Fetal ultrasound: timing, goals, and what is assessed

Fetal ultrasound is the primary prenatal imaging method used to confirm the viability and gestational age of a pregnancy, assess anatomy and growth, and facilitate early detection of abnormalities and clinical decision-making. Current protocols emphasize the value of early examination in the first trimester and mandatory detailed anatomical screening in the second trimester, with selective use of Doppler ultrasonography and additional methods in risk groups. This approach improves the accuracy of dating, the detection of abnormalities, and the quality of pregnancy management. [1]

There are no universally applicable schedules for all cases, but there is a consensus: early screening should be performed between 11 and 14 weeks and routine detailed anatomical examinations should be performed between 18 and 22 weeks. Additional examinations in the third trimester are indicated for risk factors and clinical indications, and in multiple pregnancies, according to separate, more intensive schedules. [2]

Safety is based on the principle of "minimum reasonable acoustic output": thermal and mechanical indices are controlled, scanning time is limited, and non-medical "memory" procedures are not recommended. Accumulated data confirm a favorable safety profile with proper technique and adherence to standards. [3]

When and what ultrasounds to do: optimal schedule and objectives

First trimester, 11–14 weeks: confirmation of pregnancy viability and location, accurate dating based on crown-rump length, determination of fetal number and chorionicity in twins, initial anatomical examination, and risk assessment for chromosomal abnormalities and some serious defects. This interval provides the best balance between imaging and clinical significance of results. [4]

Second trimester, 18-22 weeks: routine detailed examination of fetal anatomy, assessment of placental position, and maternal anatomy parameters. This is a key window for systemic screening: when performed professionally, it detects most clinically significant malformations. [5]

Third trimester: growth, amniotic fluid, Doppler blood flow, fetal position, placental maturity, and distress markers are assessed selectively, as indicated. In low-risk populations, routine screening is considered, while in high-risk groups and with specific findings in the second half of pregnancy, follow-up ultrasounds improve management. [6]

Specific clinical trajectories: if placenta previa, risk of fetal growth restriction, diabetes mellitus, hypertension, immune conflict, multiple pregnancy, and combined obstetric risks are suspected, the frequency of ultrasound examinations increases. Specific intervals are determined by local protocols and the condition of the mother and fetus. [7]

Table 1. Recommended windows and main objectives of surveys

Pregnancy period	Optimal time	Main tasks	Comments
First trimester	11-14 weeks	Crown-rump length dating, viability, location, fetal number and chorionicity, early anatomy review	The foundation of quality antenatal care
Second trimester	18-22 weeks	Detailed anatomy, placenta, basic biometric parameters	Routine for all pregnancies
Third trimester	Individually	Growth, water, Doppler in case of risk, presentation, fetal position	Indicated for risk factors and findings
Multiple pregnancy	According to individual schemes	Regular monitoring of growth and blood flow, taking into account chorionicity	More frequent visits and ultrasounds are necessary
[8]

What exactly is assessed by ultrasound: protocol and measurements

Dating in the first trimester is performed using the crown-rump length and is considered the most accurate, with an error margin of approximately 5-7 days. After determining the due date through early testing, subsequent biometric measurements are used to assess size and growth rates, but not to re-date the due date without compelling reasons. [9]

In the second and third trimesters, the standard biometric set includes biparietal diameter, head circumference, abdominal circumference, and femur length. This set of indicators is used to calculate the approximate fetal weight and identify growth deviations. Validated international growth standards are used for interpretation. [10]

Structural anatomy is assessed systematically by organ and segment: skull and brain, face, heart, thorax, diaphragm, abdominal cavity and organs, spine, extremities, umbilical cord, and placenta. Skilled performance improves the detection of serious defects, but diagnostic accuracy varies between diseases and depends on the quality of imaging. [11]

The location of the placenta and the condition of the cervix are indicated separately, if indicated. If the placenta is low, a follow-up scan is required to determine the delivery strategy and prevent complications. [12]

