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. Let's get into what all those numbers and measurements actually mean — there's more to it than just the size of your bump!
Methods of measuring baby's growth
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 accept or decline any or all of these scans.
Depending on how early you find out you are pregnant, you can typically expect 2-3 ultrasound scans during pregnancy. Some women may be offered more, and we'll explore that 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 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 screening 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, you won't need this scan.
The third scan is called the morphology or anatomy scan and is performed between 18 and 20 weeks. This 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). I really want to highlight 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, for 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 clinical practices within maternity care, there is no universal consensus as to which formula is the most accurate. In Australia, RANZCOG recommend 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
The sonographer takes these measurements during the scan and inputs them into the ultrasound machine, which uses the Hadlock formula to calculate the estimated fetal weight.
Concerns have been raised (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 on ultrasound scan accuracy is a bit of a minefield! There are lots of studies on this topic, all with varying conclusions. Important factors include 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 which affect accuracy. Overall, there seems to be a discrepancy between estimated birth weight and actual birth weight of 6-11% (some studies put this higher). Let's take the average of 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. Looking at a suspected "big baby" of 4000 grams: using 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 placed directly onto the skin. The cold gel used to coat the probe helps transmit the soundwaves. Images are generated by the soundwaves reflecting off structures within the body.
Whilst ultrasound scans during pregnancy are generally considered safe, there is potential for biological reactions such as heating of body tissue, and the production of small pockets of gas known as cavitation. The long-term consequences of these potential occurrences are unknown at present. For this reason, ultrasounds should only be offered during pregnancy if clinically indicated. This raises serious ethical considerations for companies which offer ultrasound scans and videography purely for pregnancy keepsakes.
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.
Abdominal palpation and fundal measurements
"Healthy well-nourished women grow healthy well-nourished babies" – Rachel Reed
Between 12 and 14 weeks of pregnancy, the uterus can usually be felt above the top of the pubic bone, 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 (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. Studies demonstrated a correlation between fundal height measurements and gestational age, leading to its widespread adoption. Clinical researchers observed that, on average, the fundal height in centimetres closely matched the gestational age in weeks after the first trimester — hence the commonly used 1 cm = 1 week rule.
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. As with everything in pregnancy, having your bump measured is an option.
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 data.
I came across a systematic review which determined that when using the 1cm = 1 week rule, measuring the fundal height results in a margin of error of +/- 43 days! The authors 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. 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.
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.
Interpreting growth measurements
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 the required measurements have been taken and estimated weight calculated, these may be recorded on a growth chart, which enables care providers to identify trends and any deviations from the expected growth pattern.
The growth chart includes percentile curves representing average growth patterns based on a large pool of data. The 50th percentile represents the average measurement. Measurements which plot outside of either the 10th or 90th percentile may indicate that the baby is smaller or larger than normal for its gestational age.
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 related, there are distinct differences.
LGA refers to a baby that is larger than expected for the number of weeks of pregnancy — typically defined as a baby whose weight is above the 90th percentile for gestational age.
Fetal macrosomia refers to a newborn with an excessive birth weight, regardless of gestational age. The most common definition is a birth weight of more than 4000 grams or 4500 grams, depending on the criteria used. This 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. Rachel Reed has written a brilliant blog post on this, which you can read here.
Is my baby too small?
Again, you may hear two different terms — "small for gestational age (SGA)" and "intrauterine growth restriction (IUGR)".
SGA is 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, indicating that the baby is not growing at the expected rate inside the womb due to an underlying problem, such as placental issues, maternal health conditions, or lifestyle factors such as smoking and alcohol consumption.
How accurate are fetal growth charts?
There are no universally accepted fetal growth charts, and even within Australia we can see differences between individual states — and even 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 factor in variables such as maternal height, weight and ethnicity, standard growth charts remain the norm.
A study conducted at King Edward Memorial 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.
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. We need to look at the bigger picture (pardon the pun!), and not just little dots on a graph.
To sum up…
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, 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.
Understanding the nuances behind these measurements can help you feel more informed and empowered during your pregnancy journey. Communicate openly with your healthcare providers, ask questions, and make decisions that align with your comfort and values. Every pregnancy is unique, and the best assessment of your baby's size and health will come after birth. 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!