Understanding Your Growth Scan at 33 Weeks: Purpose, Measurements, and Results

A growth scan at 33 weeks is a specialized third-trimester ultrasound designed to monitor fetal development, estimate weight, and assess the environment within the womb. It evaluates fetal biometrics, amniotic fluid levels, and placental function to ensure the pregnancy is progressing safely and to identify any potential late-term complications before delivery.

While many expectant parents look forward to ultrasounds as a chance to see their baby, the 33-week growth scan—often called a "position" or "well-being" scan—serves a critical clinical purpose. As you enter the final stretch of pregnancy, the data gathered during this window helps healthcare providers determine if the baby is growing at a healthy rate or if specialized birth plans are required.

Why is a Growth Scan Performed at 33 Weeks?

In many healthcare systems, including the NHS in the UK or private practices in the United States and Australia, growth scans are not always "routine" for every low-risk pregnancy. However, by 2026, they have become increasingly common as a preventative screening tool. Your midwife or obstetrician might schedule a scan at this specific interval for several reasons:

• Fundal Height Discrepancy: If the measurement of your belly (from the pubic bone to the top of the uterus) is larger or smaller than expected for 33 weeks, a scan confirms the actual size of the baby.
• High-Risk Factors: Conditions such as gestational diabetes, preeclampsia, or high blood pressure necessitate closer monitoring of fetal growth.
• Previous Pregnancy History: If you previously delivered a baby that was Small for Gestational Age (SGA) or Large for Gestational Age (LGA), providers will monitor this pregnancy more closely.
• Multiple Gestation: Twins and triplets require frequent growth checks to ensure both babies are receiving adequate nutrients.
• Maternal Health Factors: Factors such as a high BMI, smoking during pregnancy, or being over the age of 35 may prompt a 33-week assessment.

At 33 weeks, the baby is approximately the size of a pineapple. Their bones are hardening, though the skull remains soft to allow for birth, and they are practicing "breathing" by inhaling amniotic fluid. This is a pivotal time to ensure the placenta is still providing the support necessary for the final rapid growth phase.

Key Measurements Taken During the 33-Week Scan

During the appointment, the sonographer will take several specific measurements known as fetal biometrics. These numbers are then plugged into an algorithm (most commonly the Hadlock formula) to calculate the Estimated Fetal Weight (EFW).

Biparietal Diameter (BPD) and Head Circumference (HC)

The BPD is the diameter across the baby's head, from one parietal bone to the other. The HC measures the total distance around the head. At 33 weeks, these measurements help confirm neurological development and determine the baby's position. If the head is very low in the pelvis, obtaining these measurements can sometimes be challenging for the sonographer.

Abdominal Circumference (AC)

The AC is arguably the most important measurement in the third trimester. It reflects the size of the baby's liver and the amount of subcutaneous fat. This measurement is a sensitive indicator of the baby's nutritional status. A lagging AC may suggest intrauterine growth restriction (IUGR), while an accelerated AC can be a sign of macrosomia (a very large baby), often associated with gestational diabetes.

Femur Length (FL)

This measures the length of the thigh bone, the longest bone in the human body. It serves as a longitudinal marker of skeletal growth. When combined with the head and abdomen measurements, it provides a balanced view of the baby's overall proportions.

Interpreting Centiles and Growth Curves

Growth scan results are rarely given as simple "pass/fail" grades. Instead, they are plotted on growth centiles (percentiles). If your baby is on the 50th centile, they are exactly average size. If they are on the 90th, they are larger than 90% of babies at the same gestational age.

Clinical concern usually arises when:

1. The baby is below the 10th centile: This may be classified as Small for Gestational Age (SGA).
2. The baby is above the 90th centile: This may be classified as Large for Gestational Age (LGA).
3. Growth Velocity Slows: If a baby was on the 70th centile at 28 weeks but has dropped to the 20th centile by 33 weeks, this "crossing of centiles" warrants investigation, even though the baby is still within the "normal" range.

It is important to remember that ultrasound technology has a margin of error, typically around 10% to 15%. A 33-week scan provides an estimate, not an exact weight.

Assessing the Womb Environment: Fluid and Blood Flow

A growth scan is not just about the baby’s size; it is also about the "life support system" sustaining them.

Amniotic Fluid Index (AFI)

The sonographer will measure the depth of the fluid pockets around the baby. Amniotic fluid is vital for lung development and protecting the baby from umbilical cord compression.

• Oligohydramnios: Too little fluid. This can indicate placental issues or a leak in the amniotic sac.
• Polyhydramnios: Too much fluid. This can be associated with gestational diabetes or swallowing difficulties in the fetus.

