Growth Scan and Doppler: What Your Third-Trimester Scan Tells Us

You have made it to the third trimester — and a whole new set of scans begins. If your doctor has asked for a growth scan and Doppler study, you may be wondering what exactly they are looking for, what the measurements mean, and when results should prompt concern. This guide explains everything clearly, so you can walk into your scan feeling informed rather than anxious.

At Mother Hospitals & IVF Center, Boduppal, Hyderabad, third-trimester growth scans and Doppler studies are a core part of how we monitor pregnancies, particularly for women with conditions such as hypertension, diabetes, IUGR risk, IVF conception, or a previous small baby. Our obstetric team, led by Dr. E. Prashanthi Reddy, interprets every scan in the context of your individual pregnancy history — not just numbers on a chart.

Why Is a Third-Trimester Growth Scan Done?

The third trimester — from 28 weeks to delivery — is when your baby gains most of its weight and when the placenta's efficiency becomes critically important. A baby that is not growing well in the womb (a condition called intrauterine growth restriction, or IUGR) is at risk of stillbirth, oxygen deprivation during labour, and long-term health problems. Yet IUGR can be completely silent — the mother feels the baby moving and has no symptoms at all.

A growth scan is the primary tool for detecting this. It measures the baby's size, estimates weight, and — over two or more scans — reveals the growth trajectory. A single scan tells you size; serial scans tell you whether growth is on track. This is why timing and interval matter as much as individual measurements.

Common reasons your doctor may request a third-trimester growth scan include:

• Pregnancy-induced hypertension or pre-eclampsia
• Gestational diabetes or pre-existing diabetes
• Previous small-for-gestational-age baby or stillbirth
• Reduced fetal movements
• IVF/assisted conception pregnancy
• Multiple pregnancy (twins, triplets)
• Maternal smoking, low BMI, or malnutrition
• Placenta praevia or low-lying placenta
• Abdominal measurement (symphysis-fundal height) not matching dates

What Does a Growth Scan Measure? The Four Key Biometrics

During a growth scan, the sonographer takes standardised measurements of four structures. These are entered into a calculation that produces an estimated fetal weight (EFW), which is then plotted on a growth chart to see where your baby falls relative to other babies of the same gestational age.

1. Biparietal Diameter (BPD)

The width of the baby's skull at its widest point, measured across the temples. It reflects head size and is one of the earliest and most reliable biometric measurements.

2. Head Circumference (HC)

The total circumference of the baby's head. Used alongside BPD for more accurate head-size assessment. When the head is large relative to the body (asymmetric growth), it may indicate the baby is redistributing blood flow to protect the brain — a sign the sonographer and your doctor will note carefully.

3. Abdominal Circumference (AC)

The circumference of the baby's abdomen at the level of the liver and stomach. This is the most sensitive measurement for detecting growth restriction, because the liver — which stores glycogen — is one of the first organs to shrink when a baby is undernourished. A falling AC centile on serial scans is often the earliest sign that placental function is declining.

4. Femur Length (FL)

The length of the baby's thigh bone (femur). Reflects skeletal development and overall body length.

Understanding the Centile Charts

Each measurement is plotted on a centile chart that shows the expected range for babies at that gestational age. A baby on the 50th centile is exactly average — half of all babies at that age are bigger, half are smaller. A baby on the 10th centile is smaller than 90% of babies of the same age. A baby on the 90th centile is larger than 90%.

It is important to understand that small does not automatically mean unwell. A baby consistently on the 8th centile across all measurements, with normal Doppler studies and normal amniotic fluid, may simply be a constitutionally small but healthy baby — particularly if both parents are of small stature. What is concerning is a falling centile — a baby that drops from the 40th to the 15th centile between two scans — or a baby with abnormal Doppler studies at any centile.

This is why Doppler is inseparable from the growth scan in modern obstetric practice. Growth tells you size; Doppler tells you why.

What Is Doppler Ultrasound and Why Does It Matter?

Doppler ultrasound uses sound wave technology to measure the speed and direction of blood flow in specific blood vessels. In obstetrics, the most important vessels assessed are:

Umbilical Artery Doppler

The umbilical artery carries blood from the baby back to the placenta. In a healthy placenta with normal resistance, blood flows continuously forward throughout the entire cardiac cycle — during both the pumping phase (systole) and the relaxation phase (diastole). The ratio of peak systolic flow to end-diastolic flow is expressed as the S/D ratio or as the pulsatility index (PI) and resistance index (RI).

As the placenta becomes less efficient (due to pre-eclampsia, chronic hypertension, or placental insufficiency), resistance within the placental vascular bed increases. This manifests on Doppler as:

• Raised PI/RI: Flow is present throughout the cycle but resistance is elevated — early sign
• Absent end-diastolic flow (AEDF): Blood stops flowing during the diastolic phase — serious sign requiring urgent action
• Reversed end-diastolic flow (REDF): Blood actually flows backward during diastole — critical sign, often precedes stillbirth by days; immediate delivery is usually indicated

Middle Cerebral Artery (MCA) Doppler

The MCA is a major artery within the baby's brain. In a well-nourished baby, brain vessels have high resistance — the brain is not the priority recipient of cardiac output during normal foetal life. However, when a baby is starved of oxygen and nutrients by a failing placenta, a remarkable protective mechanism kicks in: the baby selectively dilates the blood vessels supplying the brain (and heart and adrenals), diverting more blood to vital organs at the expense of the gut, limbs, and skin. This is called brain sparing or cerebral redistribution.

