15 Reasons Unborn Babies Are Measured By Percentile And What It Means

The adventure of pregnancy seems to be one where a whole of new words and concepts are introduced to the vocabulary. This is great and all, it’s one big long 9 month learning moment. But really, there’s a time and place for these learning moments – and it’s not when the hormones, body, and everyday life as it was once is changing rapidly.

For this reason, hearing the doctors, midwives, and OBGYNs blurt out a bunch of words that seem to make no sense can be a little stressful. Or maybe not stressful enough. It can turn into one of those moments where pregnant women just sit back, smile and nod, and pretend that they’ve absorbed everything that was just said. Umm, not the best strategy.

This can happen a lot when percentiles are mentioned. They’re not percentages, and they’re not tiles, so what are they and what do they have to do with your baby? This term is just one of those in the pregnancy world that are important, but so confusing. And confusion is the last thing you want right now.

So percentiles are obviously important. If they weren’t, none of these professionals would mention them, right? But what’s up with there been so many percentiles and all these different numbers. What does it really mean and what does it actually look like for your baby. Find out some fun percentile facts and arm yourself with some prior knowledge before good old doc starts throwing jargon at you during your next prenatal check up.

Continue scrolling to keep reading

Click the button below to start this article in quick view

Start Now

15 Talking About Percentiles Is Like Swallowing A Dictionary

Understanding percentiles is literally like swallowing a dictionary. This is why so many times expectant moms just smile and nod at the doctor and try to go along with it. But are they talking about ABCs still or are they back onto CBAs. Who knows anymore, right?

Some common percentile jargon includes the biparietal diameter (BPD) and femur length (FL). The head circumference becomes known as the HC, which makes sense. Then there’s the abdominal circumference which is the AC and the humerus length, the HL. These are the basic measurements to get a grasp of at the gestational age.

But the fun doesn’t stop there. Then there’s the Embryonic Crown Rump Length (CRL) and the Mean Sac Diameter (MSD) and who even knows what else. These acronyms do measure important things that come together to create an understanding of the percentiles. But at the end of the day, all moms want is a HHB (healthy, happy baby).

14 Different Measurements For Different Percentiles

The growing fetus is super popular amongst OBGYNs and doctors. They want to know everything about how the baby is growing, including a bunch of different measurements. This can seem a little odd when doctors want to measure the baby from head to toe, even when the toes aren’t fully developed and the head is bigger than the body. And also, since they can’t physically measure anything with a measuring tape.

Basically, doctors use a system of measuring along the beam axis and across the axis of an ultrasound. The two most important measurements in the early stages of a fetus is the biparietal diameter and the femur length, as well as the head measurements. All these measurements come together to tell the doctor a whole lot of important stuff that is hard to translate to layman’s terms– but the main message to translate back to you is that baby is going and growing well.

13 Percentiles Start From Gestational Age

When it comes to percentiles, the fetus is measured from what is called gestational age. This is basically an ‘age’ that describes how far along the pregnancy is. It starts from the first day that the menstrual cycle ended. And women are pretty in tune with that number one indicator of pregnancy!

What this means for percentiles is that the measurements in the first ultrasound need to line up with the gestational age, so the time that the period ended. This is a major way that doctors get the estimated due date roughly correct, or at least attempt to get as close to it as possible.

Once the gestational age is established, the percentiles become important, showing that the baby is growing properly in the expected range. The percentiles are there to show that 20 weeks after gestation, a certain measurement range should be met. If it is, then baby is ticking all the boxes. This is reassuring for all involved!

12 Important To Check For Odd Growth

The main aim of percentiles is to show that the fetus is growing at a healthy and expected rate. While this isn’t a standard science tick where one can select from multiple choice answers and have only one outcome, percentiles are rough estimates. However, being inside the measurement range at a certain point is important.

If any ‘odd’ growth, that being growth outside the percentile parameters is occurring, this is when doctors become a little concerned. Sometimes it is nothing to be worried about, other times it can be a sign of health problems to come.

Percentiles are essentially a form of early detection. For babies that are very large or very small, there is an increased risk of health complications. Doctors can be switched onto this by measuring the fetus and comparing them to percentile ranges. Early detection and awareness is the number one combatant against any issues for the future in this case.