Table 2. Fetal biometry: what is measured and why

Parameter	When it is especially important	What does it show?	Clinical application
Parietal-coccygeal size	First trimester	The most accurate dating	Planning screening and pregnancy management
Head circumference and biparietal diameter	Second and third trimesters	Head growth and relation to the abdomen	Evaluation of growth harmony and weight calculation
Abdominal circumference	Second and third trimesters	Nutrition and growth, indicator of growth retardation	Monitoring the risks of hypoxia and malnutrition
Femur length	Second and third trimesters	Length of limbs	Confirmation of proportionality of growth
[13]

What ultrasound actually finds: detection of defects and limitations

Routine anatomical screening in the second trimester detects a significant proportion of serious malformations. Detection varies by organ system: structures of the anterior abdominal wall and major defects of the central nervous system are detected more frequently, while some gastrointestinal defects and mild cardiac anomalies may be missed. [14]

Early initial screening at 11-14 weeks already allows for the detection of a significant proportion of major anomalies, and a combination of early and mid-term screening increases the overall detection rate to clinically significant levels. However, some pathologies manifest later and require follow-up. [15]

In population-based programs, the rates vary by nosology: for anencephaly, anterior abdominal wall defects, and some severe heart defects, the accuracy is high, while for intestinal anomalies and some soft tissue defects, it is lower. This emphasizes the value of standardized protocols and referrals to expert centers for questionable findings. [16]

It's important to understand the limitations of this method: fetal position, maternal constitution, scars, narrow acoustic windows, and early pregnancy all reduce visualization. In such situations, a repeat examination is planned and, if necessary, fetal echocardiography or magnetic resonance imaging is included. [17]

Table 3. Detectability of defects: benchmarks based on program and survey data

Nosological group	Range of detection	Comments
Major neural tube defects	High	They are detected reliably with adequate visualization.
Defects of the anterior abdominal wall	High	Included in the priorities of screening programs
Severe heart defects	Average, varies	Improves with targeted assessment and referral for echocardiography
Gastrointestinal tract anomalies	Lower	Often appear later and require observation.
[18]

Dopplerography: when is it needed and what does it provide?

Dopplerography assesses blood flow in the maternal-placental-fetal system and is used primarily in cases of growth retardation, preeclampsia, multiple pregnancies, and other complications. The most commonly analyzed are the umbilical artery for placental resistance, the middle cerebral artery as a marker of circulatory centralization, and the ductus venosus in severe forms of fetoplacental insufficiency. [19]

In the third trimester, Doppler ultrasound is included in the assessment of high-risk pregnancies and helps determine the timing of delivery. In low-risk situations, routine use is discussed and prescribed on an individual basis. [20]

The combination of Doppler ultrasound with fetal biophysical profiling, cardiotocography, and serial biometry provides a more complete picture than any single test. The choice of monitoring frequency depends on the severity of the condition and the dynamics of the parameters. [21]

In the early stages of the first trimester, the use of pulsed Doppler is limited to clinical indications and is performed with thermal index monitoring and minimal reasonable exposure. This reduces potential biophysical effects on developing tissues. [22]

Table 4. Main vascular targets of Doppler ultrasonography and clinical conclusions

Vessel	What is being assessed	Clinical meaning
umbilical artery	Placental resistance	Risk of placental insufficiency and growth retardation
Middle cerebral artery	Cerebral vasodilation	Response to hypoxia, calculation of cerebroplacental ratio
Venous duct	Venous hemodynamics	Severity of decompensation in severe growth retardation
Uterine arteries	Uteroplacental perfusion	Prognosis for preeclampsia risks
[23]

Special situations: multiple pregnancy, cervix, placenta

In twin pregnancies, determining chorionicity and amniotic fluid status in the first trimester is critical for subsequent monitoring. In dichorionic twin pregnancies, examinations are performed regularly every few weeks, while in monochorionic pregnancies, they are performed more frequently, starting in the middle of the second trimester, to promptly identify specific complications. [24]

Second-trimester transvaginal cervical assessment is the best clinical predictor of spontaneous preterm birth in women with no history of preterm birth. In cases of short cervix, preventive interventions are discussed according to current guidelines.[25]

