Understanding Your Pregnancy Growth Scan

A growth scan is a special ultrasound performed in the third trimester to check your baby's size, growth, and overall well-being. It helps your doctor ensure your baby is developing as expected and identify any potential concerns early.

Published: October 27, 2023•Updated: October 27, 2023

What is a Pregnancy Growth Scan?

A growth scan, also known as a fetal growth scan or third-trimester ultrasound, is a diagnostic ultrasound examination typically performed after 28 weeks of pregnancy. Its main purpose is to assess how well your baby is growing and to check their overall health and well-being.

During this scan, your healthcare provider will estimate your baby's size and weight, comparing these measurements to what's expected for your stage of pregnancy. It also involves checking the amount of amniotic fluid around your baby, the position of the placenta, and how blood is flowing to your baby through the umbilical cord.

Why Might You Need a Growth Scan?

Your doctor might recommend a growth scan for several reasons, often if there are specific factors that could affect your baby's growth. These reasons are called 'indications' and can include:

Maternal Factors:

Pre-existing health conditions like chronic high blood pressure, diabetes (before or during pregnancy), kidney disease, or certain autoimmune conditions.
Being over 40 years old.
Having a low pre-pregnancy weight or not gaining enough weight during pregnancy.
Smoking, alcohol use, or use of illicit drugs.
A history of a previous pregnancy with a baby who was too small or a stillbirth.

Pregnancy-Related Factors:

If your doctor suspects your baby might be growing slower than expected (often indicated by your belly measurement, called symphysis-fundal height, being smaller than it should be).
If your doctor suspects your baby might be growing larger than expected (indicated by your belly measurement being larger than it should be).
Developing high blood pressure or pre-eclampsia during pregnancy.
Gestational diabetes.
Carrying twins or triplets.
Noticing decreased fetal movements.
Too little (oligohydramnios) or too much (polyhydramnios) amniotic fluid.
Placental issues, such as placenta previa or concerns about how well the placenta is working.
If your pregnancy goes beyond 41 weeks (post-term).

It's worth noting that guidelines on routine third-trimester scans can vary. In some regions, like the UK (NICE guidelines), regular belly measurements are used to screen for growth issues, with a scan only if concerns arise. In others, like the US (ACOG guidelines), routine third-trimester scans are not recommended for low-risk pregnancies, but are based on specific clinical indications.

What Happens During a Growth Scan?

A growth scan is a straightforward, non-invasive procedure, much like other ultrasounds you might have had during pregnancy.

The Procedure:

You'll lie comfortably on an examination table. A trained sonographer or doctor will apply a water-based gel to your abdomen. They will then move a small handheld device called a transducer across your skin. This transducer sends sound waves into your body, which bounce off your baby and create images on a screen.

Key Measurements (Fetal Biometry):

The sonographer will take several important measurements of your baby:

Biparietal Diameter (BPD): The width of your baby's head.
Head Circumference (HC): The measurement around your baby's head.
Abdominal Circumference (AC): The measurement around your baby's tummy. This is a very important indicator of how well your baby is nourished.
Femur Length (FL): The length of your baby's thigh bone.

Estimated Fetal Weight (EFW):

Using these measurements, a special formula is used to calculate your baby's Estimated Fetal Weight (EFW). This EFW is then plotted on a growth chart to see how it compares to other babies at the same gestational age. It's important to remember that EFW is an estimate and can have a margin of error, typically around ±15-20%. This means the actual birth weight could be a bit higher or lower than the estimate.

Assessing Fetal Well-being:

Beyond just size, the scan also looks at other aspects of your baby's health:

Amniotic Fluid Volume: The amount of fluid surrounding your baby is measured. Too little (oligohydramnios) or too much (polyhydramnios) fluid can sometimes indicate a concern.
Placental Position: The location of your placenta is checked.
Doppler Velocimetry: This special technique measures blood flow in specific vessels, like the umbilical artery (which assesses how well the placenta is working) and the middle cerebral artery (which can show if your baby is adapting to any challenges by redirecting blood flow to the brain, sometimes called 'brain-sparing').

Understanding Your Scan Results

After your scan, your doctor will discuss the findings with you.

Normal Growth:

If your baby is growing well and all measurements are within the expected range, you'll likely continue with your routine antenatal care. This is reassuring news!

