Doppler Ultrasound in Pregnancy

A Doppler ultrasound in pregnancy measures how blood flows through the umbilical cord, an artery in the baby's brain and the arteries that supply your womb. It is not a routine scan. It is offered from about 20 weeks when the baby is small, your blood pressure is high or a previous pregnancy had problems, and it shows how well the placenta is supplying the baby.

This page explains what each Doppler measurement means, gives the published reference values for every week from 20 to 41, and has a checker for the numbers on your report. It also covers who needs a Doppler and who does not, how the scan is done and why it is safe in a clinic.

What a Doppler scan measures

An ordinary scan shows the baby's size and shape. A Doppler scan adds movement: it picks up the speed of blood in a vessel and draws it as a waveform, a peak with each heartbeat and a lower level of flow between beats. From that waveform the machine calculates an index of how much resistance the blood is meeting. ISUOG recommends the pulsatility index (PI) for clinical use, because it keeps working when flow between beats falls to zero, where the older ratios break down.

Four measurements cover nearly every pregnancy Doppler report:

• Umbilical artery (UA). Measured in a free loop of cord, it reflects resistance in the placenta. As the placenta grows, resistance falls, so the normal PI drops week by week.
• Middle cerebral artery (MCA). An artery in the baby's brain. When oxygen runs short, a baby can widen these vessels to protect the brain, and the MCA PI falls.
• Cerebroplacental ratio (CPR). The MCA PI divided by the UA PI. It combines the two signals in one number.
• Uterine arteries (UtA). Your own arteries that carry blood to the womb, reported as the mean PI of the left and right side. High resistance here means the placenta is not being supplied as well as it should be.

Specialist units also measure the ductus venosus, a small vein that carries blood from the cord towards the baby's heart and matters in early, severe growth restriction, and the peak speed of blood in the MCA, which checks a baby for anaemia. Those belong to fetal medicine care and have no table here.

Normal Doppler values by week

The table gives the median and the 5th to 95th percentile range for the three fetal measurements from the Fetal Medicine Foundation (FMF), based on 72,387 pregnancies scanned in two London-area hospitals. The umbilical artery PI falls steadily from 20 weeks. The MCA PI rises to a peak at around 32 weeks and the CPR at around 34 weeks, and both fall towards term.

Source: Ciobanu et al., Ultrasound in Obstetrics & Gynecology 2019, Table 2. Each row is calculated at the middle of the week (20+3, 21+3 and so on). The FMF scanned at 20–22, 31–33, 35–36 and 41 weeks and fitted curves through those data, so the weeks in between come from the paper's curves. The study included every routine scan, which makes these reference ranges rather than ranges for healthy pregnancies only.

Not every unit uses this chart. Older references such as Acharya (2005) give slightly different limits, so the same PI can fall inside the range on one report and outside it on another. Your report's own percentile, if it gives one, comes from the chart your unit chose.

Uterine artery PI by week

For the uterine arteries, the reference most guidelines point to is Gómez and colleagues in Barcelona, who measured 620 women, twenty in each week from 11 to 41. The value is the mean of the left and right artery. It falls quickly through the first half of pregnancy and levels off after about 34 weeks.

Source: Gómez et al., Ultrasound in Obstetrics & Gynecology 2008, Table 2, as reproduced in the NHS England North West fetal growth restriction guideline (2025). The guideline uses the 95th percentile as its cut-off.

Doppler checker

Enter the week of the scan and any of the values on your report. If you enter both the umbilical and the MCA PI and leave the CPR blank, the checker works it out. Each value is placed against its week's 5th to 95th percentile.

Reading a Doppler report

Our guide to ultrasound results decodes the rest of the report, from BPD and HC to the estimated weight.

When a Doppler is done, and why

Doppler earns its place when there is a question about the placenta. The ISUOG 2020 guideline on growth restriction and the Royal College of Obstetricians and Gynaecologists (RCOG) guideline of 2024 both build monitoring of a small baby around the umbilical artery Doppler. The usual reasons for one are:

• ✓A baby measuring small on the biometry chart, or growth slowing across scans
• ✓A bump that measures small, or stops growing, on the tape measure
• ✓High blood pressure or pre-eclampsia
• ✓A previous baby born very small, or born before 34 weeks because of growth restriction or pre-eclampsia

The NHS England North West guideline, written to RCOG 2024, shows how this works in practice. Women at increased risk have a growth scan with umbilical artery Doppler and a fluid measurement every three weeks until birth, starting at a week set by their level of risk. Women at high risk also have a uterine artery Doppler at 18 to 24 weeks, and a bump measuring small leads to a scan with Doppler within 72 hours. What the same report says about where the placenta lies is explained on our placenta position page.

In a low-risk pregnancy, trials show no benefit from a Doppler. A Cochrane review of 5 trials with 14,624 women found that routine umbilical artery Doppler did not change perinatal deaths or other outcomes. In high-risk pregnancies the result reverses: a second Cochrane review of 19 trials with 10,667 women found fewer perinatal deaths when Doppler guided care (1.2% against 1.7%), with fewer inductions and caesareans too.

