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This is a transcription of a portion of a Healthful Woman podcast.

Nicole’s Question

Our next question is from Nicole: “Hello, big fan and toaster here. I had a scan at 28 weeks and the femur and humerus are measuring 25 to 26 weeks and they are behind. The rest of the baby’s measuring 27 to 28 weeks. They said his bones are mineralizing and not evident for skeletal dysplasia, but will monitor me again for placental insufficiency. I’m concerned about skeletal dysplasia based on these numbers. Do you have a cause for concern, or is it more likely a normal variant? It could be maybe fetal growth restriction. Thank you.”

Dr. Fox’s Response

All right. So, Nicole, to answer your question, just some background. So, when we’re evaluating fetal size on ultrasound, what we’re doing is, at the end, we’re estimating a fetal weight, and we’re also saying how big is the baby measuring week wise, meaning you’re 28 weeks: is the baby measuring 28 weeks or 27 weeks or 26 weeks or 29 weeks, 30 weeks? So we usually get two results. One of them is an actual weight in grams or pounds, which comes along with that percentile. If you’re average weight, you’re about the 50th percentile. If you’re larger, you may be 70, 90, whatever. And if you’re smaller, lower numbers. And we also get sort of the gestational age equivalent. Those are obviously related to each other, but those are the data we get.

Now, in order to get the weight, we don’t actually weigh the baby on a scale, obviously. What’s typically done is you get several measurements of the baby. We typically, most commonly, take two measurements of the head. One of them is the circumference around the head. What I tell people, it’s sort of like your hat size. And we also get the distance between one side of the head at the other. Think of it as a distance, not exactly, but from ear to ear. So, that’s sort of like your headphone size is what I tell people. Then we get one measurement of the baby’s belly. It’s like a waistline but a little bit higher. We call that the abdominal circumference. And then we measure the length of the femur, which is the thigh bone. So, the baby’s thigh bone, we measure that length. We’ll sometimes also measure the humerus, which is the upper arm bone. But it doesn’t typically go into the calculation to estimate the baby’s weight. Fine. So, we get those measurements, and then we get an overall weight and a percentile. But with each individual measurement, you will also get a certain percentile or certain week.

So, for you, when they measured the femur and then the humerus, they said that they were measuring a little bit smaller. And the question you’re asking is, “Is that concerning?” Obviously, I would have to see the details to know the answer. But what I would tell you is that typically it’s not concerning, meaning most of the time we see this, it’s not concerning. And why is that? There is variation in how babies grow and how bones grow, and some babies are going to have a normal-sized head and shorter legs. And some babies are going to have a smaller head and longer legs, just like there’s a difference in humans and how tall we are and how large our waists are and how broad our shoulders are and how big our heads are. That’s going to differ between adults. It’s going to differ between children. It’s going to differ between fetuses as well.

So, most of the time we see sort of differences or some things are measuring bigger, something smaller. It’s usually just this is how people are built. They’re all built differently. It’s perfectly fine. Now, there obviously are situations that can be concerning related to having short limbs. What you mentioned was sort of a catch-all diagnosis called skeletal dysplasia. So, skeletal dysplasia is basically a category for conditions where the baby has a disorder that the bones aren’t going to grow normal. And there are many, probably hundreds of known conditions where that can happen. Some are more severe than others. In fact, some of them are lethal. Some of them, the babies don’t even survive. So, this can cause a lot of anxiety, obviously, for people if they’re told that the limbs are a little bit short, and then they Google skeletal dysplasia and they’re like, “Oh, my God, some of these conditions are absolutely horrible.”

What you were mentioning in your question is that typically if the baby has one of these conditions where it’s a real diagnosis, there are usually going to be other things seen on the ultrasound related to how the bones look themselves, not just the size. Like, are they the right shape? And are they straight as opposed to bent? Are they bright white, which is what we normally like to see, or not as white? And so there are certain features that we might find that would indicate, hey, this is more likely to be not a problem, or this is more likely to be a problem. So, that’s one thing. And also, how profound is the shortness, right? So, if you’re 28 weeks and the bones are measuring 15 weeks, that is a massive difference. And that’s much, much more likely to be a significant problem or a real diagnosis versus something that’s lagging by a week or two. Most babies whose bones are lagging by a week or two are going to be perfectly fine.

And so sometimes this is an issue with just how the percentiles come out. Sometimes we’ll check a femur length, and it comes out in the third percentile. And that seems pretty significant and pretty profound. But when you actually look at it, it’s measuring maybe just a week behind, which isn’t such a big deal. Or if the femur is supposed to measure, let’s say, 3 centimeters, and it measures 2.6 centimeters, right, which is a 4-millimeter difference, meaning even by the eye, it’s hard to see the difference between those, it could be a massive difference in percentiles. And so when these situations come up, what I normally do is I try to really get into the weeds of what’s going on. So, we’ll obviously look at the measurements and see how they compare to what they should be. Number two, we’ll look at the bones themselves to see if they look normal, which is again what your doctors did. We’ll look for any other issues going on with the baby, any other parts of the baby that may or may not be lagging in growth. And then we’ll sort of see how profound it is. And then we’ll try to get a sense. Is this something that’s more likely to be problematic or more likely to be really nothing? And then if it’s at the time appearing problematic, obviously, we talk about which one it might be. What does it mean? What kind of testing do we need to do? Sometimes we’ll do an amniocentesis to check for certain genetic conditions and so forth.

But if it looks like it’s likely or probably or most likely, just nothing and just the bones are lagging by a little bit, but probably not an issue, what we’ll typically do is say come back in two weeks. and we will measure them again. And so two weeks later, if the bones are measuring about two weeks larger, then they grew normally over those two weeks. And again, then the chance that it’s going to be an issue goes down. So, as an overall answer to your question, I don’t know, but I would say what you’re describing does not sound particularly concerning. The fact that the baby looks perfectly fine otherwise, but the bones are measuring a week or two behind, usually that’s not much, and probably they’re going to have you come back in a couple of weeks to check again. And if the bones are growing and they’re staying about a week or two behind, it’s more likely going to be nothing significant. But obviously, if things come up and change or look more concerning than based on exactly what’s going on, they’ll do that. And for people who are sort of potentially going to be in this situation, it’s very, very important not to just look at the percentile or just look at what’s going on but to make sure that whoever is giving this information really knows what they’re doing and understands this so they can find out what’s important but also not scare you to death unnecessarily because these are definitely situations where a lot of people get scared probably unnecessarily.

Carnegie Imaging for Women blogs are intended for educational purposes only and do not replace certified professional care. Medical conditions vary and change frequently. Please ask your doctor any questions you may have regarding your condition to receive a proper diagnosis or risk analysis. Thank you!