Suspected Fetal Growth Restriction (FGR) / Small for Gestational Age (SGA):

This means your baby is estimated to be smaller than expected for their gestational age (typically below the 10th percentile for weight or abdominal circumference). If this is suspected, your doctor might recommend:

Increased Monitoring: More frequent growth scans (usually every 2-4 weeks) to track growth velocity, along with regular checks of fetal well-being using Doppler studies, biophysical profiles, or nonstress tests.
Specialist Consultation: You might be referred to a Maternal-Fetal Medicine (MFM) specialist for further evaluation and management.
Management: Your doctor will work to address any underlying maternal health issues. In some cases, if early delivery is anticipated, antenatal corticosteroids might be considered to help your baby's lungs mature. The timing of delivery will be carefully considered, balancing the risks of prematurity against the risks of continuing the pregnancy if the baby is not thriving inside the womb.

Suspected Large for Gestational Age (LGA) / Macrosomia:

This means your baby is estimated to be larger than expected for their gestational age (typically above the 90th percentile for weight). While a big baby can sound good, it can sometimes lead to challenges during birth. Your doctor will discuss:

Risks: Potential risks like shoulder dystocia (where the baby's shoulder gets stuck during delivery), birth trauma, and an increased chance of needing a C-section.
Managing Underlying Conditions: If you have gestational diabetes, strict control of your blood sugar levels is crucial.
Delivery Planning: Your doctor will discuss the timing and mode of delivery. Suspected macrosomia alone is generally not a reason to induce labor before 39 weeks. A planned C-section might be considered if the estimated fetal weight is very high (e.g., over 5,000g for non-diabetic mothers or over 4,500g for diabetic mothers).

Is a Growth Scan Safe? What Are the Limitations?

Safety:

Diagnostic ultrasound has been used safely in pregnancy for many decades. When performed by trained professionals using appropriate settings, there are no known harmful effects to either the mother or the baby from standard diagnostic ultrasound exposure.

Limitations:

Accuracy of Estimated Fetal Weight (EFW): As mentioned, EFW is an estimate with a margin of error (±15-20%). This means it's not an exact prediction of birth weight. The accuracy can be lower for very small or very large babies, and can also be affected by factors like maternal body size.
Operator Dependency: The quality and accuracy of the scan depend on the skill and experience of the sonographer performing it.
Not a Perfect Predictor: While growth scans are excellent tools for identifying babies at higher risk for certain outcomes, they are not perfect. Some babies identified as small or large may have completely normal outcomes, and occasionally, complications can arise even when growth parameters appear normal.

A growth scan is a valuable tool that provides important information about your baby's development. It helps your healthcare team make informed decisions to support a healthy pregnancy and delivery. If you have any questions or concerns about your scan results, always have an open discussion with your doctor.

Frequently Asked Questions

What is the difference between a growth scan and an anatomy scan?

A growth scan is typically performed in the third trimester (after 28 weeks) to assess your baby's size, estimated weight, amniotic fluid, and blood flow. An anatomy scan, usually done earlier in pregnancy (around 18-22 weeks), focuses on checking your baby's organs and body structures for normal development.

Why do I need a growth scan?

You might need a growth scan if your doctor has concerns about your baby's growth based on your belly measurements, if you have certain medical conditions (like diabetes or high blood pressure), if you're carrying multiples, or if there are other factors that could affect your baby's development. It helps ensure your baby is growing appropriately.

Is the growth scan safe for my baby?

Yes, diagnostic ultrasound has been used safely in pregnancy for many decades. When performed by trained professionals, there are no known harmful effects to either the mother or the baby from standard diagnostic ultrasound exposure.

How accurate is the estimated weight of the baby?

The estimated fetal weight (EFW) from a growth scan has a margin of error, typically around ±15-20%. This means it's an estimate, not an exact prediction, and the actual birth weight could be a bit higher or lower. Accuracy can also be lower for very small or very large babies.

What do the percentile numbers mean?

Percentile numbers compare your baby's measurements to those of other babies at the same gestational age. For example, if your baby's weight is at the 50th percentile, it means 50% of babies at that stage are smaller and 50% are larger. If your baby is below the 10th percentile, they are considered small for gestational age (SGA) or may have fetal growth restriction (FGR). If they are above the 90th percentile, they are considered large for gestational age (LGA).

Does a growth scan hurt?

A growth scan is a non-invasive procedure. A water-based gel is applied to your abdomen, and a transducer is moved across your skin. You might feel some pressure, but it should not be painful.

How long does the procedure take?