Where Doppler fits in growth restriction

A small baby is not always a baby in trouble, and Doppler is what tells the two apart. The 2016 Delphi consensus, which ISUOG adopted, uses Doppler in its definition of fetal growth restriction:

• Before 32 weeks: absent end-diastolic flow in the umbilical artery is enough on its own. A baby whose abdomen or estimated weight is below the 10th percentile also counts if the umbilical or uterine artery PI is above the 95th percentile.
• From 32 weeks: Doppler is one of three supporting signs. A CPR below the 5th percentile or an umbilical artery PI above the 95th counts when combined with a baby below the 10th percentile or one whose growth has dropped by more than two quartiles.

That is why a single Doppler number on its own rarely settles anything. The full picture, and what happens next, is on our page about fetal growth percentiles.

Absent or reversed flow

Between heartbeats, blood should keep moving forward through the cord towards the placenta. When resistance in the placenta climbs high enough, that forward flow at the end of each beat disappears (AEDF) or turns back (REDF). These are the findings a fetal medicine team acts on: the scans become much more frequent, the baby's heart rate is monitored, and the team plans the timing of birth around them. If your report mentions either, ask for the plan before you leave the unit.

How the scan is done

A Doppler is part of an ordinary abdominal scan and feels the same: gel, the probe on your bump, sometimes some pressure. The sonographer switches on colour Doppler to find the vessel, places a small sample box on it and records the waveform. ISUOG asks for the recording to be made while the baby is still and not making breathing movements, so you may wait a little or be asked to hold your breath. The PI comes from several similar waveforms in a row. You hear a whooshing sound while it records; that is the blood flow, made audible.

Is Doppler safe?

Doppler is a standard part of care in the second half of pregnancy when there is a clinical reason for it. ISUOG treats it with more caution than ordinary imaging in early pregnancy: no routine Doppler before 11 weeks, and from 11 to 13+6 weeks a displayed thermal index of 1.0 or less with the exposure kept short. At every stage it asks sonographers to keep exposure as low as reasonably achievable, the ALARA principle. A uterine artery Doppler carries no fetal safety concern as long as the baby is outside the beam.

A Doppler scan is not a home heartbeat monitor

The handheld "fetal Doppler" sold for home use only plays a heartbeat sound. It measures nothing about blood flow or the placenta. The NHS advises against using one, because hearing a heartbeat does not show that a baby is well and can delay a call that matters. Our fetal heart rate guide explains why, and your baby's movements are the sign to watch.

When to call your maternity unit the same day

A Doppler number on a report is never on this list by itself. If a result surprises you, ask the person who scanned you or your doctor what it means for the plan; read our guide to baby movements for what a change in movements looks like.

Frequently Asked Questions

Doppler ultrasound in pregnancy at a glance

A pregnancy Doppler measures blood flow in the umbilical artery, the baby's middle cerebral artery and your uterine arteries, and reports it as a pulsatility index. It is offered from about 20 weeks when the baby is small, blood pressure is high or a past pregnancy had problems; routine Doppler in low-risk pregnancies shows no benefit. In the FMF reference the umbilical artery PI falls from 1.218 at 20 weeks to 0.797 at 40, and a value above the 95th percentile suggests placental strain. A low CPR or absent flow leads to closer monitoring in specialist care. Fewer movements, bleeding or pre-eclampsia symptoms need a call the same day.

Read next

Sources

• Ciobanu et al., Ultrasound in Obstetrics & Gynecology 2019 — Fetal Medicine Foundation reference ranges for umbilical artery and middle cerebral artery pulsatility index and cerebroplacental ratio
• Gómez et al., Ultrasound in Obstetrics & Gynecology 2008 — Reference ranges for uterine artery mean pulsatility index at 11–41 weeks of gestation
• ISUOG Practice Guidelines (2020) — Diagnosis and management of small-for-gestational-age fetus and fetal growth restriction
• Gordijn et al., Ultrasound in Obstetrics & Gynecology 2016 — Consensus definition of fetal growth restriction: a Delphi procedure
• RCOG Green-top Guideline No. 31 (2024) — Investigation and care of a small-for-gestational-age fetus and a growth restricted fetus
• ISUOG Practice Guidelines (updated 2021) — Use of Doppler velocimetry in obstetrics
• ISUOG statement on the safe use of Doppler for fetal ultrasound examination in the first 13+6 weeks of pregnancy (updated 2021)
• Alfirevic et al., Cochrane Database of Systematic Reviews 2015 — Fetal and umbilical Doppler ultrasound in normal pregnancy
• Alfirevic et al., Cochrane Database of Systematic Reviews 2017 — Fetal and umbilical Doppler ultrasound in high-risk pregnancies
• NHS England North West — Regional guideline for the detection and management of fetal growth restriction (2025)
• NHS — Your baby's movements
• NHS — Pre-eclampsia

Baby Novum is an informational and tracking tool, not a medical device, and does not provide medical diagnosis or treatment. Always consult your doctor or midwife about your pregnancy.