Ultrasound Imaging in Pregnancy

Ultrasound imaging in pregnancy uses high-frequency sound waves to visualise the fetus, placenta, and uterus. It is used to confirm and date the pregnancy, assess the number and viability of fetuses, screen for and assess structural anomalies, and monitor fetal growth and wellbeing, making it a central tool of antenatal assessment.

Definition

Obstetric ultrasound is the use of sonography during pregnancy to image the fetus and surrounding structures in order to establish gestational age, assess viability and number, screen for structural anomalies, evaluate growth, and assess fetal and placental blood flow.

Scope

This topic covers the purposes and main applications of obstetric ultrasound: early pregnancy assessment and dating, anomaly screening, growth assessment, and Doppler studies of blood flow. It explains what these examinations are for and how their results are interpreted as part of antenatal care, and treats them as reference concepts rather than as instructions for performing or acting on any individual scan.

Core questions

• What are the main purposes of ultrasound across pregnancy?
• How does early ultrasound contribute to dating and viability assessment?
• What is the role of ultrasound in anomaly and growth assessment?
• What does Doppler ultrasound add in higher-risk pregnancies?

Key concepts

• Dating and gestational age assessment
• Viability and fetal number
• Anomaly screening scan
• Fetal biometry and growth assessment
• Fetal growth restriction
• Umbilical and fetal Doppler studies
• Placental localisation

Mechanisms

Ultrasound transmits high-frequency sound into the tissues and reconstructs an image from the reflected echoes, allowing visualisation of the fetus, placenta, and uterus without ionising radiation. Early scans establish viability, number, and gestational age from crown-rump length; later scans assess anatomy and measure biometry to estimate growth against reference standards, with consensus criteria used to define fetal growth restriction. Doppler ultrasound measures blood flow in the umbilical and fetal vessels and, in high-risk pregnancies, is used in surveillance of fetal wellbeing.

Clinical relevance

Ultrasound provides much of the objective information used in antenatal assessment, from dating to growth surveillance, and reading ultrasound findings is part of antenatal care for midwives and other clinicians. This entry describes the purposes and interpretation of obstetric ultrasound; it is not a protocol for performing scans or for managing any individual pregnancy based on imaging findings.

Epidemiology

Early-pregnancy ultrasound improves gestational age estimation and detection of multiple pregnancy compared with assessment without it; in high-risk pregnancies, umbilical artery Doppler is associated with changes in perinatal outcomes, and consensus criteria support consistent definition of fetal growth restriction across studies and practice.

History

Obstetric ultrasound developed from mid-twentieth-century work applying sonar principles to the gravid uterus and became routine in antenatal care over subsequent decades. Its applications expanded from confirming pregnancy and dating to detailed anomaly screening, biometric growth assessment, and Doppler studies of fetoplacental blood flow, with consensus efforts later standardising definitions such as fetal growth restriction.

Debates

What is the role of routine versus indicated ultrasound?

Ultrasound clearly aids dating, viability, and anomaly detection, but the optimal number, timing, and indications for scans, and the balance between routine and selective use, vary by setting and remain a subject of guidance and debate.

Seminal works

• whitworth-2015
• gordijn-2016

Frequently asked questions

Why is an ultrasound done early in pregnancy?

Early ultrasound helps confirm that the pregnancy is viable, establish how many fetuses are present, and estimate gestational age from measurements such as crown-rump length, which improves the accuracy of dating compared with assessment without a scan.

What is Doppler ultrasound used for in pregnancy?

Doppler ultrasound measures blood flow in the umbilical and fetal vessels and is used, mainly in higher-risk pregnancies, as part of surveillance of fetal wellbeing; this entry describes its purpose rather than directing its use for any individual.