How accurate are baby weight estimates by ultrasound?

[LeilaM]

I don't post here very often, but I hope the collective wisdom of the group can offer me some reassurance...

I had an ultrasound yesterday (38 weeks, 3 days) to check the position of my baby because the midwife couldn't tell 100% if it was head-down. Luckily, it is, and the ultrasound tech said that, in her opinion, the baby looks like it's ready to go. Yay! I was relieved that the baby is in the right position, but she also estimated the baby's weight to be 8 lbs 12 oz. This is my first baby, so I know it's more likely to be overdue, and I'm getting very worried that it's going to be too big. I want to have a home birth, and I desperately do not want a cesarean.

How likely is it that the baby actually weighs as much as the tech estimated? My fundal height measurements are bang-on average, so I was surprised to hear that my baby might be so large. I do not have gestational diabetes. People are actually always commenting about how small my belly looks, but I think that's because I'm 5'8" tall, so my belly is probably taller and a bit less rounded than most.

That being said, my mother gave birth to my older brother on his due date, and he weighed 9 lbs 9oz, and I was born two weeks early weighing 8 lbs 10 oz, so there is a family history of largish babies. My mom is several inches shorter than me and gave birth to both of us vaginally, so I hope I'm also predisposed to be able to give birth to large babies vaginally.

Do I need to worry about losing my natural home birth? Is there anything I can *safely* do to prevent the baby from gaining too much more weight? (I doubt it.) I'm hoping that lots of sex with hubby and walking around will help bring the baby along sooner before it gets too gigantic.

Thanks for any advice or reassurance you can offer me!

[2sweetboysmom]

Depending on the skill of the tech, her familiarity with measuring babies (does she just do babies, or is she a general US tech?) and the measurements she chose to make the determination. An estimate can be off by 1-2 lbs in either direction.

The best estimates take into account HC (head circumference) AC (abdominal circumference) and FL (femur length) they measure each of those 3 times, average the measurements and the us machine combines the data to generate an estimate. Even then,the very best estimates are + or - 1lb.

Your midwife has a good idea of your pelvic dimensions and I'm sure is not at all concerned with you growing a healthy baby. We only think of 8+ as big because so very many babies are induced early. You will do great!!

[Tiffanoodle]

Ultrasound estimates are pretty unreliable. They told my aunt (she's a large woman) that my nephew was going to be over 9 pounds. He came out at 6#10oz.

Family history is a fairly decent indicator. My children have all been about the same size relative to their edd's. Which is pretty spot on with what my brother and I were.

Don't worry about size. I'm sure you've heard the stories of tiny women delivering 12 pound babies naturally. It's more in positioning. Having a homebirth as opposed to going to an intervention-happy hospital will really help your odds. You just need to trust your body to do what it's made to do and try not to worry. At this point, I don't think you can (or should) try to stop your baby from gaining. Obviously, eat the right kinds of foods, but big babies are good.

Walking is good for keeping you in shape and more prepared to give birth, just don't overdo it and make yourself tired and worn out. You baby knows when to come and your body knows what to do. Trust that.

[livinlovinlaughin]

Your baby will come when it is ready. I had the same worries before our homebirth. They tried telling me that my baby was over 8lbs many weeks before I delivered. My son was barely over 7lbs when born. Trust your body. Your body knows how to birth this baby. You will do fine. Even if your baby is large have faith. There have been plenty of 10lb plus babies born at home.

[bananabee]

My OB says they can be up to 20% off.


(So much for lurking...it's so boring to just sit around doing nothing)

[sunflower.mama]

My son's was off by 2 pounds within a week of his birth (they said 6.5, he was 8.5) I don't put a lot of stock in them personally.

[sharr610]

One thing my midwife told me before the birth of my 11lb babe was once I reached 39 weeks, to lay off the sugar. Like cookies, ice cream, even too much fruit. that all goes straight to the babe once you are really preggers.

As someone who had an 11lb babe and is now carrying a totally normal sized babe, I can tell you, for me at least, sugary treats are limited to 2 a week, carbs are kept to a minimum, my milk consumption is much lower than last time(I was downing two gallons a week easily...this time, maybe a quart a week), and even protein, that they push so much, is kept at a modest, but obviously increased, level.

