Fetal monitoring is a routine medical practice used during pregnancy and labor to assess the health status of an unborn baby. This non-invasive or minimally invasive procedure involves tracking two primary physiological signals: the baby’s heart rate and the mother’s uterine contractions. By observing the relationship between these factors, healthcare providers gain real-time insight into how the fetus is tolerating the stresses of pregnancy and birth. The continuous recording of these signals is often referred to as cardiotocography (CTG). This practice is a screening tool that helps medical teams make timely decisions regarding care and potential interventions.
The Primary Goal of Monitoring
The fundamental purpose of fetal monitoring is to detect early signs of distress, which most often relates to an inadequate supply of oxygen to the fetus. Uterine contractions naturally compress maternal blood vessels within the muscular wall of the uterus, which can temporarily reduce the flow of oxygenated blood to the placenta and, subsequently, the baby. The average fetal heart rate ranges between 110 and 160 beats per minute, and a healthy fetus will maintain a specific pattern of heart rate variability and response to contractions.
When a baby’s oxygen supply is reduced, the fetal nervous system attempts to compensate, which is reflected in changes to the heart rate pattern. Monitoring looks for specific patterns, such as a drop in heart rate called a deceleration, or a lack of beat-to-beat variability. These irregularities can signal a state of hypoxia, or low oxygen, which can progress to metabolic acidosis if prolonged. Acidosis is a condition where excessive acid builds up in the blood, potentially compromising cell function. The ultimate objective is to identify and resolve these issues before they cause lasting harm while also avoiding unnecessary interventions.
Distinguishing External and Internal Methods
Fetal monitoring is broadly divided into external and internal methods based on the invasiveness of the procedure. External monitoring is the most common and non-invasive approach, performed by placing two separate devices on the mother’s abdomen, secured with elastic belts. The fetal heart rate is tracked using a Doppler ultrasound transducer, which uses sound waves to detect the fetal heart motion and relays the signal to the monitor.
The second external device is a tocodynamometer, often called a toco, which is a pressure sensor placed over the top of the uterus. This sensor detects changes in the abdominal wall caused by uterine contractions, providing a record of the frequency and duration of the contractions. While convenient and safe, external monitoring can be affected by factors like maternal movement, body mass, or fetal position, which can make the heart rate tracing unreliable or difficult to interpret.
When external monitoring proves insufficient or when a more precise reading is required, internal monitoring may be used. This method requires the amniotic sac to be ruptured and the cervix to be partially dilated for the devices to be inserted. For a highly accurate heart rate measurement, a Fetal Scalp Electrode (FSE) is attached directly to the baby’s scalp through the cervix.
The Intrauterine Pressure Catheter (IUPC) is a thin catheter inserted into the uterine cavity. The IUPC provides an exact measurement of the pressure inside the uterus, allowing healthcare providers to determine the precise strength and intensity of the contractions. While internal methods offer a more consistent and accurate reading, they carry a slight risk of infection and require that labor is already underway.
Common Scenarios for Use
Fetal monitoring is employed both during the pregnancy period, known as ante-partum monitoring, and throughout the labor and delivery process, which is intra-partum monitoring. Ante-partum testing is typically reserved for pregnancies considered high-risk due to conditions like gestational diabetes, high blood pressure, or when the pregnancy extends past the due date.
A common test is the Non-Stress Test (NST), which uses external monitors to track the fetal heart rate’s response to the baby’s own movements over a period of time. Another test is the Contraction Stress Test, which observes the fetal heart rate pattern in response to induced contractions to assess placental function.
During active labor, intra-partum monitoring is used to assess the baby’s ability to tolerate the uterine contractions. For low-risk pregnancies, intermittent auscultation, which involves a healthcare provider listening to the heart rate periodically with a handheld Doppler device, is an option. High-risk labors, or those involving interventions like induction or pain medication, often require continuous electronic fetal monitoring. This continuous surveillance ensures that any changes in the heart rate pattern that might signal fetal acidosis are identified quickly, allowing for immediate interventions to improve fetal well-being or expedite delivery if necessary.