How accurate are ultrasounds at guessing size and weight?

nina8381

I had an ultrasound today (36 w 4 d) because I was put on medication for GD. Was told his size is measuring at 38 to 39 weeks and weighs about 8 lbs already. Everything else looks good, just a big boy . I have 19days before my scheduled c section and I'm wondering if that's means he will be 10plus lbs. Anyone have experience with ultrasounds guessing weight and how accurate they are?

golfingdarwinfish

There are a ton of threads about this if you use the search feature.

They are not 100% accurate - purely estimates.  People have had them be a pound or more off in either direction, others have had the estimate be in the ballpark.  

Ultrasound measurements are subject to human error and the limitations of the technology - you won't know how accurate yours are until LO arrives, and in the meantime you'll hear that they're accurate and that they're inaccurate. 

trisharenee0316

There are something like 30 different ways that doctors use to estimate fetal weight.  If your doctor doesn't use one of the more accurate measures, then you have a less likely chance of it being accurate.

Also, I doubt your baby will gain 2+ lbs in the next less-than-3 weeks.

golfingdarwinfish

I always wonder why people don't ask their doctor how accurate the measurements are once they hear them.  That was the first thing I asked my MFM when he came in the room (literally, as he was walking in) post ultrasound, and he answered the question with no issue.

Dubsgirlie

Mine was off by 2 ounces.

trisharenee0316

golfingdarwinfish said:

I always wonder why people don't ask their doctor how accurate the measurements are once they hear them.  That was the first thing I asked my MFM when he came in the room (literally, as he was walking in) post ultrasound, and he answered the question with no issue.

Same.  And in fact, even though I had growth ultrasounds once a month and they were able to track this baby consistently over many months, my doctor still admitted that fetal estimates are crap shoots and they'd have to be very skewed in any one direction for him to begin to take it seriously.  So many factors go into this that having one data point from one ultrasound is probably not very helpful.

AmandaJean12

My OB wouldn't even have the tech do those measurements when they checked of LO was breech this week because she said they are off so often all they so is give moms one more thing to obsess and worry over. So unless your Dr is telling you to be concerned, I wouldn't give it another thought.

Emerald27

In three weeks, it's more likely that your baby will gain 1-2 lbs.

BreandMikewedding

They can be way off and depending on the ultrasound tech can be farther off than others. Some techs are just better than others. My friend's was 5lbs off.

melaniedel

They told me DS1 would be over 9lbs. He was five days late and 6lb 15oz. With DS 2, I had. US the day before delivery, and thy told me 7.2. He was 7.3, so spot on tht time.

Justabean3

The told me today that

1. The more u/s you have the better the estimate

2. Before 36 weeks estimates are more accurate bc they have some room left

3. Mothers weight/fluid levels can effect an estimate bc a sonogram may be measuring bones but it still has to go through water, tissue and muscles to get it. Think about dropping something is water and measuring it,
It can be destorted.

kimmybaby412

My son was off by 2 pounds! They told me he would be only 4 pounds and i was induced. he ended up being 6 pounds 14 ounces. They told me my daughter weighs about 4 pounds right now based on the ultrasound i got at 34 weeks. Why are you doing a c section? I am going to wait for my second to come naturally, instead of being convinced to use drugs and get induced. i went through a horrible painful labor from the induction, i don't recommend it unless that baby has a lot of issues and high risk.

Sweetiepops

I had an ultrasound on Wednesday to check on baby- MW told me weight estimate could be about a pound off either way, so I didn't take the estimate too seriously. 

[Deleted User]

Had an ultrasound yesterday (34w3d) and was told the little guy weighs about 4lbs 14oz. On the smaller side of average, but I'm not putting too much thought into it. I won't know for sure until he gets here!

cmcookaln

I have GD as well so I have been concerned about baby's size.  I mentioned it at my last dr's appt because I had gone to see an perinatologist who said the baby weighed 4lb 10oz and the next week my regular OB said the baby weighed 5lb 9oz.  I was shocked that the baby could gain one pound in only a week, especially since my sugar levels have been regulated.  He told me that ultrasound readings can be 2lbs off in either direction.  I wouldn't worry too much especially if you already know you are not delivering the baby vaginally. 

CNJ4EVA

With my DD, at 40w they said she may be close to 10lbs. She was born at 41w weighing 7lb 10 oz.

This time, they estimated 7lb 1 oz at 37w.

kaniela17

Estimates of current weight at time of ultrasound are really accurate, not 100% but pretty close. Future weight predictions are NOT at all accurate.
I had an ultrasound at 36 weeks with my first and they said he was already 7lbs and said he'd be over 9lbs or more at birth (scared the crap out of me as it was supposed to) but at my next ultrasound the day I went into labour right at 40weeks he was only 7lbs12oz, and that was his exact birth weight.

sagegoodenough

They were off on my first. At 32 weeks they said he was measuring just shy of 3 lbs, had him later in the same week, he was 4lbs 5 1/2 oz. I don't remember measurements with my second, I was told low 7lb range with my last and he was almost 8lbs. And this time, at 34 weeks she was measuring 4lbs 9oz. I have another ultrasound on the 30th for growth and to check my uterus, and then whatever she is at birth. This time I'm with MFM so I trust their measurements a bit more but it's still just an estimate.

deepnasingh

Thank God I read this post.. Feeling better dat I am not alone .. I went in for the scan today at 34weeks 4days and the baby is measuring small at only 4lbs 12oz. I have GD and it's been a struggle managing it. I felt guilty of starving my baby. Infact, was depressed the entire day.. I hope and pray the baby has growth spurts in the coming weeks!! [-O<

Justabean3

another thing with estimates is if your edd is exact

I know the day I got pregnant so I trust estimates a little more. If they would have gone off my LMP and my due date was a week earlier then his measuments would be off

BeanNut

My baby's measuring 7.9 lbs as of yesterday. I asked my dr about margin of error and she said one lb either way.