Does my baby look big in this?!
Once you reach that magical time in pregnancy when your bump is finally showing, the unsolicited comments about the size of your belly are sure to start! Whether it's "Wow, your bump is tiny!" or "Are you sure there's only one in there?!", these remarks can make you feel quite anxious (and probably quite annoyed!) about just how big your little one is growing. So, let's dive into the world of measuring baby's growth during pregnancy and uncover what all those numbers and measurements really mean. Spoiler alert: there's more to it than just the size of your bump!
Methods of Measuring Baby's Growth
First things first, how can your care provider measure and track your baby’s growth? There are a couple of different ways to do this, and we’ll start with ultrasound scans, as most women will likely have a scan well before their first midwife appointment.
A couple of abbreviations I’ll use throughout:
AFW = Actual Fetal Weight (fetus = baby!)
EFW = Estimated Fetal Weight
IUGR = Intrauterine Growth Restriction
LGA = Large for Gestational Age
SFH = Symphysis-Fundal Height
SGA = Small for Gestational Age
Ultrasound scans:
There’s more to ultrasound scans than getting keepsake pictures!
How many scans will I receive?
Before we get stuck in, I just want to highlight (as always!) that every test offered to you during pregnancy is your choice. You can choose to accept or decline any/all of these scans.
Depending on how early you find out you are pregnant, you can typically expect to receive 2-3 ultrasounds scans during pregnancy. Some women may be offered more than this, and we’ll explore that in more depth later on. The first scan is called a dating scan and is offered between 6 and 12 weeks. The sonographer will take key measurements of the baby, which will help them to determine how many weeks pregnant you are, and whether that correlates with the date of your last menstrual period.
The second scan is offered as part of a screening process for chromosomal abnormalities, such as Down Syndrome. It is called the Nuchal Translucency Scan and is done between 11 and 14 weeks. If you have chosen to decline this screening, then you won’t need to have this scan performed.
The third scan is called the morphology or anatomy scan and is performed between 18 and 20 weeks. This ultrasound checks the baby's physical development, including the heart, brain, spine, and other organs. It can also determine the baby's gender if you wish to know and will provide an estimate of the baby’s weight. This scan will also locate the position of the placenta (I have made an informative video on low-lying placentas, which you can view here). Having this scan is usually a very exciting time, as it may even be the first time you have actually seen an image of your baby! I really want to highlight here that this scan is a screening tool, meaning that the sonographer is checking for abnormalities. If this is something you do not wish to know, you can either decline the scan altogether or inform the sonographer of your wishes.
Some women are offered additional scans later on in pregnancy, and this can be for a number of reasons, such as monitoring the baby’s growth or checking the placenta.
How do sonographers estimate the baby’s weight?
During my research for this blog post I discovered that there are over 20 different formulas for estimating fetal weight! As is the case with many other clinical practices within maternity care, there is no universal consensus as to which formula is the most accurate. In Australia, the Royal Australian and New Zealand College of Obstetricians and Gynaecologists (RANZCOG) recommend the use of the Hadlock formula, which uses the following fetal measurements to estimate weight:
Biparietal Diameter (BPD): The width of the fetal head.
Head Circumference (HC): The circumference of the fetal head.
Abdominal Circumference (AC): The circumference of the fetal abdomen.
Femur Length (FL): The length of the fetal femur bone.
I won’t bore you with the maths but the sonographer takes these measurements during the scan and inputs them into the ultrasound machines, which have in-built software that uses the Hadlock formula to calculate the estimated fetal weight.
Concerns have been highlighted by researchers (for example, in this study) regarding a blanket approach to using EFW formulas, as they are all based on normal fetal growth rates and therefore should not theoretically be applied to women with medical conditions or pregnancy complications which may impact the baby’s size.
How accurate are ultrasound scans?
