Internal fetal monitoring (IFM) is a medical technique used during labor and delivery to obtain more precise and continuous measurements of a baby’s heart rate and the strength of uterine contractions. It is more direct than external monitoring, which uses sensors placed on the mother’s abdomen. By providing clear and consistent data, IFM helps healthcare providers assess the baby’s well-being and the progression of labor when external methods fall short. It is not used routinely but is reserved for situations requiring a higher degree of surveillance.
When Internal Monitoring Becomes Necessary
The decision to switch from external to internal monitoring is typically driven by a need for more reliable data about the fetal heart rate (FHR) or uterine activity. External monitoring uses a toco-dynamometer for contractions and an ultrasound transducer for FHR, but it can be inadequate under certain circumstances. Factors like maternal obesity, excessive movement during labor, or the baby’s position can interfere with the external sensors, leading to ambiguous or unreadable tracings.
If external readings are unclear or intermittent, providers cannot accurately assess the baby’s response to labor, raising concerns about fetal distress. Internal monitoring offers a beat-to-beat analysis of the heart rate, eliminating signal interference caused by movement or maternal tissue. This clarity is also important during specific interventions, such as the induction or augmentation of labor with medication like oxytocin.
In cases where labor is not progressing as expected, internal monitoring can determine the exact strength and frequency of contractions, providing data that the external monitor cannot measure precisely. The ability to monitor both the fetal heart rate and the true strength of the uterine contractions simultaneously allows for more informed clinical decisions. This enhanced surveillance is particularly helpful in high-risk pregnancies or when a non-reassuring fetal heart rate pattern is observed on the external monitor.
The Procedure and Required Conditions
Internal fetal monitoring is an invasive procedure that requires certain conditions to be met before it can be performed safely. The two main prerequisites are that the mother’s amniotic sac must be ruptured, and the cervix must be partially dilated, usually at least two centimeters. These conditions allow the necessary devices to be safely guided through the cervical opening into the uterus.
One component of IFM is the Fetal Scalp Electrode (FSE), a thin wire with a small spiral tip. This tip is gently twisted onto the baby’s presenting part, typically the scalp, avoiding the fontanelles, to directly detect the electrical signals of the fetal heart. The FSE provides a direct, continuous, and highly accurate reading of the fetal heart rate and its variability, offering a much clearer picture of the baby’s cardiac activity than external sensors.
The second device is the Intrauterine Pressure Catheter (IUPC), a thin, flexible tube inserted through the cervix and positioned between the baby and the uterine wall. The IUPC measures the pressure inside the uterine cavity in millimeters of mercury (mmHg). This measurement gives an objective reading of the actual strength, duration, and frequency of uterine contractions, as well as the resting tone of the uterus between contractions. The IUPC is superior because external monitors can only measure the frequency and duration of contractions, not their true intensity.
Important Considerations and Risks
Internal fetal monitoring provides superior data, but it is an invasive procedure that carries certain risks. Since the procedure involves introducing devices into the uterine cavity, there is an increased potential for infection for both the mother and the baby. The break in the protective barrier of the amniotic sac and the use of the IUPC can create a pathway for bacteria, potentially leading to maternal fever or uterine infection.
The Fetal Scalp Electrode, by attaching to the baby’s scalp, carries a slight risk of minor trauma. This can result in a small scratch, puncture wound, or bruise at the site of placement, although these marks typically heal quickly after birth. In rare instances, there is a risk of a small collection of blood or an abscess forming on the scalp.
There are specific medical situations, known as contraindications, where IFM must be avoided entirely to prevent harm. Internal monitoring is not used if the mother has active maternal infections, such as genital herpes, HIV, or Hepatitis B or C, as the procedure could increase the risk of transmission to the baby. Additionally, it is avoided if the baby is known or suspected to have a bleeding disorder. Both the FSE and IUPC are removed immediately following delivery.