Standard Obstetric Ultrasound: Minimal Fetal Anatomy According to AIUM 2024
What the AIUM 2024 Revision Changed
The document AIUM Practice Parameter for the Performance of Standard Diagnostic Obstetric Ultrasound Examinations, 2024, prepared jointly by AIUM, ACR, ACOG, SMFM, and SRU, sets the minimal scope of standard diagnostic obstetric ultrasound. It is not a protocol for detailed anatomical examination but a basic level that should be reproducible in everyday practice.
The key logic of the parameter: the standard examination answers questions of viability, gestational age, number of fetuses, growth, placenta, amniotic fluid, and mandatory anatomical screening appropriate to the gestational age. In cases of technical limitations or suspicion of pathology, the standard protocol does not 'silently expand' but becomes the basis for targeted further examination.
Trimester Boundaries in the Protocol
AIUM uses obstetric grading by gestational age: the first trimester is up to 13+6 weeks inclusive, the second is 14+0–27+6 weeks, and the third is from 28+0 weeks. This is important for protocol formulation: the volume of anatomy in the first trimester is not equal to the mandatory anatomical survey of mid-pregnancy.
| Period | Gestational Age According to AIUM 2024 | Main Task of Standard Ultrasound |
|---|---|---|
| First Trimester | up to 13+6 weeks | Localization of pregnancy, number of gestational sacs/embryos, viability, CRL if an embryo is present, uterus and adnexa |
| Second Trimester | 14+0–27+6 weeks | Biometry, growth, placenta, amniotic fluid, minimal fetal anatomical survey |
| Third Trimester | from 28+0 weeks | Growth, presentation, amniotic fluid, placenta, anatomy to the extent technically and clinically justified |
First Trimester: What Must Be Documented
In the first trimester, standard ultrasound should establish the presence, localization, and number of gestational sacs. If the yolk sac and embryo/fetus are visualized, they are described; if an embryo is present, the crown-rump length is measured. Cardiac activity is documented, and in cases of multiple pregnancies, chorionicity and amnionicity are assessed if possible at this gestational age.
The uterus, including the cervix if clinically necessary, adnexa, and the posterior cul-de-sac are also evaluated. The anatomy of the embryo/fetus in the first trimester can be assessed to the extent allowed by gestational age and visualization, but the AIUM document does not replace the mandatory anatomical survey of the second trimester.
Second–Third Trimester: Basic Elements Before Anatomy
Before the anatomical list in the standard examination, the number of fetuses, cardiac activity, position/presentation, volume of amniotic fluid, localization of the placenta, and its relation to the internal os, when relevant, are recorded. The uterus and adnexa are evaluated if there are clinical indications or findings.
The biometric minimum in the second–third trimester includes biparietal diameter, head circumference, abdominal circumference, and femur length. Based on biometry, gestational age or fetal weight is calculated if it corresponds to the clinical task of the study and local protocol.
Minimal Fetal Anatomical Survey in the Second–Third Trimester
The main practical list for standard obstetric ultrasound in the second–third trimester is provided below. It reflects the mandatory anatomical areas according to AIUM 2024 and should be performed as fully as gestational age, fetal position, maternal body mass index, scars, amount of amniotic fluid, and acoustic conditions allow.
| Area | Minimal Structures/Views |
|---|---|
| Head and Brain | Skull, lateral ventricles, choroid plexuses, falx cerebri, cavum septi pellucidi, cerebellum, cisterna magna |
| Face and Neck | Upper lip; neck area if visualization is available |
| Chest and Heart | Lungs, 4-chamber heart view, left and right outflow tracts, diaphragm |
| Abdomen | Stomach and situs, kidneys, bladder, umbilical cord insertion into the anterior abdominal wall |
| Umbilical Cord | Number of umbilical vessels when technically possible |
| Spine | Cervical, thoracic, lumbar, and sacral regions in available planes |
| Limbs | Upper and lower limbs; presence of major segments is documented |
Heart: Minimum Standard Examination
For standard obstetric ultrasound, the mere presence of a heartbeat is insufficient. In the second–third trimester, the anatomical survey includes a 4-chamber view and evaluation of the outflow tracts. If the outflow tracts are not obtained due to fetal position or other limitations, this should be explicitly noted in the protocol, not interpreted as normal.
Standard ultrasound is not equivalent to fetal echocardiography. In cases of an abnormal 4-chamber view, suspicion of outflow tract pathology, arrhythmias, or other clinical risk factors, specialized fetal heart assessment is required.
When the Standard Protocol Becomes Insufficient
AIUM emphasizes: the standard examination is not intended to exhaustively exclude all developmental defects. Transition to a detailed anatomical examination is justified when an anomaly is detected or suspected, biometry and gestational age do not match, screening results are pathological, significant maternal diseases, teratogenic exposures, a complicated obstetric or family history, and other clinical circumstances.
A practically important formulation in the conclusion: 'structure not visualized' differs from 'structure without features.' For non-visualized mandatory elements, the reason is indicated if obvious, and repeat evaluation or referral for expert examination is recommended.
Third Trimester: Features of Anatomical Survey
In the third trimester, standard ultrasound is often performed for indications of growth, presentation, placenta, or amniotic fluid. The anatomical survey is conducted within the capabilities, but late gestational age, ossification, oligohydramnios, and fetal position may limit the assessment of the head, face, spine, and heart.
If anatomy was not previously assessed in the second trimester, this should be reflected in the referral and protocol. A late standard ultrasound does not always compensate for a missed optimal anatomical survey, so incomplete visualization requires a follow-up plan.
How to Format the Conclusion Without Redundancy
The optimal protocol contains separate blocks: gestational age and dating method, number of fetuses, viability, biometry, amniotic fluid, placenta, cervix if indicated, anatomy by systems, study limitations, and recommendations. For multiple pregnancies, elements should be linked to each fetus.
Do not include unconfirmed 'norms' for each structure in the standard protocol if they were not measured and are not required by the parameter. It is sufficient to indicate the visualization of mandatory structures and the absence of identified features within the standard examination. This form reduces the risk of both under-examination and the false impression that a detailed expert ultrasound was performed.
Frequently asked questions
Is a full anatomical protocol of the fetus required in the first trimester?
No. According to AIUM 2024, the first trimester is up to 13+6 weeks; the standard examination primarily confirms localization, number of embryos/fetuses, viability, CRL, and evaluates the uterus/adnexa. Anatomy is assessed as possible, but the mandatory systematic survey pertains to the second–third trimester examination.
What cardiac views are included in the standard ultrasound in the second trimester?
The minimum includes a 4-chamber heart view and evaluation of the left and right outflow tracts. If they are not visualized technically, this needs to be documented, and repeat or specialized examination should be scheduled if necessary.
When to refer for a detailed anatomical ultrasound?
In cases of suspected or detected anomaly, incomplete visualization of mandatory structures, growth discrepancies, pathological screening, significant risk factors from the mother, fetus, or family history. Standard ultrasound does not replace a detailed examination.