The duration of a growth scan can vary, but it typically takes around 20-40 minutes, depending on how easily your baby can be measured and what specific assessments are needed.

Do I need a full bladder for a third-trimester growth scan?

Generally, a full bladder is not required for a third-trimester growth scan, unlike some earlier pregnancy scans. However, it's always best to confirm any specific preparation instructions with your clinic before your appointment.

What happens if my baby is measuring small for its age?

If your baby is measuring small, your doctor will likely recommend increased monitoring with more frequent growth scans and Doppler studies to check blood flow. You might also be referred to a specialist. Management will focus on addressing any underlying causes and carefully planning the timing of delivery.

What does 'fetal growth restriction' mean?

Fetal Growth Restriction (FGR) means your baby is not growing as expected and is estimated to be significantly smaller than average for their gestational age (e.g., estimated fetal weight or abdominal circumference below the 10th percentile, or even lower, sometimes with abnormal blood flow patterns).

What happens if my baby is measuring large for its age?

If your baby is measuring large, your doctor will discuss potential risks during delivery, such as shoulder dystocia or the need for a C-section. If you have gestational diabetes, strict blood sugar control will be emphasized. Delivery planning will be carefully considered.

What is amniotic fluid and why is it measured?

Amniotic fluid is the protective liquid surrounding your baby in the womb. It's crucial for your baby's development and movement. Measuring its volume (using methods like AFI or SDVP) helps assess your baby's well-being, as too little (oligohydramnios) or too much (polyhydramnios) fluid can sometimes indicate a concern.

What are Doppler studies and why are they done?

Doppler studies measure the speed and direction of blood flow in your baby's and your blood vessels, particularly in the umbilical artery and middle cerebral artery. They are done to assess how well the placenta is functioning and if your baby is receiving adequate oxygen and nutrients, or if they are adapting to any challenges.

Will I need more than one growth scan?

If there are no concerns, you might only need one growth scan. However, if your doctor identifies any risk factors or concerns about your baby's growth or well-being, you may need serial growth scans to monitor the situation over time.

How often will I need to have scans if there is a concern?

If there is a concern about your baby's growth, such as suspected fetal growth restriction, you might need repeat scans typically every 2-4 weeks to monitor growth velocity and fetal well-being more closely.

Can a growth scan tell me the exact birth weight?

No, a growth scan provides an estimated fetal weight (EFW) with a margin of error (typically ±15-20%). It cannot tell you the exact weight your baby will be at birth.

Does a small baby mean I will have to deliver early?

Not necessarily. The decision to deliver early for a small baby depends on various factors, including the severity of the growth restriction, gestational age, and findings from Doppler studies. Your doctor will balance the risks of prematurity against the risks of continuing the pregnancy if the baby is not thriving inside the womb.

Does a large baby mean I will need a C-section?

Not always. While a very large baby (macrosomia) can increase the likelihood of a C-section, especially if the estimated weight is above certain thresholds (e.g., >5,000g for non-diabetic mothers or >4,500g for diabetic mothers), it does not automatically mean you will need one. Your doctor will discuss the best delivery plan based on your individual circumstances.

Can the scan detect all potential problems?

No, while a growth scan is a powerful diagnostic tool, it cannot detect all potential problems. It primarily focuses on growth, fluid levels, and blood flow. Some conditions or complications may not be visible or detectable through ultrasound.

What is the 'brain-sparing' effect?

The 'brain-sparing' effect is a fetal adaptation mechanism. If a baby is not getting enough oxygen or nutrients, their body may redistribute blood flow to prioritize vital organs like the brain. This can be detected by Doppler studies of the middle cerebral artery (MCA) and indicates the baby is trying to cope with a challenging environment.

What does it mean if there is a problem with the blood flow in the umbilical cord?

Problems with blood flow in the umbilical cord, often detected by umbilical artery Doppler, can indicate that the placenta is not functioning optimally (placental insufficiency). This can lead to increased resistance in the blood vessels, and in severe cases, absent or reversed end-diastolic flow, which is associated with significant concerns for the baby's well-being and may require closer monitoring or early delivery.

Can I do anything to help my baby grow better?

If there are concerns about your baby's growth, your healthcare provider will discuss any modifiable maternal factors that could be addressed. This might include managing pre-existing conditions like diabetes or hypertension, ensuring adequate nutrition, and avoiding substances like smoking or alcohol. Always follow your doctor's specific advice.