Placental Position and Grannum Grade

The scan confirms that the placenta is not blocking the cervix (placenta previa). By 33 weeks, most placentas have moved upward as the uterus expands. The "grade" of the placenta refers to its maturity; while calcification is normal near 40 weeks, an overly "aged" placenta at 33 weeks might require earlier delivery.

Umbilical Artery Doppler

If there are concerns about growth, the sonographer will use Doppler ultrasound to measure the resistance of blood flow in the umbilical cord. This ensures the baby is getting enough oxygen and nutrients from the placenta. High resistance or "absent end-diastolic flow" is a serious clinical finding that usually leads to hospitalization or immediate delivery.

Standard Fetal Measurements at 33 Weeks

The following table provides the average ranges for fetal measurements during the 33rd week of pregnancy. Please note that individual variations are common and should be discussed with a medical professional.

What Happens if the Scan Detects an Issue?

If the 33-week growth scan indicates that the baby is smaller or larger than expected, or if fluid levels are abnormal, several steps may follow.

For a Small Baby (SGA/IUGR)

If the baby is tracking below the 10th centile, you may be scheduled for "serial growth scans" every two weeks. Your doctor may also order more frequent Doppler checks and Non-Stress Tests (NSTs) to monitor the baby's heart rate. In cases of growth restriction, the goal is often to keep the baby in the womb as long as it is safe, but if the placenta begins to fail, a 33-week or 34-week delivery may be recommended.

For a Large Baby (LGA/Macrosomia)

If the baby is tracking very large, the primary concern is often gestational diabetes. You may be asked to repeat a glucose tolerance test. While a large baby is not inherently a "problem," it can influence the discussion around the mode of delivery (e.g., induction of labor or elective C-section) to avoid complications like shoulder dystocia.

Breech Presentation

At 33 weeks, many babies are already "head down" (cephalic). However, about 25% are still in a breech or transverse position. While most babies will turn on their own by 36 weeks, knowing the position at 33 weeks allows you to research External Cephalic Version (ECV) or pelvic exercises to encourage turning.

Preparation and What to Expect During the Appointment

The 33-week scan is transabdominal, meaning the sonographer will move a transducer over your stomach.

1. Full Bladder: Unlike the first-trimester scans, you usually do not need a painfully full bladder. A partially full bladder can sometimes help "lift" the baby for a better view, but follow your specific clinic’s instructions.
2. Duration: The scan typically takes 20 to 45 minutes, depending on the baby's position. If the baby is tucked away or moving constantly, it may take longer to get accurate measurements.
3. Visualization: At 33 weeks, the baby is quite crowded. You will see parts of the baby—a foot, a beating heart, a profile—rather than the whole body in one frame. Many clinics also offer 3D/4D imaging during this scan, though this is often for bonding rather than medical diagnosis.
4. Reporting: In most cases, the sonographer cannot give you a definitive "diagnosis." They gather the data, which is then reviewed by a radiologist or your OB/GYN.

Common Questions About the 33-Week Growth Scan

Is an ultrasound at 33 weeks safe for the baby?

Yes. Diagnostic ultrasound has been used in pregnancy for decades. According to the American Institute of Ultrasound in Medicine (AIUM), there is no evidence that ultrasound, when used for medical indications by trained professionals, causes harm to the developing fetus.

How accurate is the estimated fetal weight?

Weight estimates are calculations based on bone and tissue measurements. They can be off by roughly 10% to 15% in either direction. It is best to look at the trend of growth over several weeks rather than a single number.

What if my baby is in the 95th centile?

Being in the 95th centile means your baby is larger than average. This isn't always a cause for alarm; it may simply be genetics. However, your provider will likely rule out gestational diabetes and discuss the potential for a larger baby during delivery.

Can I find out the gender at 33 weeks?

If you haven't already found out, and the baby's legs are positioned correctly, the sonographer can usually identify the biological sex. However, at this stage, the baby is very cramped, making it occasionally more difficult than at the 20-week anatomy scan.

Why does the sonographer keep measuring the same thing?

Accuracy is paramount. Sonographers often take three measurements of the AC or BPD and average them to ensure the data isn't skewed by a slight angle or a baby's sudden movement.

Next Actions and Clinical Follow-up

Following your 33-week scan, your healthcare provider will review the growth charts. If everything is within the normal range, you will likely proceed with your standard prenatal appointment schedule.

If there were findings that require attention, such as low fluid or growth slowing, you might be referred to a Maternal-Fetal Medicine (MFM) specialist. These specialists have advanced training in high-risk pregnancies and use high-resolution equipment to get a more detailed picture of the baby's health.

Disclaimer: This article provides general information regarding 33-week growth scans and should not replace professional medical advice. Always consult with your healthcare provider for guidance specific to your pregnancy.