11 Head Percentile Measures Brain Growth

Some say that a big head means big brains and this means bigger intelligence. Maybe, maybe not. Either way, measuring the head in terms of percentiles is super important and something that docs are going to pay a lot of attention to.

The tricky thing with the head percentile is that it's different to the rest of the body’s percentiles. So baby's head percentile could be in the 5th percentile at 27 weeks and the 55th percentile in terms of bodily growth. This is why they are hard to keep up with, but thankfully doc knows exactly where healthy head growth and brain development sits in relation to the rest of the body.

A lot of moms do tend to be alarmed when their baby’s head is in the percentile for 27 weeks but their gestational age is 30 weeks. This can be concerning, of course, but in the majority of cases it's just a matter of baby interpreting the percentiles as guidelines. In most cases, the head growth will even out over time and the brain will fit nicely in the head.

10 Percentiles Really Mean Patterns

After the confusing concept of percentiles has been thrown around, the doctor is going to measure them all and place them on a plot graph that ends up in a chart. These charts are important to see how a fetus is growing in relation to other fetuses, which is where the idea of ‘normal’ development largely comes from.

What this means is that basically percentiles reveal how your fetus is growing compared to the national survey. Therefore, if your baby girl is growing in the 25th percentile, she is bigger than 25 percent of other baby girl fetuses and smaller than 75 percent.

This is nothing to be worried about at the end of the day. Rather, this pattern is one that emerges over time. Since fetuses grow at a fairly predictable rate, there is a steady curve that happens along the growth chart. And at the end of the day, each unique baby grows to their own pattern, just as they march to their own beat once they’re out of the womb.

9 Patterns Are More Like Guidelines, Though

Pretty much most of parenting advice and baby information ends up being a guideline. From the ‘right’ way to talk to a child to the ‘correct’ amount of screen time a toddler should have, parenting is so much more about taking the suggested ideas and following them as a guideline.

In terms of percentiles, they are certainly important. They do help doctors find out if a fetus is growing at a predicted and steady rate. They do come in handy for early detection of any growth abnormalities. But all in all, they are more useful as guidelines.

This is why a doctor probably isn’t going to tell you that your baby is growing two percentiles below the expected for their gestational age. They are probably just going to tell you that your baby is growing healthily and well. This is because it’s literally no cause for concern most of the time. Babies catch up to the percentiles in their own time. They don’t take the guidelines too seriously, and you don’t have to either.

8 Babies' Percentiles Can Change Everyday

This is the other reason why it is so hard to take percentiles too seriously. Again, they are important and they are needed for doctors. But since babies think of percentiles as guidelines, they have no issue with bouncing from percentile to percentile. This might give mom-to-be a near heart attack to think about, but it is no real drama.

So growing fetuses do tend to drop from one percentile to the next from ultrasound to ultrasound. Sometimes if a doctor notices a big change, they will want to schedule more regular scans. This is more or less a precautionary measure so that the growth can be tracked closer.

However, there are so many factors that impact on how a fetus is measured in a particular scan. For instance, in the latter months of pregnancy, babies can be more active in the womb and if they are wriggling, it is harder to get an accurate measurement. This can impact on where they sit in the percentiles.

7 Percentiles Aren’t Fortune Tellers

We know that percentiles can indicate if a baby is going to be particularly short or long. For instance, a fetus in the 10th percentile for weight and the 25th percentile for height is probably going to have a petite frame. As long as it is relative and comfortable, there is nothing wrong with these discrepancies.

However, just because your baby boy is in the 95th percentile for height, don’t get too excited about an all-star basketball career ahead of him. Growth patterns in the womb don’t give an exact picture to how the baby is actually going to come out.

Sure, they can suggest patterns of growth, but there is nothing guaranteed. Really, there is nothing guaranteed with anything about the way a baby is going to grow. So don’t get too excited when the doctor says ‘looks like he’s going to be a tall one!’. Wait and see how the growth really happens before your eyes before planning baby's future career.

6 The Fetal Weight Is Generally Wrong

Here is another reason why percentiles really are more like guidelines. Getting an exact fetal weight is really difficult and there is no formula at the moment proven to be entirely accurate. Fetal weight errors tend to be about 10% higher or lower of the estimated weight. So it’s not a huge margin of error, but something to be aware of.

Doctors might be able to tell you that your baby is a little heavier than their expected percentile, or weigh a little less. But unless there is a dramatic discrepancy, they aren’t going to call for too many intervention measures.

