How to Dictate an OB Growth Ultrasound: Biometry, the EDD You Never Touch, and Placental Wording

The third-trimester growth ultrasound looks like the easiest study on the OB schedule — four measurements, a fluid check, a placenta glance — and it contains the single most consequential error a resident can dictate: changing the due date. Everything in this report hangs off one principle. Dating is established once, early, and never revised by a growth scan. The growth scan's entire job is to measure the fetus against that established date. Blur the two and the report stops meaning anything.

Biometry: the order and the output

The standard set, dictated in the standard order:

• Biparietal diameter (BPD)
• Head circumference (HC)
• Abdominal circumference (AC)
• Femur length (FL)

Each with its measurement and the gestational age it corresponds to. The four combine — Hadlock, in nearly every department — into the estimated fetal weight (EFW), in grams, and the EFW is then expressed as a percentile for the established gestational age. That percentile is the deliverable. The referring obstetrician will read three things in your report: the percentile, the fluid, and the presentation; the rest is supporting documentation.

Two numbers frame the percentile's meaning, both widely taught: EFW below the 10th percentile raises fetal growth restriction and triggers the surveillance pathway (umbilical artery Doppler, closer-interval follow-up — per department protocol, which your impression can invoke by name); EFW at or above the 90th percentile raises large-for-gestational-age with its delivery-planning implications. The report's language should be the measured fact — "EFW 1,850 g, 8th percentile for 32 weeks 4 days by established dating" — followed by an impression that names the category. Asymmetry is worth a sentence when present: an AC lagging the head measurements is the classic late-growth-restriction pattern and should be stated, not left for the reader to derive.

The dating paragraph: write it once, copy it forever

Every growth report states its dating basis explicitly: "Established EDD 2027-01-14 by first-trimester ultrasound, corresponding to 32 weeks 4 days today." That sentence does three jobs — it tells the reader which clock the percentiles run on, it documents that you used the established date rather than today's biometry, and it makes the serial reports comparable.

And then the cardinal rule: never redate. A third-trimester fetus measuring 29 weeks when the established dating says 32 is not "29 weeks"; it is a 32-week fetus measuring small — which is to say, possibly growth-restricted. Redating to match the biometry converts a sick fetus into a "normal" one with a new due date, erases the growth restriction from the record, and resets every downstream decision to a wrong clock. Biometry assigns dates only when no reliable earlier dating exists, and the later in pregnancy that happens, the wider the error bars — a fact your report should state when forced into it. On a routine growth scan with an established EDD, the composite "ultrasound age" is a measurement of the fetus, never a correction of the calendar.

The BPP line exists only when the BPP was done

A biophysical profile is a scored examination: fetal breathing, gross body movement, tone, and amniotic fluid, two points each, observed over a defined window, often paired with a non-stress test for the fifth component. If those components were formally assessed and scored, the report carries the line — "BPP 8/8" — with the components named.

If they were not, the report contains no BPP line at all. The trap is the helpful reflex: the fetus moved during biometry, fluid is normal, so "BPP 8/8" gets appended as a courtesy. That line is now a documented antenatal surveillance result that nobody performed, in the chart of a patient whose management may lean on it. Incidental observation of movement can be stated as exactly that — "fetal movement observed during the examination" — which claims nothing it didn't earn.

Placenta and previa: wording with numbers

The placenta gets a location (anterior, posterior, fundal, right or left lateral) and a relationship to the internal cervical os, and the relationship is numeric whenever it is close:

• Placenta previa: the placenta covers the internal os. Say "covers," not "marginal," not "partial" — the modern binary is covers or doesn't.
• Low-lying placenta: the inferior edge lies within 2 cm of the internal os without covering it. Give the measured distance in centimeters.
• Normal: edge more than 2 cm from the os — and once it is, stop measuring it on every subsequent scan.

Two caveats belong in the same breath. A full maternal bladder and lower uterine contractions both manufacture false previas on transabdominal imaging; when the transabdominal view is equivocal or the placenta looks low, the standard of care is transvaginal ultrasound, and the report should either include it or recommend it. And a previa or low-lying placenta found at the mid-trimester anatomy scan usually resolves — the follow-up is a repeat at around 32 weeks, which your impression should schedule rather than imply. In a patient with a prior cesarean and an anterior previa, add the sentence about accreta-spectrum evaluation; that combination is the one the reader cannot afford to miss.

Round out the report with the remaining standing items: amniotic fluid — single deepest pocket or AFI, with the number, since "normal fluid" without a number cannot be trended; presentation (cephalic, breech, transverse — the 36-week report's most-read word); cord insertion when seen; and maternal cervix when assessed.

The phrases attendings correct

• "Size less than dates, recommend redating." The forbidden sentence. Size less than dates is the finding; the recommendation is growth surveillance.
• "BPP 8/8" on a scan where no BPP was scored.
• "Fluid is adequate." Number, or nothing.
• "Low-lying placenta" without the centimeter distance — the 32-week follow-up decision is made on that number.
• Percentiles without the dating basis. A percentile is only as good as the clock it was computed against; name the clock.

A model report

EXAMINATION: Ultrasound OB, third trimester growth
HISTORY: 29-year-old G2P1 at 32 weeks 4 days. Fundal
height lagging. Established EDD 2027-01-14 by 9-week
ultrasound.
COMPARISON: Anatomy ultrasound 2026-08-21.
FINDINGS:
Single live intrauterine pregnancy, cephalic
presentation. Fetal heart rate 142 bpm.
Biometry: BPD 8.1 cm (32w2d), HC 29.6 cm (32w1d),
AC 26.2 cm (30w0d), FL 6.1 cm (31w5d).
EFW 1,640 g — 7th percentile for 32 weeks 4 days by
established dating. AC lags head biometry.
Amniotic fluid: Single deepest pocket 3.4 cm. Normal.
Placenta: Posterior, no previa; inferior edge 4 cm from
the internal os. Normal appearance.
Fetal movement observed during the examination. No BPP
performed.
IMPRESSION:
1. EFW 7th percentile against established dating with
   lagging AC — concerning for fetal growth restriction.
   Umbilical artery Doppler and growth surveillance per
   protocol recommended. Established EDD unchanged.
2. Normal amniotic fluid. Posterior placenta, no previa.

The impression names the category, schedules the pathway, and says out loud that the EDD was not touched. For the normal-study scaffold, start from our normal OB growth ultrasound template — the template library keeps the dating paragraph and fluid-number discipline built in. And to watch a growth-scan dictation assemble itself with the percentile computed against the established date automatically, there's a live OB case in the demo — no signup required.