What Are Normal Umbilical Artery Doppler Values?

Normal umbilical artery Doppler values are not a single fixed number but a range that shifts steadily downward as pregnancy progresses. The most commonly reported index, the pulsatility index (PI), starts above 1.5 in the early second trimester and falls to roughly 0.8–1.0 near term in an uncomplicated singleton pregnancy. This decline reflects the placenta building out an ever-expanding network of tiny blood vessels, which lowers resistance to blood flow. Understanding what counts as “normal” requires knowing the gestational age, which reference chart your provider is using, and a handful of technical details that can nudge readings up or down.

What the Numbers Actually Measure

When an ultrasound probe captures blood flow in the umbilical artery, it records a waveform that peaks when the fetal heart contracts (systole) and dips when the heart relaxes (diastole). Clinicians boil that waveform down into a few summary numbers. The pulsatility index (PI) is the difference between the peak systolic velocity and the lowest diastolic velocity, divided by the mean velocity over the entire cardiac cycle.¹ The resistance index (RI) uses only the peak systolic and end-diastolic velocities. The systolic-to-diastolic (S/D) ratio is the simplest: peak velocity divided by end-diastolic velocity. All three track placental resistance and tend to move in the same direction, so a pregnancy with a high PI will usually also have a high RI and a high S/D ratio.

Of the three, PI is the most widely used in research and clinical practice because it incorporates the full waveform shape rather than relying on just two points. That said, your report may list any or all of these indices, and they are all considered clinically useful for identifying elevated downstream resistance in the placenta.²

How Normal Values Change Across Pregnancy

The single most important thing to know about umbilical artery Doppler values is that they are not static. In the first trimester, resistance in the placental bed is relatively high, and diastolic flow may be barely detectable. From the first trimester onward, resistance drops significantly as the placental vascular tree matures.³ By the late second and third trimesters, there is a continuous, roughly linear decline in PI, RI, and S/D ratio.⁴ A small but significant drop in the S/D ratio continues through the last trimester right up to delivery.⁵

The INTERGROWTH-21st Project, which followed pregnancies across multiple countries, produced international centile charts for PI, RI, and S/D at each gestational week. All three indices decreased with advancing gestational age, and the study published 3rd through 97th percentile values along with equations that let clinicians convert any individual measurement into a centile or z-score.⁶ That means a PI of 1.2 at 24 weeks can be perfectly normal, whereas the same 1.2 at 36 weeks would land well above the 95th percentile and raise concern.

Why There Is No Single “Normal” Cutoff

If you search for a definitive threshold separating normal from abnormal, you will find that the numbers differ from chart to chart. A review of multiple published reference ranges found that the 95th percentile for umbilical artery PI ranged between 1.28 and 1.48 at 32 weeks and between 1.03 and 1.40 at 39 weeks, depending on which study you consult.⁷ That is a wide spread, and it matters clinically because a value judged borderline on one chart could look clearly abnormal on another.

These discrepancies arise from differences in population demographics, ultrasound equipment, the exact spot on the cord where the measurement was taken, and how many pregnancies were included. The practical takeaway is that your clinician interprets your reading against the specific reference range their institution uses, and the gestational-age centile matters more than any raw number in isolation.

Where on the Cord the Probe Is Placed

The umbilical cord has two ends—one near the baby’s belly (the fetal or “para-vesical” end, measured near the fetal bladder) and one at the placenta—plus a freely floating middle section. In the second and third trimesters, PI tends to read higher at the fetal end than in a free loop of cord.⁸ Earlier in pregnancy, during the first trimester, one study found no significant site-related difference, suggesting the effect becomes more pronounced as the cord lengthens and the placental bed grows.⁹ Most guidelines recommend sampling a free loop of cord to keep things standardized, but in practice, the sonographer sometimes has to work with whatever segment of cord is accessible.

What Drives the Normal Decline in Resistance

The steady drop in umbilical artery PI throughout pregnancy is not just a number on a chart—it reflects a physical remodeling of the placenta. The small muscular arteries inside the placental villi progressively thin their walls as pregnancy advances. One morphometric study found that vessel wall thickness in the tertiary-stem villi decreased at roughly half a micron per week, and that thinning correlated strongly with the decline in RI.¹⁰ When that remodeling goes wrong and small arteries become obliterated instead of opening up, the downstream resistance rises and the Doppler waveform reflects it: the diastolic component drops, and the PI climbs. Pathology studies have confirmed that placentas from pregnancies with high A/B ratios had far fewer small muscular arteries in their villi than normal placentas.¹¹

When Values Are Elevated but Not Extreme

A PI above the 95th percentile for gestational age signals increased placental resistance and is often the first Doppler abnormality seen in pregnancies complicated by fetal growth restriction. In those pregnancies, an elevated PI can precede other signs of fetal distress by days or even weeks.¹² Progressively worsening Doppler patterns are linked to earlier delivery, lower Apgar scores, longer stays in the neonatal intensive care unit, and higher perinatal mortality.¹³

An important nuance: in growth-restricted pregnancies, an intermittently elevated PI that pops above the 95th percentile on one scan but returns below it on another does not appear to carry the same risk as a persistently elevated reading. One study found that these intermittent elevations were neither uncommon nor linked to increased neonatal illness, stillbirth, or cesarean delivery compared to growth-restricted pregnancies with consistently normal Doppler values.¹⁴ That finding matters because it can spare families unnecessary alarm over a single borderline scan.

