Ultrasound in the Third Trimester: ISUOG 2024 Checklist

Why ISUOG Established a Separate Standard for the Third Trimester

ISUOG Practice Guidelines 2024 describe not an 'advanced expert ultrasound' but a minimally reproducible routine scan for the third trimester. The goal is not only to confirm a live fetus but also to identify clinically significant conditions that may have appeared or become visible after the second trimester screening: growth disorders, abnormal fluid volume, incorrect presentation, low placentation, and late-manifesting anomalies.

The main practical idea of the document is a uniform checklist for each examination. If an item is not assessed due to fetal position, obesity, scars, oligohydramnios, or late gestational age, it is recorded in the protocol as a limitation, not replaced with the phrase 'unremarkable.'

Minimum Examination Checklist

• confirm fetal cardiac activity;
• determine the number of fetuses, position, and presenting part;
• perform biometry: BPD, HC, AC, FL;
• calculate estimated fetal weight, EFW, and indicate the percentile;
• compare measurements with gestational age and, if previous measurements are available, assess growth dynamics;
• evaluate amniotic fluid volume, preferably through the deepest vertical pocket, DVP;
• describe placental location and its lower edge's relation to the internal os if low-lying;
• perform a targeted anatomy review achievable in the third trimester;
• document technical limitations and the need for further investigation.

Threshold Values to Include in the Protocol

Biometry and Weight Calculation: What Matters for the Ultrasound Physician

ISUOG 2024 retains the standard set of measurements: BPD, HC, AC, and FL. 'Corresponds to gestational age' alone is insufficient: the report must include numerical values, estimated weight, and percentile according to the chosen reference scale. Percentile assessment is especially important in the third trimester because absolute millimeters without reference to gestational age poorly reflect risk.

EFW or AC below the 10th percentile is interpreted as small for gestational age and requires clinical correlation: verification of gestational age, review of growth chart, assessment of placenta, fluid, and, if indicated, Doppler studies. EFW or AC above the 90th percentile indicates large for gestational age; this does not equate to a diagnosis of birth trauma or diabetic fetopathy but should be clearly stated in the report.

Amniotic Fluid: Not Just a Subjective Assessment

In a routine third-trimester scan, qualitative assessment of fluid is permissible, but if deviation is suspected, DVP should be measured. The protocol is convenient with the formula: 'fluid is normal' only when there are no signs of reduction or increase, and the deepest vertical pocket does not fall into pathological ranges. DVP <2 cm corresponds to oligohydramnios, DVP ≥8 cm — polyhydramnios.

An abnormal fluid volume changes the value of the entire examination: with oligohydramnios, visualization of anatomy worsens, and the likelihood of placental insufficiency increases; with polyhydramnios, a repeated search for structural causes is required, primarily from the gastrointestinal tract, CNS, and face, as technically possible at a late gestational age.

Placenta, Internal Os, and Vessels

Placental location should be described in every examination. If the placenta is low-lying or covering the internal os, transabdominal access is often insufficient; transvaginal assessment of the distance to the internal os is preferred. The threshold <20 mm from the internal os is used for low-lying placenta, and covering the os is for placenta previa.

Signs that increase the risk of abnormal placental attachment are assessed separately, especially with a uterine scar and anterior low placentation. If vasa previa, abnormal attachment, or unclear placental relation to the os is suspected, the examination should be expanded and/or referred to a center with appropriate expertise.

Anatomy Review in the Third Trimester

The late scan does not replace the detailed second-trimester screening but must include a review of accessible anatomy. Practically, the head and intracranial structures, face if accessible, chest, heart at least at a basic level, stomach, anterior abdominal wall, kidneys, bladder, spine, and limbs within visualization are checked.

In the third trimester, some anomalies become more noticeable: ventriculomegaly, some obstructive uropathies, intestinal dilation, tumor-like formations, skeletal growth disorders, signs of heart failure. Therefore, the phrase 'anatomy previously assessed' should not completely exclude a late targeted review.

Doppler Studies: When to Include in the Routine Protocol

ISUOG 2024 does not make Doppler of all vessels a mandatory item for every low-risk scan with normal growth and fluid. However, if small for gestational age, growth restriction, placental insufficiency, or other clinical risk factors are suspected, Doppler studies become part of the diagnostic algorithm. Minimally, umbilical artery blood flow is discussed; the choice of additional vessels depends on the local protocol and clinical task.

It is important not to mix levels of conclusion: 'small for gestational age' by percentile and 'growth restriction' as a clinical diagnosis require different data sets. The ultrasound specialist should provide measurements, percentile, fluid, placenta, and Doppler parameters when indicated, not just an overall statement.

What Must Be Included in the Report

• gestational age at which measurements are interpreted;
• live fetus, position, presenting part;
• BPD, HC, AC, FL, EFW, and EFW/AC percentile;
• fluid assessment with DVP if deviation or doubt;
• placental location, if low-lying — distance to the internal os;
• brief result of anatomy review and list of unassessed structures;
• suspicion of SGA/LGA, oligohydramnios, polyhydramnios, low placentation, or previa;
• recommendation for further investigation if the result affects pregnancy management.

Common Protocol Errors

The first error is the absence of percentiles: EFW without a percentile does not allow categorization into <10th or >90th percentile groups. The second is subjective fluid description with obvious deviation without DVP. The third is the phrase 'placenta low' without the distance to the internal os. The fourth is undocumented visualization limitations: if the heart, face, or spine are not assessed, this should be documented.

A practical protocol according to ISUOG 2024 is not a lengthy text but a verifiable structure. It should allow the obstetrician to quickly understand three things: whether the fetus corresponds to gestational age, whether there are conditions requiring a change in monitoring, and whether there are reasons for referral for expert ultrasound.