From the Guidelines
Ultrasound in the third trimester is most commonly indicated for assessing fetal growth, evaluating amniotic fluid volume, and determining placental location, among other critical clinical scenarios, as evidenced by recent guidelines and studies 1, 2, 3. The most common indications for ultrasound in the third trimester include:
- Assessment of fetal growth, particularly for suspected intrauterine growth restriction or macrosomia
- Evaluation of amniotic fluid volume, such as in cases of oligohydramnios or polyhydramnios
- Placental location assessment, especially for placenta previa
- Fetal position determination before delivery
- Biophysical profile testing in high-risk pregnancies
- Evaluation of fetal well-being when decreased fetal movement is reported Additional common indications include:
- Monitoring multiple gestations
- Follow-up of previously identified fetal anomalies
- Assessment of cervical length in patients at risk for preterm labor
- Evaluation of maternal complications, such as preeclampsia or gestational diabetes Doppler studies may be performed to assess umbilical artery blood flow in cases of growth restriction, as recommended by the Society for Maternal-Fetal Medicine 2. These ultrasound examinations are crucial for clinical decision-making regarding timing and mode of delivery, need for intervention, and overall pregnancy management, with the goal of minimizing morbidity, mortality, and improving quality of life for both the mother and the fetus 1, 3. While routine third trimester ultrasound in low-risk pregnancies remains controversial, targeted scanning based on clinical indications has become standard practice in modern obstetric care, as supported by the American College of Radiology and other professional organizations 1.
From the Research
Common Indications for Ultrasound in Third Trimester Prenatal Care
The most common indications for ultrasound (US) in third trimester prenatal care include:
- Fetal growth restriction (FGR) screening 4, 5, 6
- Detection of small-for-gestational-age (SGA) infants 4, 6
- Abnormalities of fetal growth or amniotic fluid 5, 7
- Large for gestational age (LGA) detection 7, 6
- Oligohydramnios or polyhydramnios detection 5, 7
Benefits of Universal Third Trimester Ultrasound Examination
Universal third trimester ultrasound examination has been shown to:
- Increase detection of SGA infants 4
- Identify abnormalities of fetal growth or amniotic fluid 5, 7
- Attenuate composite neonatal and maternal adverse outcomes 5
- Improve detection of fetal growth restriction and smallness for gestational age 6
Limitations and Controversies
However, some studies have found that routine third trimester ultrasonography may not reduce adverse perinatal outcomes in low-risk pregnancies 8, and may even lead to increased interventions such as induction of labor 8. The effectiveness of universal third trimester ultrasound examination in reducing adverse perinatal outcomes is still a topic of debate, and more research is needed to fully understand its benefits and limitations 4, 5, 8.