Absent and Reversed End-Diastolic Flow

The most alarming Doppler findings are not just “high” values but the complete loss—or reversal—of blood flow during diastole. Absent end-diastolic flow (AEDF) means that between heartbeats, blood essentially stops moving forward through the umbilical artery. Early research flagged this as a serious marker of fetal compromise: in one series of 24 high-risk pregnancies with AEDF, 22 of the 24 babies weighed below the 5th percentile and four died.¹⁵ Growth-restricted babies with absent or reversed end-diastolic flow also carry a higher risk of permanent neurologic injury compared to growth-restricted babies who maintain forward diastolic flow.¹⁶

Reversed end-diastolic flow (REDF) is one step worse. Here, blood actually flows backward through the umbilical artery during diastole, indicating extreme placental resistance. In one cohort, perinatal mortality for REDF was roughly 333 per 1,000 births, compared to about 94 per 1,000 for AEDF alone.¹⁷ Both absent and reversed diastolic flow are associated with intraventricular hemorrhage, lung disease, and other serious neonatal complications, often prompting urgent delivery planning.¹⁸

As with mildly elevated PI, the distinction between intermittent and persistent AEDF matters. In growth-restricted pregnancies, intermittent AEDF was seen in about a third of cases categorized as having absent flow at some point, and after adjusting for gestational age at delivery and steroid use, the odds of serious composite outcomes were not statistically different from pregnancies that merely had elevated Doppler values without absent flow.¹⁹ Persistent AEDF, by contrast, carried far higher odds of problems. The clinical lesson is that a single scan capturing absent flow does not automatically mean the worst-case scenario; serial monitoring determines the trajectory.

Things That Can Temporarily Shift Readings

Several fetal and maternal factors can nudge Doppler values up or down independently of any real change in placental health. Fetal breathing movements disrupt the regularity of the waveform and can make a single measurement unreliable, which is why standard practice calls for recording during a quiet fetal state with no visible breathing or body movements.²⁰ Fetal heart rate also plays a role: PI drops slightly as heart rate rises, by about 0.0075 per beat per minute, a small but measurable effect that some researchers correct for by normalizing to a standard heart rate of 140 beats per minute.²⁰

Corticosteroids given to the mother to accelerate fetal lung maturation can transiently lower the umbilical artery PI. In growth-restricted pregnancies with abnormal Doppler, steroids such as betamethasone or dexamethasone caused a significant drop in PI that peaked around one to two days after administration, then returned to pre-treatment levels by about day four.²¹ In some cases, betamethasone even temporarily restored forward end-diastolic flow in arteries that previously showed absent or reversed flow, and those babies tended to have better outcomes.²² Clinicians are generally aware that a Doppler scan taken within 48 hours of a steroid dose may look falsely reassuring and schedule follow-up accordingly.

Does Doppler Monitoring Actually Improve Outcomes?

This question has been tested directly. A Cochrane review pooling data from 18 trials involving more than 10,000 high-risk pregnancies found that using umbilical artery Doppler was associated with about a 29% reduction in perinatal deaths, fewer labor inductions, and fewer cesarean deliveries compared to care without Doppler monitoring.²³ The benefit was clearest in pregnancies complicated by suspected growth restriction or hypertensive disease—precisely the situations where placental resistance is most likely to be elevated.²⁴ In low-risk pregnancies with no signs of growth restriction, routine Doppler has not shown the same benefit, which is why most guidelines reserve it for pregnancies with identified risk factors.

The Cerebroplacental Ratio

Umbilical artery Doppler is increasingly interpreted alongside a measurement from the baby’s middle cerebral artery (MCA). The ratio of MCA-PI to UA-PI, called the cerebroplacental ratio (CPR), captures whether the fetal brain is receiving a disproportionately large share of blood flow—a sign that the baby is redistributing circulation in response to placental insufficiency. A systematic review and meta-analysis found that adding CPR to the assessment improved the prediction of poor outcomes beyond what umbilical artery Doppler alone could achieve.²⁵ Reference ranges for CPR have been published alongside those for UA-PI and MCA-PI, and a low CPR for gestational age raises a flag even when the umbilical artery values by themselves still look borderline.²⁶

Twin Pregnancies Need Their Own Reference Ranges

If you are carrying twins, singleton reference charts may not apply to your scans. Umbilical artery PI and RI tend to run higher in twins than in singletons, while MCA-PI and the cerebroplacental ratio tend to be lower.²⁷ These differences are not trivial: applying singleton cutoffs to twins would flag a lot of perfectly healthy pregnancies as abnormal. Dedicated twin reference charts, including specific centile curves for monochorionic (shared-placenta) twins, have been developed to address this gap.²⁸

Single Umbilical Artery

Most umbilical cords contain two arteries and one vein. About 1% of pregnancies have a cord with only one artery, a variant that is usually detected during a routine anatomy scan. In fetuses with a single umbilical artery, PI readings run roughly 20% lower than in fetuses with the normal three-vessel cord, simply because all of the blood volume passes through one larger vessel instead of being split between two.⁷ A study comparing single-artery and two-artery fetuses at 18–22 weeks confirmed that all Doppler parameters were significantly lower in the single-artery group at every sampling level.²⁹ Using standard reference charts for these babies would make their Doppler look artificially reassuring, so some centers apply separate norms.

Doppler During Labor

You might wonder whether contractions distort the readings. Research on Doppler during normal labor found that the umbilical artery S/D ratio stayed essentially the same before, during, and after uterine contractions, whether in the latent or active phase, after membrane rupture, or with oxytocin augmentation. The S/D ratios hovered around 2.0 regardless of contraction timing.³⁰ In practice, intrapartum Doppler is not part of routine labor monitoring, but the finding confirms that normal term umbilical flow is resilient to the pressure swings of contractions.