Ultrasound in the Third Trimester: Biometry, Placenta, Amniotic Fluid, Doppler ISUOG 2024

What the ISUOG 2024 Document Changed

ISUOG Practice Guidelines 2024 is the first standalone international guideline for performing third-trimester obstetric ultrasound. The document does not replace clinical pregnancy management protocols but standardizes what exactly the ultrasound physician should assess: viability, position and presentation, biometry and estimated fetal weight calculation, placenta, amniotic fluid, anatomical structures that may manifest late, and Doppler indicators in the appropriate context.

The third trimester in the document is considered from 28+0 weeks until delivery. Gestational age at this stage is not recalculated based on biometry: dating should rely on earlier ultrasound or reliable obstetric data, and third-trimester measurements are used to assess growth.

Minimum Examination Protocol

In the protocol, it is advisable to sequentially record: number of fetuses, cardiac activity, fetal position, presenting part, placental location relative to the internal os, amount of amniotic fluid, standard biometric parameters, and EFW. In cases of multiple pregnancies, suspected IUGR, macrosomia, placental complications, or abnormal fluid volume, the scope of the examination is expanded.

• Position: longitudinal, transverse, oblique.
• Presentation: cephalic, breech, shoulder/transverse.
• Biometry: BPD, HC, AC, FL; EFW calculation using the chosen formula and plotting on the centile curves used.
• Placenta: anterior, posterior, fundal, lateral; relationship of the lower edge to the internal os.
• Amniotic fluid: preferably the deepest vertical pocket — DVP.
• Doppler: umbilical artery, uterine arteries; other vessels as per clinical task.

Biometry: How to Assess Growth

ISUOG emphasizes that third-trimester biometry is not for refining gestational age but for identifying small fetuses, IUGR, and large fetuses. Measurements are performed in standard sections: head — BPD and HC, abdomen — AC at the level of the stomach and intrahepatic portion of the umbilical vein, femur — FL along the diaphysis axis. EFW is indicated in grams and in the centile for the given term.

It is practically important not to mix different normative scales during dynamic observation. If previous measurements are available, not only the current centile is assessed but also the growth trajectory; however, a single ultrasound does not prove growth rate. The conclusion should separate the fact of small size from the diagnosis of IUGR: EFW or AC below the 10th percentile is a screening signal requiring clinical interpretation and, as a rule, Doppler.

Threshold Values in the Conclusion

Fetal Presentation

In the third trimester, presentation is not a formality: it affects the delivery plan, especially closer to term. The conclusion should specify the presenting part, not just the position. In transverse or oblique positions, the placenta, fluid volume, fibroids, uterine anomalies, and other possible causes of incorrect positioning are additionally assessed.

Breech presentation in the late third trimester should be clearly reflected in the protocol, as it requires an obstetric decision: observation, external version if no contraindications, or planning the mode of delivery.

Placenta, Scar, and Vasa Previa

Placental localization is described relative to the uterine walls and internal os. If the placental edge is low or the os is covered, transabdominal access is often insufficient: transvaginal ultrasound is safer and more accurate for measuring the distance to the internal os. The low-lying threshold is less than 20 mm.

For a placenta in the lower segment in a patient with a uterine scar, two tasks are especially important: exclude signs of placenta accreta spectrum and trace vessels in the area of the internal os with color Doppler. Vasa previa should be sought with low placentation, previa, accessory lobe, bilobed placenta, velamentous cord insertion, and after assisted reproductive technologies. The protocol should indicate whether unprotected fetal vessels are visible near the internal os.

Amniotic Fluid: DVP Instead of Subjectivity

The amount of fluid should not be described only with words like "normal" or "moderately reduced" if there is a clinical question. ISUOG allows for a standardized assessment of the maximum vertical pocket: DVP is measured perpendicular to the probe in a free pocket without cord loops and fetal parts. DVP <2 cm corresponds to oligohydramnios, DVP ≥8 cm to polyhydramnios.

An abnormal fluid volume should trigger a search for the cause: in oligohydramnios — IUGR, placental insufficiency, fluid leakage, urinary system pathology; in polyhydramnios — diabetes, anatomical anomalies, infections, fetal anemia, and other conditions. The ultrasound conclusion does not establish etiology but should guide further diagnostics.

Doppler: Umbilical and Uterine Arteries

The Doppler part of the study is especially significant in cases of small fetus, reduced growth gain, hypertensive disorders, suspected placental insufficiency, and unfavorable obstetric history. In the umbilical artery, PI and the presence of end-diastolic flow are assessed; it is critically important to separately indicate absent or reversed diastolic flow. Measurements are performed in the absence of active fetal movements and breathing movements, on stable uniform curves.

Uterine arteries reflect uteroplacental resistance. Usually, indicators are recorded on the right and left and the average PI is used; a value above the 95th percentile is interpreted as increased resistance. In the third trimester, an isolated Doppler finding should not be interpreted outside of biometry, blood pressure, history, and laboratory data.

Anatomy in the Third Trimester

Late ultrasound is not a replacement for second-trimester screening, but some anomalies become visible or clinically significant only later. If technically possible, the ventricles of the brain, posterior cranial fossa, four-chamber heart view, and outflow tracts, stomach, kidneys, bladder, anterior abdominal wall, spine, and limbs are assessed. If visualization is limited by obesity, fetal position, oligohydramnios, or late term, this should be explicitly stated.

How to Formulate a Conclusion

A good third-trimester conclusion answers obstetric questions: where is the placenta, what is the presenting part, does growth correspond to the term, is there oligohydramnios/polyhydramnios, are there Doppler signs of increased resistance. Recommended format: term based on initial dating; presentation; biometry with centiles; EFW with centile; DVP; placenta and distance to internal os if low-lying; Doppler with PI/centiles and nature of diastolic flow; study limitations.

The terms "IUGR," "macrosomia," "placental insufficiency" are better used with a caveat if ultrasound criteria are incomplete. For example: "EFW below the 10th percentile, umbilical artery Doppler normal; small for gestational age fetus, clinical correlation required."