The research for ultrasound scan accuracy is a bit of a minefield! There are lots of studies on this topic, all with varying conclusions. There are a number of important factors which need to be taken into consideration in terms of assessing accuracy, including the clinical experience of the person conducting the scan, how close to the due date the weight estimation was done, and maternal BMI. All of these factors result in less accurate estimations. Overall, there seems to be a discrepancy between estimated birth weight and actual birth weight of 6-11% (although some studies state this percentage to be higher). Let’s take the average between these two numbers, which is 8.5%. If your baby is estimated to weigh 3500 grams, the actual birth weight is likely to fall between 3203 grams and 3798 grams (+/- 298 grams). The heavier the estimated fetal weight, the larger the discrepancy becomes. Let’s look at a suspected “big baby” of 4000 grams. Using our average percentage of 8.5%, the parameters become 3660 grams - 4340 grams (+/- 340 grams).
Are there any risks associated with having multiple scans?
Ultrasound scans produce images using high-frequency soundwaves, emitted via a probe which is placed directly onto the skin. The cold gel used to coat the probe helps to transmit the soundwaves. The images are generated by the soundwaves reflecting off structures within the body, such as our organs or in this case, a baby!
Whilst ultrasound scans during pregnancy are generally considered to be safe, there is the potential for biological reactions to occur, such as heating of body tissue, and the production of small pockets of gas known as cavitation. However, the long-term consequences of these potential occurrences are unknown at present. For this reason, it is recommended that ultrasounds are only offered during pregnancy if clinically indicated to reduce any potential risk to the baby’s well-being. This raises serious ethical considerations for companies which offer ultrasound scans and videography purely for pregnancy keepsakes, and services such as these continue to grow in popularity.
Multiple scans can sometimes lead to overdiagnosis and unnecessary anxiety for expectant mothers, particularly if they begin to feel a loss of control over their care as demonstrated in this study on women who received a “big baby” diagnosis during pregnancy.
A quick summary of our exploration of ultrasounds…
Ultrasound scans are typically offered 2-3 times during pregnancy, with each scan serving different purposes like confirming pregnancy dates and assessing physical development. Additional scans may be provided for specific concerns. The Hadlock formula, used to estimate fetal weight, has a margin of error influenced by various factors. While generally safe, ultrasounds can pose potential risks and should be used only when necessary to avoid overdiagnosis and unnecessary stress. Understanding these aspects can help you make informed decisions about your pregnancy care.
Abdominal Palpation and Fundal Measurements:
Tradition versus technology
Between 12 and 14 weeks of pregnancy, the uterus (womb) can usually be felt above the top of the pubic bone (when a midwife or obstetrician has a feel of your tummy, it is called an ‘abdominal palpation’), and from around 20 weeks onwards measurements are usually taken at each antenatal appointment to assess the baby’s growth. This measurement is called the symphysis-fundal height (SFH), or more commonly referred to as simply the fundal height and is done by using a tape to measure from the top of the pubic bone to the top of the uterus (which is called the fundus) - it doesn’t sound very scientific does it?!
Midwives and traditional birth-keepers have been feeling pregnant bumps for centuries, but in the 1950s and 1960s as part of the medicalisation of pregnancy and birth, fundal height measurement was introduced as a standard practice. It was a straightforward, non-invasive method to assess fetal growth and development during pregnancy. Studies demonstrated a correlation between fundal height measurements and gestational age, leading to its widespread adoption. During these studies, clinical researchers observed that, on average, the fundal height in centimetres closely matched the gestational age in weeks after the first trimester.
This is where the commonly used 1 cm = 1 week rule stems from - it simplified the monitoring process, allowing healthcare providers to quickly estimate fetal growth and gestational age during routine check-ups. It’s not an exact science of course, and measurements can vary from practitioner to practitioner. Not everyone agrees with this clinical rule of thumb either, with variations used across the globe and no universal agreement on which is most accurate.
As with everything in pregnancy, having your bump measured is an option! You can use the information in this post to help you make the choice which feels right for you.
How accurate is Symphysis-Fundal Height measurement?
As I was researching this question, I was astounded to learn that there are 19 different recorded methods for measuring SFH, and 12 different ways that SFH can be converted into gestational age! This makes navigating the research very tricky, as different studies use different methods for collecting and interpreting their data, which in turn means that it is difficult to make comparisons.