Most of the time, the fetal weight is an educated guess. So while it is important to pay attention to the percentile, don’t get too caught up in it. It’s not worth announcing to Facebook that your growing fetus is in the 40th percentile weight range and you’re so proud. Most people probably won’t understand you anyway!

5 Percentiles Can Drop And You Don’t Need To Panic

As baby bounces around in the womb, percentiles bounce with it. Not literally, but there are so many factors that cause the percentiles to drop during fetal development. Unless there is a major and dramatic drop from, for example, the 95th percentile to the 25th, most doctors aren’t going to show too much concern.

It is pretty normal for growing fetuses to jump from, for example, the 50th percentile to the 60th percentile and back down within the space of ultrasounds. This is just because so much growth is occurring and it's hard for the whole body to keep up. For instance, the weight percentile can be affected by certain organs getting bigger and heavier before the limbs are fully developed.

Remember, fetuses grow in their own time and to their own pattern, not factoring in the doctor’s opinion too much. Percentiles aren’t something they are thinking of, even though they’re being measured against them.

4 Percentiles Come With Ultrasounds

Throughout pregnancy, ultrasounds become extremely important. They are not only a check up on the health of baby growing inside the womb, but also a wonderful memento of pregnancy. Everyone loves sharing an ultrasound photo with family and friends, even if it just looks like a peanut hidden amongst black and white waves to the untrained eye.

During ultrasounds, doctors will be frantically figuring out the measurements of the baby. From measuring the head to the stomach and finding out the length of the baby as well as the width; there is whole lot more going on while you’re just staring at the screen with gooey loved-up eyes.

From ultrasound to ultrasound, doctors measure the length and weight of the baby, getting a percentile range. These are compared each time to get an overlook at the growth of the baby. This helps to inform your individual baby’s percentile growth and get a predicted growth rate for them.

3 So Many Things Influence The Percentile Measurement

As with anything in terms of a baby’s development, there are so many influences and factors to be aware of. Obviously, genetics play a pretty big role regarding how a baby is growing. This has the first influence over how tall, short, skinny, or big boned a baby will be. And doctors can get a sense of this just from measuring fetuses.

Other factors that are important include nutrition, diet, and lifestyle. For example, small head growth and a head circumference less than the average percentile can be linked to Fetal Alcohol Syndrome. Therefore, alcohol and drug use while growing a baby has obvious negative consequences.

This one is just a reminder that everything you do while growing a fetus impacts them, including where they sit on the percentile charts. So keep up that healthy diet and regular exercise as best as you can for these super important 9 months!

2 Percentiles Don’t Stop In The Womb 

That’s right, the percentile fun doesn’t stop once the baby is born. In fact, percentiles remain to be really important to monitor early childhood growth and development. Between being a newborn and having their first birthday, babies grow at exponential rates. This is when percentiles change in leaps and bounds.

Getting regular check ups with a paediatrician is really important in that first year in particular. Some babies can triple their birthweight in this time, which means those go from the 50th percentile up to the 75th. This can seem extreme, but it is actually just a sign of good growth.

Unless there is a major drop in the percentile, doctors won’t be too worried. Drops normally occur from illness or nutritional problems, even hormonal problems. But typically it’s up, up, and away with percentiles. Going from the 50th to 75th percentile means that your baby is following the curve of percentiles, which is a good thing.

1 Percentile Five Is The Magic One

If there can be a magic number with percentiles that are so flexible, unsteady, and acting more as guidelines, it is percentile 5. Ideally, babies will be born well above percentile 5. Basically, percentile 5 means that they are just scraping into the healthy medium with percentiles.

Although the 5th percentile isn’t ideal, it means that babies are going to be born fully functioning and healthy. They might need some lung boosting injections and additional oxygen as well as closer monitoring, but all in all percentile 5 is feeling fine.

Anything below the 5th percentile is when there is an increased risk of health problems and, sadly enough, a more likely chance of the baby becoming an angel instead of a child. However, if your baby is measuring up at the 5th percentile, just remember this is a reflection of how your baby is choosing to grow. After all, someone’s gotta keep percentile 5 on the charts!

Sources: Babble.com, Webmd.com, Baby2See.com, Essentialbaby.com

More in Did You Know...