 2 April]. Then at the 12-week scan I got an EDD of 15 march, so that early scan was completely out.
"From then on he was smack bang on average size for babies EDD on 15 March. I wouldn’t take an early scan as read, as it’s not your official dating scan anyway.”
Does it get more accurate as you get more pregnant?
Well, you'd think so, wouldn't you. And yet, bizarrely, after your dating scan at 12 weeks, your due date can get less accurate the more pregnant you get.
Sonographer Jan Steward explains, “As your baby gets bigger all of the usual differentials start to happen between big and small babies so it’s much harder to be accurate. At around 12 weeks you can get more accurate measurements.”
What can you do if you believe your scan dates are wrong?
“If your dates are wildly out then it may mean there is an issue with your baby’s growth so you should certainly talk to your midwife or doctor,” says Jan Steward.
“You will be offered another scan if the sonographer feels there is an issue. By talking to the health care professionals you should get a clearer picture as to what is happening.”
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