I came across a systematic review of using SFH as a way to determine gestational age (so not focusing on the baby’s weight, but it is indirectly related). The researchers determined that when using the 1cm = 1 week clinical rule of thumb, measuring the fundal height results in a margin of error of +/- 43 days! The authors of this paper go as far as to say that SFH should be abolished as part of routine antenatal care due to the high margin of error and the potential consequences associated with overdiagnosis of problems and the subsequent clinical interventions used to manage them. However, this paper only collated evidence from studies done in low-to-middle income countries, where access to technology such as ultrasound scans is limited and the level of professional skill may vary, so therefore the findings may not be applicable to higher income countries. You can read the paper here.
The clinical studies I’ve read are generally in agreement that measuring SFH should not be the only tool used to estimate fetal weight. SFH by itself is considered a poor method for determining babies that are on either extreme of the weight scale - i.e. very small babies and very big babies. This could be down to the technique itself, practitioners’ levels of experience, or that practitioners tend to be quite conservative in their measurements.
The World Health Organization recommends that any variations beyond +/- 2 cm from the expected fundal height should be investigated further as it may indicate potential growth issues.
How can a baby’s weight be estimated from this?
It’s hello maths once again! There are several different mathematical formulas which care providers can use to estimate a baby’s weight, and once again there is no universal agreement as to which one is the most accurate. Johnson’s Formula and Insler’s Formula feature most prominently in the research, and each use measurements from abdominal palpations in a slightly different way to estimate fetal weight (again, I won’t bore you with the actual maths!).
Interestingly, there have been several studies (this paper here has great references) which have shown Insler’s Formula to be slightly more accurate at estimating fetal weight than Hadlock’s Formula (which is the formula used by most ultrasound machines), but the researchers have still concluded that ultrasound scans should be considered the ‘gold-standard’ method of estimating fetal weight. Baffled? Me too!
A quick summary of our exploration of hands-on estimates…
Abdominal palpation and fundal height measurements are non-invasive ways of tracking a baby's growth during pregnancy. Starting around 20 weeks, midwives measure from the pubic bone to the top of the uterus, with the general rule of thumb being that one centimetre equals one week of pregnancy. While widely used, this method isn't always precise, and opinions on its accuracy vary. It is an optional procedure, and you can decide whether to have it done.
Research shows that there are many ways to measure and interpret fundal height, making it hard to draw conclusions about which method is most accurate. Measuring fundal height alone is not reliable for identifying very small or large babies. Although there are various formulas to estimate baby weight from these measurements, ultrasounds are generally considered to be more accurate.
Ultimately, as my midwifery idol Rachel Reed says, the best way to assess a baby’s weight is to actually weigh the baby after the birth!
Interpreting Growth Measurements
“Healthy well-nourished women grow healthy well-nourished babies” - Rachel Reed
If you have been offered additional scans to monitor your baby’s growth, you might be wondering what your care providers do with all this information! Once all of the required measurements have been taken and estimated weight has been calculated, these may then be recorded on a growth chart, which enables care providers to identify trends and any deviations from the expected growth pattern.
To assist care providers with their assessments, the growth chart includes curves or percentiles that represent average growth patterns based on a large pool of data. These percentiles help to determine whether a baby's growth is within the normal range. The 50th percentile represents the average measurement; half of babies will plot above this line and half below. Measurements which plot outside of the either the 10th percentile or the 90th percentile may indicate that the baby is smaller (below the 10th percentile) or larger (above the 90th percentile) than normal for its gestational age.
You can check out some examples of fetal growth charts here (around halfway down the page). There may even be a growth chart in your handheld pregnancy notes.
Is my baby too big?
There are two terms you may hear when care providers talk about “big babies”. Large for gestational age (LGA), and fetal macrosomia. Whilst the terms are related, there are distinct differences.
LGA refers to a baby that is larger than expected for the number of weeks of pregnancy. LGA is typically defined as a baby whose weight is above the 90th percentile for gestational age, either via estimations during pregnancy or by the actual birth weight. LGA can either be diagnosed during pregnancy or after the birth.