Low-lying placenta and placenta previa detected in the second trimester require routine reassessment in the second half of pregnancy, as the situation often improves as the uterus grows. In the presence of a uterine scar and a low-lying placenta, a targeted examination for the spectrum of placenta accreta is necessary. [26]

If fetal growth restriction or placental complications are suspected, an individualized schedule of serial measurements and Doppler monitoring is developed. The choice of intervals and tactics is determined by a combination of biometrics, Doppler ultrasound, and the clinical picture. [27]

Table 5. Who needs special studies and when

Situation	What to add	For what
Monochorionic twins	Frequent follow-up ultrasound examinations with Doppler	Early detection of twin-to-twin transfusion syndrome
Shortened cervix	Cervicometry in dynamics	Prevention of premature birth
Low lying placenta in the second trimester	Monitoring in the second half of pregnancy	Clarification of presentation and delivery plan
Suspected growth retardation	Serial biometrics, Doppler	Assessing the risk of hypoxia and choosing the timing of delivery
[28]

How the study is conducted and how to prepare

No special preparation is required. It is recommended to review the results of previous examinations and discuss any questions you may have with the specialist. In early pregnancy, a moderately full bladder may be necessary for better visualization with a transabdominal probe, and for detailed anatomy and assessment of the cervix, a transvaginal approach is often used for medical reasons. [29]

The duration depends on the gestational age, fetal position, and objectives. In cases of complex visualization or unfavorable positioning, a repeat visit may be scheduled to complete the protocol without compromising quality or safety. This is a standard practice that improves the overall diagnostic yield. [30]

During the examination, the specialist sequentially evaluates the anatomy, biometry, placenta, and amniotic fluid, records findings, and forms a conclusion. If suspicious signs are detected, repeat imaging, additional fetal echocardiography, or a consultation with a specialized center is prescribed. [31]

Results are interpreted in a clinical context: values for one parameter rarely determine treatment decisions without taking into account other data and dynamics. It is consistent measurements and comparison with international standards that make conclusions robust. [32]

Table 6. Reasons for a return visit or referral to experts

Situation	Why is this important?	Typical actions
Incomplete visualization of organs	Fruit or conditions interfered with the assessment	Repeat ultrasound soon
Suspected developmental defect	Verification required	Expert ultrasound, fetal echocardiography
Risks of growth and hypoxia	Dynamic control is needed	Serial biometrics and Doppler
Low placenta, scar on the uterus	The tactics of childbirth depend on the location	Follow-up ultrasound in the third trimester
[33]

Safety: principles, indices, reasonable limits

Diagnostic ultrasound imaging during pregnancy has a favorable safety profile when the principle of minimal sound power and on-screen indicators are observed. Major organizations emphasize the absence of confirmed harmful effects in humans when the equipment is used correctly without contrast agents. [34]

Key safety parameters are the thermal and mechanical indices. Up to approximately 10 weeks, the soft tissue thermal index is monitored, and after the onset of ossification, the bone-specific index is monitored. Overexposure and unnecessary regimens should be avoided at any time. [35]

Pulsed Doppler is used in the first trimester only when indicated, with a thermal index no higher than one and a minimal exposure time. For heart rate recording, the M mode and short recording are preferred. [36]

Non-medical "memorial" studies are not recommended by regulators and industry societies. If images are released, it should be within the context of a clinical trial, without additional exposure, and without replacing medical diagnostics with entertainment. [37]

Frequently asked questions

Is frequent ultrasound harmful?
Provided safety standards are met and there is reasonable clinical need, no. Information from serial examinations in risk groups helps promptly adjust tactics and improve outcomes. [38]

Why is early dating so important?
Because it is the early fetal size that allows for the most accurate dating, and is then used to plan screenings and assess growth without constant "re-dating." [39]

Should everyone have a third-trimester ultrasound?
In low-risk pregnancies, routine comprehensive screening is discussed, and the decision is individualized. In high-risk groups, additional monitoring is warranted. [40]

What if something isn't seen on the ultrasound?
This may be due to timing factors. Repeat imaging as planned and, if necessary, referral to an expert center is a standard, safe approach. [41]