What is a fetal non-stress test (NST), also known as a cardiotocography (CTG)?

From the Guidelines

A fetal non-stress test (NST) is a prenatal monitoring procedure that evaluates fetal well-being by measuring fetal heart rate in response to movement, with a reactive result indicating adequate oxygenation and a healthy nervous system 1.

Definition and Purpose

The test involves placing two belts around the mother's abdomen to measure fetal heart rate and detect contractions. During the 20-30 minute test, the mother presses a button when she feels the baby move, and healthcare providers look for accelerations in the baby's heart rate in response to movement.

Interpretation of Results

A "reactive" or normal result shows at least two accelerations of 15 beats per minute lasting 15 seconds within a 20-minute period 1. NSTs are typically performed after 28 weeks of pregnancy and are recommended for high-risk pregnancies, including those with conditions like gestational diabetes, high blood pressure, decreased fetal movement, or past-due pregnancies.

Recommendations for Use

The test is non-invasive, painless, and poses no risk to mother or baby. If results are "non-reactive," additional testing such as a biophysical profile may be ordered, though this doesn't necessarily indicate a problem but may require further evaluation 1. According to the Society for Maternal-Fetal Medicine, antepartum fetal surveillance, including nonstress tests, should start at 32 to 34 weeks of gestation for women with pregestational diabetes mellitus 1.

Key Points to Consider

• The test is used to evaluate fetal well-being and detect potential complications
• It is recommended for high-risk pregnancies, including those with gestational diabetes, high blood pressure, and decreased fetal movement
• A reactive result indicates adequate oxygenation and a healthy nervous system
• Additional testing may be ordered if results are non-reactive
• The test is non-invasive, painless, and poses no risk to mother or baby

From the Research

Definition and Purpose of Fetal Non-Stress Test

• The fetal non-stress test (NST) is a widely accepted method of screening for fetal distress in high-risk pregnancies 2.
• It is a non-invasive test that records fetal movements and fetal heart rate, indicating fetal biophysiological well-being 3.
• The purpose of the NST is to assess fetal well-being and identify potential complications in high-risk pregnancies 4.

Procedure and Interpretation

• The NST is a technically simple procedure that can be performed by family physicians 2.
• The test involves recording fetal heart rate and movements over a period of time, usually 20 minutes 3.
• The results of the NST can be reactive (normal) or non-reactive (abnormal), with reactive results indicating fetal well-being 2, 4.
• Non-reactive results may require further evaluation with other tests, such as the contraction stress test (CST) 4.

Factors Affecting Test Results

• Maternal position during the test can affect the results, with some positions more comfortable for the mother than others 3.
• However, the position of the mother does not significantly affect the reactivity of the NST 3.
• Other factors, such as fetal movements and heart rate, can also affect the results of the NST 3, 5.

Clinical Significance and Predictive Value

• The NST has been found to be clinically efficacious in evaluating fetal well-being in high-risk pregnancies 4.
• The test has a high predictive value for identifying healthy fetuses and those at risk of complications 5.
• The biophysical profile, which includes the NST, is a more accurate method of identifying hypoxic fetuses than any single test 5.