Fetal macrosomia refers to a newborn with an excessive birth weight, regardless of gestational age. The most common definition of fetal macrosomia is a birth weight of more than 4000 grams or 4500 grams, depending on the criteria used. Again, there is debate as to what the exact definition should be. Fetal macrosomia is typically diagnosed after birth when the actual weight is known.
This can be a tricky diagnosis to navigate, as care providers can become fixated on potential problems associated with big babies during labour and birth, and suddenly you’re heading down a path you didn’t envision being on! Rachel Reed has written a brilliant blog post on this, which you can read by clicking here.
There are several reasons why women grow big babies - medical conditions such as diabetes may contribute, lifestyle factors such as diet and exercise, and genetics also play a role.
Is my baby too small?
Again, you may hear two different terms being used when discussing small babies - ‘small for gestational age (SGA)’ and ‘intrauterine growth restriction (IUGR)’. Whilst they are related, there are some key differences between the two.
SGA is a descriptive term used after birth and refers to babies whose birth weight is below the 10th percentile for their gestational age. SGA babies are usually perfectly healthy, just naturally small.
IUGR is a clinical diagnosis made during pregnancy and indicates that the baby is not growing at the expected rate inside the womb due to an underlying problem preventing normal growth, such as placental issues, maternal health conditions such as high blood pressure and diabetes, and lifestyle choices such as smoking and alcohol consumption. Babies with IUGR are at higher risk for complications both before and after birth.
How accurate are fetal growth charts?
Wow, I opened a Pandora’s Box with this blog! There are no universally accepted fetal growth charts, and even within Australia we can see differences between individual states (and even differences between hospitals in the same state) depending on local recommendations and care provider preferences. Despite recommendations from RANZCOG for the use of customised growth charts, which also factor in variables such as maternal height, weight and ethnicity, standard growth charts remain the norm. A study conducted at King Edward Memorial Hospital, Perth’s principal tertiary maternity hospital, plotted data from almost 3000 women using 4 different fetal growth charts and noted significant variations, with each chart resulting in different percentiles for the exact same measurement.
In the same study, researchers identified two growth charts which best fit our local population characteristics (but they weren’t without their flaws), but this doesn’t mean that these recommendations have been implemented by other hospitals.
Personally, I think this is a harmful approach to maternity care, as we are expecting each and every woman to fall within a specific set of parameters, with little to no consideration of individual factors or circumstances. As with everything, we need to look at the bigger picture (pardon the pun!), and not just little dots on a graph.
A quick summary of our exploration of growth charts…
When additional scans are done to monitor your baby's growth, the measurements are recorded on a growth chart to track trends and deviations from expected patterns. These charts use percentiles to show whether a baby's growth is within the normal range, with measurements outside the 10th or 90th percentiles indicating smaller or larger than average growth for the gestational age.
Fetal growth charts are not universally standardised, and variations can exist between regions and hospitals, making it essential to consider a more individualised approach to maternity care.
To sum up everything we’ve covered (and there was a LOT!)…
As you can see, measuring your baby's growth during pregnancy involves a variety of methods, each with its own set of complexities and potential inaccuracies. While ultrasound scans and fundal height measurements are common tools, it's important to remember that they are not infallible. Factors such as the experience of the sonographer, maternal BMI, and even the specific formula used can all influence the estimated measurements. Ultimately, these methods provide a way to monitor growth trends and identify any potential concerns, but they should be viewed as part of a broader picture of maternal and fetal health.
Understanding the nuances behind these measurements can help you feel more informed and empowered during your pregnancy journey. It's crucial to communicate openly with your healthcare providers, ask questions, and make decisions that align with your comfort and values. Remember, every pregnancy is unique, and the best assessment of your baby's size and health will come after birth. If you ever feel overwhelmed or unsure, don't hesitate to seek additional support or a second opinion. Your peace of mind and well-being are just as important as the numbers on a growth chart.
Where can I go for additional information?
Rachel Reed’s blog post on “big babies”
Dr Sarah Buckley has written an interesting article on the routine use of ultrasound scans during pregnancy, which you can read by clicking here.
Click here to read a great summary of routine screening tests offered during pregnancy on the Pregnancy, Birth & Baby website.