But yea, the u/s can be off by 15% I've heard. They start pushing the c/s if the big babe seems like it might be diabetic, which GD babies are easily diagnosed by looking at ratios. Most GD babes aren't only big, but have extra large abdomens or torsos in comparison to the rest of their bodies. Its those kind of deliveries that make for a greater increase of shoulder dystocia (the scary term they tend to throw around for big babes), not necessarily just big babies, which many of us just tend to have.

Don't worry too much about it(I know, so much easier said than done )! Keep walking and keep taking care of yourself!

Good luck!

[pippasmum]

When I was pregnant with my daughter, they told me at 35 weeks she was almost 8 lbs and in the 98th percentile and when she was born at 40 weeks, she was 9 lbs, 1 oz - there is no way she only grew a pound in five weeks. My o.b. told me with my daughter that once they hit 4 kgs (about 8 and a half lbs), the ultrasounds are very, very unreliable in terms of size. On the flip side, even if the baby is big, that doesn't necessarily mean anything. My husband is English and his aunt, who is not a large woman, had homebirths with all four of her children, no pain meds and each of them was over 11 lbs. From what I hear, a good midwife will be able to guide you through.

That being said, I understand the worry. At my 36 week appt, I was told that the baby was all of a sudden measuring 7 cm oversize and was sent for a longer glucose screen and an ultrasound. Luckily, the glucose test was totally fine and the ultrasound was showing the baby was 7 lbs 8 oz (this was at 37 weeks, 1 day) and everything was in proportion so all is fine (some people might worry about that size but since dd was 9 lbs, 1 oz and our family grows big babies, that size seems fairly reasonable and my o.b., who had seemed concerned up to that point, seemed to relax quite a bit). Is there extra fluid around the baby? A friend is measuring big and they think her baby will be big and she just had to have another g.t.t. because the extra fluid from her ultrasound supposedly indicates the possibility of diabetes.

Try not to worry (which I know is easier said than done). From what I hear, stress slows things down - give the baby a nice, relaxed environment and hopefully the baby will decide to arrive in a timely manner :-).

[wavybrains]

DDCC from July I truly believe that our bodies won't grow babies bigger than we can birth, absent medical factors or other complications. DD was estimated to be "big" our whole pregnancy, but she was only 8lbs 1 oz at 42 weeks gestation. Had I induced at 40 weeks b/c of big baby fears, I'm not sure she would have been ready, and I doubt my body would have been ready either. My mother had 3 large, late babies--9lbs, 10lbs, and almost 11 lbs, and she is only five foot even. It sounds like your family makes nice big babies, but I wouldn't scare myself too much with that--celebrate it. Remember too that by aiming for a natural birth, you are actually increasing your chances of successfully delivering a larger baby--you can move around and try positions to open the pelvis up. Rather than try to reduce growth, I would spend a little time on spinning babies and reading other things about positioning of the baby and positions for labor and pushing that might be optimal for you.

[at_the_hip]

Late term u/s are pretty unreliable when it comes to weight just because of the range (1-2 lbs!!) that they can be off by. My mw agrees that regardless of baby's size, the best outcomes are when things are left alone (short of a medical reason) rather than inducing.

[Pinoikoi]

My second son was induced at 39 weeks 5 days because they said he was almost 9 lbs.. um, nope. He was 7 lbs even.

[heartandhands]

I just wanted to add my experience. I had an u/s at 40 wks and they estimated dd to be 8.5lbs. I got a stern talking to from an ob about how she was probably even bigger considering I was basically "overdue" and how I'd likely be risking a c section if I didn't agree to being induced then and there.
DH and I decided to wait and let nature do it's thing and we stayed w our mw and had our planned homebirth at 42 wks plus 3. DD ended up being only 7 lbs even, at "17 days overdue"

[LeilaM]

Thanks, everyone, for your stories and encouragement. I'm feeling a lot better. I haven't been having much in the way of sweets throughout my pregnancy, aside from the occasional little indulgence. I'm going to be extra careful about avoiding simple carbs and getting plenty of exercise for the rest of my pregnancy. I have a strong suspicion that the u/s tech's estimate of 8 lbs 12 oz must have been high, but considering my family history of largish babies, it might not be too far off the mark. I feel like I am capable of delivering a large baby, anyway. I just don't want to get pressured into being induced or having a c-section unnecessarily, and the sooner I can have the baby, the better.