How accurate are baby scans?

[BiscuitLover09876]

With my DS I was told my actual due date was much earlier than my dates stated. I remember thinking it was ridiculous because my supposed ovulation date was about a week before we'd even had sex (and we hadn't had sex for months before- dh had been abroad).

Low and behold ds came 2 weeks late when predicted. I deliberately avoided the pressure to induce.

Any medics know why there is so much on these scans? Because I feel like everything is measured against that initial one!

(No offense intended btw - generally very thankful for very hard working medical professionals!)

It depends on when the scan is. An early ultrasound scan (8-11 weeks)is very accurate but it declines after that.

They have to use something to give you a date and the measurements taken around 12 weeks are the most accurate for dating. It's only ever an estimation though. They obviously can't tell the future and give an exact date. They just have to use their size as a guide as not everyone knows when they conceived or last ovulated

2 of mine were out by nearly 3 weeks. My oh works away so we knew when we conceived. Good job he trusts me as any man even slightly suspicious would have demanded a paternity test with the dates that they gave me

I think it’s because most very early babies stick very close in size against an average. So the difference between 2 different babies both of a 12 week gestation is more aligned than later scans where size can vary between babies a lot more. As most women probably don’t know when they ovulated or more importantly implanted, it’s just easier for them to go off scans and averages

Is this your second post?

Dating scans won't be that far out because development changes so quickly in the early weeks. Theres an enormous difference between say a 9 week embryo and a 12 week one. Later growth scans that predict birth weight are often wrong but it's harder to be accurate with a large fully formed baby. It's normal to have the EDD change by a few days because it will be going back to ovulation rather than your last period.

You can't be sure by your own dates unless you had IVF or accurately tracked ovulation.

If your DH was away that suggests he may not be the father if the hospital EDD was that far out.

My date changed by 10 days....IVF baby so no way was the scan right. But the EDD was changed anyway 🤷‍♀️.

Neither of my pregnancies have been correct with due dates. I had IVF both times and time stamped videos of the eggs fertilising haha so I definitely know exactly how pregnant I was/am. First pregnancy they brought me forward 10 days and I went 8 days over; this pregnancy I’m a little over 2 weeks ahead of where I should be and I know there’s absolutely no way that’s possible. Babies are just long

Sonographer here. Like a PP your due date is only ever an estimation. Some ladies give birth earlier some later. Your due date estimation is calculated on the first date of your last period (not your ovulation date) and the rough measurements of the foetus at your 12 week scan. This will never be 100% accurate as it is impossible. It's just a uniform way of calculating things because every body is different. Everyone ovulates at a different time - not always the average day14, you could conceive up to 7 days after having sex and things develop slightly slower or slightly faster in some pregnancies.
Unfortunately a lot of things during pregnancy are one of life's great mysteries.

What do you mean by 'supposed ovulation date'? Do you mean your LMP date?

@Patapouf

Is this your second post?

Dating scans won't be that far out because development changes so quickly in the early weeks. Theres an enormous difference between say a 9 week embryo and a 12 week one. Later growth scans that predict birth weight are often wrong but it's harder to be accurate with a large fully formed baby. It's normal to have the EDD change by a few days because it will be going back to ovulation rather than your last period.

You can't be sure by your own dates unless you had IVF or accurately tracked ovulation.

If your DH was away that suggests he may not be the father if the hospital EDD was that far out.

He definitely is 😂 isn't it more likely there was an error?

I was convinced they'd given me the wrong date with DS2. He turned up bang on time, so I hold my hands up to the sonographer!

@Godmothered

Sonographer here. Like a PP your due date is only ever an estimation. Some ladies give birth earlier some later. Your due date estimation is calculated on the first date of your last period (not your ovulation date) and the rough measurements of the foetus at your 12 week scan. This will never be 100% accurate as it is impossible. It's just a uniform way of calculating things because every body is different. Everyone ovulates at a different time - not always the average day14, you could conceive up to 7 days after having sex and things develop slightly slower or slightly faster in some pregnancies. Unfortunately a lot of things during pregnancy are one of life's great mysteries.

What do you mean by 'supposed ovulation date'? Do you mean your LMP date?

Yep that's what I meant. So I would've had to have ovulated a fair bit before ttc which made no sense.

My first child was given an inaccurate date based.on the first scan measurements.

My cycle was liked clockwork, to the exact day and we had sex once. I knew exactly what my due date was but they wouldn't have it. They brought it forward by a week as apparently the baby was big.

Lo and behold, according to my dates, she was 3.5 weeks early but obviously technically full term by their dates so they didn't count her as prem.

She was 5lb when born and was tiny. They discharged us and that night we ended up back in hospital for a week as we had a baby who couldn't feed. You know, behaving like a prem baby!

My second child I was certain of my dates again. I went into labour 2 hours after his due date.

Yanbu. I find it very insulting how doctors dismiss the actual conception date where known. I knew exactly when mine was conceived, but no one would use that date, only the fictional one they’d made up. You can see another thread today where someone had a clearly premature baby but no one would listen to her or trust her dates and now she can’t get the medical support her child needs because the ‘computer says no’.

They want ‘objective’ data ie they don’t trust anything the mother says. It comes from the Victorian view of women as ignorant idiots who need to be managed for their own good, a view which is still alive and well in the medical profession…

I think when you have extra information such as husband being away, it's worth adding the information every time they are looking at due dates. A friend regrets not doing that, because they were then putting pressure on her to be induced when she didn't think she was much overdue.

Every time they asked me for the first day of my last period, I used to add that my cycle was very long, and as a result they went more on the scan (at that time they tended to go mostly off the period date).

My mil was told she'd got her dates wrong when my husband was born, as he was bigger than her dates would suggest. Then he turned blue, and they realised his lungs were not fully developed...

I only DTD once the month i conceived DD so knew for sure. The due date based on online calculators from LMP or conception date were on or within a day of her birthday. The scan based due date was 3 days earlier (and I had a scan at 8 weeks!)

I wasn't given dates that I disagreed with but the estimates on the size of my babies was wrong.
They told me dd was measuring huge, she was born at 37 weeks weighing 7lb6oz
On the opposite side they said DS was measuring really small, I was having extra scans and monitoring due to having hyperemesis, he was born at 39 weeks weighing 7lb13

The day you have sex isn’t necessarily the day you conceive. Sperm can live inside the body for several days, so you might actually conceive up to a week after you have sex. That’s why they don’t go by the ‘known’ conception date; even if you only had sex once that month, you don’t actually know when you conceived.

My scans were accurate for both my pregnancies.

First I was having very regular scans (twins) and they were born at 36 weeks and within half a pound of their estimated weight. Second pregnancy I can’t remember my last scan but might have been the 20 week one - he was born the day before due date and the midwife who measured my bump the week before got his birth weight bang on!

My DS was 2 weeks late (induced) but a little under average weight. As we weren’t TTC and the due date was changed after the second scan, I don’t think we’ll ever know if DS was early, late or on time! I’m glad I didn’t opt for an earlier induction though.

With DD1, they put her due date forward by a week. She ended up being 8 days late according to them but only a day late for me.

With DD2, my dates only differed by 1 day with the EDD they gave me and she arrived a day earlier than predicted.