Baby measuring a week ahead at 20 weeks

This is a common confusing point I am seeing on this subreddit and IRL. Unless you are at your Dating Ultrasound, which happens in the first trimester, if your ultrasound technician says baby is measuring ahead it means general size, not developmentally. Some people are taller than average; therefore, some babies measure ahead (taller than average). It does not change your due date. It does not call into question the date of conception.

For example: I am shorter than average, my husband is tall, and all our children/fetuses measure ahead (taller than average). I also have high risk pregnancies and will likely deliver early with a larger than average baby for gestational age, but that does not change the organ development or maturity.

I hope this explanation was helpful and I'm happy to edit for clarification.

Edit: This does not refer to fundal height measurements, which are notoriously inaccurate. This refers to actual measuring ultrasounds.

Edit 2: The same concept of measuring ahead is similar to measuring behind. Not every fetus will measure at the average for her/her gestational age, so by default some will be ahead and behind the length curve. Gestational age is the time from conception to birth.

If you're ever confused by what a technician or doctor is saying - get clarification. Don't be afraid to speak to them. They are there for you and your baby.

Like a teenager, fetuses can have growth spurts where one week they are 4w ahead, next measurement they are only 2w ahead, and on the third measurement back to 3w or 4w ahead. They don't stop growing during that time, but merely slow their length growth.

I got extra scans due to GD. At 32 weeks she was measuring over 34 weeks (34+4 I think)—5something pounds, and like 99th percentile for abdomen.

I was convinced she was going to come out a toddler, or a linebacker. My doctor wasn’t worried the least bit.

At 36ish weeks she was measured only 1 week and 4 days ahead instead of 2 weeks +4, and about 7lbs (which was less than they would have expected her to gain)

At 38 weeks I went into labor and she popped out 5lb 13oz and 2.5 percentile.

The ultrasound tech told me ultrasounds can be off by 1lb either way. My doctor said they can be off by 2 pounds routinely. Mine was clearly off by more than that.

All it means is that doctors may want to monitor you and your baby more closely, and see if you need any tests.

How can they tell my baby's measuring big?

Your baby's size is checked by your midwife measuring your bump (called 'fundal height') during a check-up.

If they think you're measuring large for your dates, you'll likely be asked to go for what's known as a wellbeing or growth scan - or they may suggest some tests.

What the experts say about babies measuring big

Our resident GP, Dr Philippa Kaye (read more about Philippa on her dedicated website), tells us you may be checked for gestational diabetes if your baby's measuring large, as this could be a possible reason why.

(We know a few of our mums have had a couple of other blood tests to check various things, like thyroid function, after being told their baby's measuring big.)

There's NO need to be alarmed if you need these tests. They're just precautionary, to check there's nothing underlying going on.

Professor Basky Thilganathan, a Consultant Obstetrician and spokesperson for the RCOG, seems to agree, and stresses to us: "Most women who have a small baby or a large baby are having a healthy small baby or large baby."

He adds: "Bigger babies are very unlikely to have placental dysfunction. I think the major concern with bigger babies is: could it potentially lead to difficulties at birth?"

Yep, we're sure that thought's crossed your mind. Oh my gosh, is my baby going to be massive? Will I struggle to give birth, or even need a C-section?

Expert midwife/doula duo Beccy Hands and Alexis Stickland advise any woman who's found out her baby's measuring large not to worry.

"Your birth team will be keeping an eye on you," they note, "and if there are any concerns about babies’ size and the safety of a vaginal delivery, this will always be discussed and all birthing options will be presented to you – with you and the baby’s safety at the heart of all decisions."

Professor Basky also reassures us that there is no one 'big baby size' that's known to cause problems with a vaginal delivery or that definitely requires a C-section - because every pregnant woman's circumstances are different.

He also reckons the NHS is running a study at the mo to look at exactly this. (Of course, when that study is done - we'll update you here.)

"On the whole, the vast majority will have a perfectly healthy, straightforward pregnancy," Basky stresses again.

"These wellbeing scans are being done to reassure us that despite whatever risk factors [women] have, that their baby’s health is all going to be well.

"It’s a way for us to screen out, or pick up on, the few babies that may have a problem."

Anyway, just because you're 'measuring big', doesn't necessarily mean you'll go on to have a big baby!

It's well known that there's a margin of error (thought to be about 15%) between the measurements taken from midwives' fundal height checks and growth scans, and a baby's actual birth weight.

Dr Philippa also notes that if you're expecting twins or multiples, your bump will be a bit bigger - and therefore fundal height measurements won't be exact.

MFM mums share their experiences of babies measuring big

"There are lots of stories out there of women being told they will have bigger babies, but they end up with relatively normal weight babies!" says cupcakemummy.

"My mum was told my brother would be over 10lb. He was born less than 7lb - he was very long though. He's 6ft 6 now! Enjoy your pregnancy and don't worry too much."

Indeed, we've had loads of similar stories to this one pop up in our (reassuring) forum discussions on the subject.

"At my 20 week scan he was off the charts, and I always measured around 3-4 weeks ahead of dates," shares moominmummy.

"Firstly, it's likely they will check you for thyroid dysfunction and diabetes - glucose tolerance test and blood tests - just to check there's no reason why the baby has grown big.

"In my case, these were all fine so it was just one of those things. They didn't induce me or discuss C-section etc and I ended up going 5 days over and going into labour naturally.

"He was 10lb at birth so they were correct, although I understand it's not always accurate.

"I was going for a home birth but did end up transferring in as he did get stuck so ended up with a forceps delivery.

"But this wasn't his size - but the fact he was facing sideways (instead of front to back) and due to me having low amniotic fluid (another no-cause coincidental finding that was picked up during all my growth scans) he couldn't turn."

Linzi1234 found her baby's size was well over-estimated: "I had this with my 2nd pregnancy - I was told that she would be well over 10lb.

"I was worried throughout my pregnancy as wanted a natural birth and they kept on about a C-section. I was measuring weeks in front.

"She was born naturally the day before she was due and weighed in at 6lb 10oz so they are not always [bang-on] xxxxxxx"

Linzi shares that her advice is to avoid becoming consumed with worry. "It took away the excitement for me towards the end and there was no need for it.

"You just have to remember that even if baby's on the big side, you are in the right place - but don't let it scare you as you might end up with a 6lber like mine :)"

Amethyst had quite the bump with both her pregnancies, but found her babies weren't high birth weights:

"I was huge - gained 3.5 stone and measured 2 weeks ahead, was told at 37 weeks the baby was 8lb already but she came on her due date weighing 6lb9oz!!

"Very similar story with my second, but that time I gained 4 stone and he was 7lb 8 x."

Indeed, Katt1906 found exactly the same: "From growth scans I was told my son would be 8lb 5oz... He was actually 6lb 13oz at birth."

What MadeForMums says

We totally empathise with the fact that being told your baby's measuring big can cause lots of worry and panic.

However, any tests are just a precaution. If you do end up needing to discuss an induction or C-section, your docs will talk you through everything.

At the end of the day, it's better to know that your baby's being monitored closely, just to be safe. Try your best not to overly worry - we know it's tricky, but hang in there ?

Images: Stock image via Getty Images

Read more:

• Growth scans in pregnancy: what you need to know
• What does it mean if your baby's measuring too small?
• Scans and tests during pregnancy - what's coming?

Is it normal for a fetus to measure a week ahead?

How Big Should a baby be measuring at 20 weeks?

Does my due date change if baby is measuring ahead?

Can due date change at 20 week scan?