What are the indications for ultrasound in pregnant patients?

Indications for Ultrasound in Pregnant Patients

Ultrasound should be performed for all pregnant patients presenting with abdominal pain and/or vaginal bleeding in the first trimester to evaluate for possible ectopic pregnancy, regardless of β-hCG level. 1

First Trimester Indications

First trimester ultrasound (≤13 weeks 6 days) is indicated for:

• Confirmation of pregnancy and establishing the presence of an intrauterine pregnancy 1, 2
• Dating of pregnancy based on crown-rump length measurements 1, 3
• Assessment of pregnancy viability through visualization of cardiac activity 1, 2
• Determining pregnancy location (intrauterine vs. ectopic) 1, 2
• Determining pregnancy number (singleton vs. multiple gestation) 1, 3
• Evaluation of patients with vaginal bleeding and/or abdominal pain 1
• Suspected pregnancy complications including threatened abortion, complete abortion, or ectopic pregnancy 1

Important Clinical Considerations in First Trimester

• Transvaginal ultrasound is the preferred approach for early pregnancy assessment due to superior resolution 1, 2
• Ultrasound should be performed regardless of β-hCG level in symptomatic patients, as ectopic pregnancies can be detected even with β-hCG below traditional discriminatory thresholds 1
• The presence of a yolk sac confirms a definite gestational sac/pregnancy 1
• Cardiac activity is typically visualized on transvaginal ultrasound at approximately 6 weeks gestational age 1

Second and Third Trimester Indications

• Anatomical survey (typically 18-22 weeks) to assess fetal anatomy and screen for structural anomalies 4
• Assessment of fetal growth and detection of intrauterine growth restriction 1
• Evaluation of amniotic fluid volume 1
• Assessment of placental location and detection of placental abnormalities 5
• Cervical length measurement in patients at risk for preterm birth 1
• Evaluation of fetal well-being through biophysical profile (BPP) or modified BPP in high-risk pregnancies 1
• Follow-up assessment of previously identified soft markers for aneuploidy 1

Specific Second/Third Trimester Follow-up Indications

• Isolated echogenic bowel requires evaluation for cystic fibrosis, cytomegalovirus infection, and third-trimester ultrasound for growth assessment 1
• Isolated single umbilical artery requires third-trimester ultrasound for growth evaluation 1
• Isolated urinary tract dilation requires follow-up ultrasound at 32 weeks 1
• Isolated shortened humerus or femur requires third-trimester ultrasound for growth assessment 1

High-Risk Pregnancy Indications

• Maternal medical conditions affecting pregnancy (e.g., diabetes, hypertension) 1
• Previous adverse pregnancy outcomes 1
• Advanced maternal age 1
• Abnormal serum screening results 1
• Suspected fetal anomalies on previous imaging 1
• Assessment of fetal well-being in post-term pregnancies 1

Pitfalls and Caveats

• Ultrasound in early pregnancy may not detect all ectopic pregnancies, particularly with β-hCG levels below 1,500 mIU/mL (sensitivity approximately 25%) 1
• The term "normal first-trimester pregnancy" should be avoided as anomalies may be missed early in pregnancy 1
• Ultrasound findings must be interpreted in conjunction with clinical presentation and β-hCG levels 1
• Follow-up ultrasound may be necessary when initial findings are inconclusive 1
• Transvaginal ultrasound is contraindicated in cases of ruptured membranes or placenta previa with active bleeding 1

Ultrasound is a safe, non-invasive, and valuable diagnostic tool throughout pregnancy that provides critical information for optimal maternal and fetal outcomes 6, 5.