Select a vessel, gestational age and Doppler measure to see a published reference range and compare your value. Umbilical-artery PI and RI also show an estimated percentile within the supported age range.
* References are from singleton pregnancies and differ between studies. A value inside the band does not establish that a pregnancy is healthy. If your age is outside a source range, the nearest reference is shown without an age-matched classification. Your ultrasound team should interpret the full scan, including absent or reversed flow, symptoms and the chart used by your service.
How to use this calculator
- Choose the vessel (Umbilical artery or MCA) and Doppler measure (PI, RI, or PSV).
- Enter gestational age (weeks+days). Optional: add your measured value.
- Tap Lookup to see the typical band for that GA and how your value compares.
PI/RI are unitless; PSV is usually reported in cm/s.
Methodology and sources
The range shown is the 5th to 95th percentile in the named reference study, with its median. Being outside that range is not a diagnosis, and being inside it does not rule out a problem.
Umbilical artery PI and RI: the INTERGROWTH-21st equations use your exact weeks plus days/7, from 24w 0d to 40w 0d. Drukker et al. followed 431 low-risk singleton pregnancies with 1,243 measurements. The equations generate the range and, when you enter a value within the supported age range, its estimated percentile. PI and RI are unitless.
Middle cerebral artery PI, RI and PSV: the displayed 5th, 50th and 95th values come directly from Tarzamni et al. (2008), Table 2, for completed weeks 20-40. These are weekly observations from a regional singleton-pregnancy cohort, not an exact-day model. Days do not change a completed-week reference. No individual percentile is inferred between these three table values.
Umbilical artery PSV: the displayed age-band values come from Table 3 of Fetal Hemodynamic Parameters in Low Risk Pregnancies (2016). This smaller longitudinal study started with 63 women and had fewer observations near term. Its observed age bands run from 16-19 through 40-41 weeks. PSV is in cm/s. These bands are not an anemia screening threshold.
At ages outside a source's range, the nearest reference stays visible with its actual comparison age. No out-of-range percentile or age-matched classification is calculated. Different reference studies and measurement techniques produce different results; use the reference chart and clinical advice supplied by your ultrasound service. These comparisons do not account for multiple pregnancy, fetal movement, technique or the rest of the scan.
- Drukker et al. (2020), INTERGROWTH-21st: Tables 3, 4 and 6, umbilical artery PI/RI.
- Tarzamni et al. (2008): middle cerebral artery reference values, Table 2.
- Fetal Hemodynamic Parameters in Low Risk Pregnancies (2016): Table 3, umbilical artery systolic velocity.
- ACOG: fetal growth restriction and clinical surveillance.
- ISUOG: use of Doppler velocimetry in obstetrics.
Last updated: October 10, 2026.
Support if you have experienced a loss
Reading about miscarriage, stillbirth, or infant loss can be difficult. If you have experienced a loss, you do not have to use this calculator or read the statistics. You may find it more helpful to talk with your care team or a support organization.
In the United States, groups such as:
offer information and bereavement support. If you are elsewhere, your local health service or a pregnancy and baby loss charity may be able to guide you to support in your region.
Each vessel and measurement has reference ranges for completed weeks of pregnancy. The result identifies the exact comparison band. Ages below or above the available bands use the nearest endpoint band, with an asterisk; no new age-specific range is extrapolated.