Brain sparing is detected on MCA Doppler as a fall in MCA PI below the 5th centile for gestation. The cerebroplacental ratio (CPR) — MCA PI divided by umbilical artery PI — captures this relationship elegantly. A CPR below 1.0 (or below the 5th centile) indicates significant brain sparing and is an important marker of fetal compromise even in babies whose size appears borderline rather than severely small.

Uterine Artery Doppler

The uterine arteries supply blood to the placenta itself. In a normally implanted placenta, uterine artery resistance is low and bilateral. High resistance or bilateral notching in the uterine arteries (typically assessed at 20–24 weeks but sometimes repeated in the third trimester) is associated with pre-eclampsia and placental insufficiency risk. When uterine artery Doppler is abnormal alongside growth restriction, the overall picture is more serious.

Ductus Venosus (DV) Doppler

The ductus venosus is a small vessel within the fetal liver that connects the umbilical vein to the central venous circulation. When placental insufficiency becomes severe and the fetal heart begins to struggle, this is reflected as abnormal DV flow — absent or reversed a-wave. DV Doppler abnormality is a very late sign indicating the baby is decompensating and delivery is required, often regardless of gestational age.

What Is Small for Gestational Age (SGA)?

A baby is classified as small for gestational age (SGA) when the estimated fetal weight or abdominal circumference is below the 10th centile for gestational age. SGA is further subdivided:

• Mild SGA: 3rd–10th centile — close monitoring; most are constitutionally small
• Severe SGA: Below the 3rd centile — higher risk; Doppler and amniotic fluid assessment essential

Not all SGA babies have IUGR (intrauterine growth restriction). IUGR implies the baby has failed to reach its growth potential due to placental insufficiency — it is a pathological process. A constitutionally small but healthy baby (with normal Dopplers and normal amniotic fluid) is SGA but not IUGR. Making this distinction matters enormously, because it determines whether early delivery is necessary or whether it would only expose a healthy small baby to the risks of prematurity unnecessarily.

Amniotic Fluid — The Other Half of the Picture

A growth scan also assesses amniotic fluid volume, typically reported as the amniotic fluid index (AFI) or the deepest vertical pool (DVP). When a baby's kidneys are well-perfused, they produce normal amounts of urine (which forms most of the amniotic fluid in the second and third trimesters). When placental blood flow is compromised and the baby redirects blood away from the kidneys, urine output falls — and amniotic fluid decreases (oligohydramnios).

Normal AFI in the third trimester is 8–24 cm. An AFI below 5 cm (or DVP below 2 cm) is oligohydramnios and, in combination with growth restriction or abnormal Doppler, is a significant warning sign. Conversely, excess fluid (polyhydramnios, AFI above 24 cm) can indicate gestational diabetes, fetal anomaly, or certain fetal conditions and warrants its own investigation.

When Are Growth Scans Done in the Third Trimester?

In low-risk pregnancies in India, many hospitals do not routinely perform third-trimester growth scans — the standard anomaly scan at 18–20 weeks and clinical monitoring (fundal height measurement) are considered sufficient. However, in high-risk pregnancies at Mother Hospitals, Hyderabad, we recommend:

• 28 weeks: Baseline third-trimester growth scan and Doppler — establishes a reference point
• 32 weeks: Repeat growth scan — important window for detecting early-onset growth restriction
• 36 weeks: Pre-delivery assessment — confirms presentation (head-down or breech), estimates weight, assesses placenta, checks for late-onset growth restriction

If growth restriction or Doppler abnormalities are detected, the interval between scans shortens — sometimes to every 1–2 weeks, or even twice weekly in severe cases. The frequency of monitoring is tailored to the degree of compromise.

What Happens If the Growth Scan Shows a Problem?

Discovering that your baby is small or that Doppler is abnormal is frightening, but early detection is the whole point of growth scanning — and in most cases, there is time to act. Depending on findings, your doctor may:

• Increase scan frequency to monitor closely
• Admit you to hospital for close observation
• Give corticosteroid injections (betamethasone or dexamethasone) to mature the baby's lungs if delivery before 34 weeks is anticipated
• Administer magnesium sulphate for neuroprotection if delivery before 32 weeks is likely
• Plan and time delivery — either by induction or caesarean section — at the safest possible gestation balancing the risks of prematurity against the risks of continuing pregnancy

For a baby with absent or reversed end-diastolic flow in the umbilical artery, delivery is usually planned promptly at a hospital with neonatal intensive care capability. For a baby with brain sparing (low MCA PI) but otherwise acceptable growth, delivery is typically planned at 37 weeks or earlier depending on the clinical picture.

Growth Scan and Doppler at Mother Hospitals, Hyderabad

At Mother Hospitals & IVF Center, Boduppal, Hyderabad, all third-trimester growth scans and Doppler studies are performed by experienced sonographers and interpreted by Dr. E. Prashanthi Reddy alongside your complete obstetric history. We do not scan and send you home with a report — we discuss findings with you at the time, explain what we see, and give you a clear plan.

For women with high-risk pregnancies — those who have conceived through IVF, who have hypertension, diabetes, PCOS, thyroid disease, or a previous small baby — we build regular third-trimester surveillance into the pregnancy management plan from the outset. You will always know when your next scan is, what we are watching for, and what the findings mean.

If you have concerns about your baby's growth, if you have been told your symphysis-fundal height is behind dates, or if you simply want expert monitoring for a high-risk pregnancy, call Mother Hospitals on 97059 93366 or WhatsApp @motherhospitals to book your growth scan and Doppler assessment in Hyderabad.