Internal fetal monitoring is a medical procedure used during labor to obtain a continuous measurement of the fetal heart rate and the strength of uterine contractions. Unlike external monitoring, which uses sensors placed on the mother’s abdomen, this method involves placing small devices directly inside the uterus. This direct measurement is only performed after labor has begun and the amniotic sac has ruptured. Internal monitoring provides data that is more precise and consistent because it is not affected by factors like maternal movement or body size.
Indications for Internal Monitoring
Internal monitoring is indicated when external methods fail to provide reliable data, such as when the mother has significant body mass or the fetus is moving frequently. External monitors struggle to maintain a clear signal of the fetal heart rate in these situations. Internal monitoring offers a clearer, beat-to-beat assessment of the fetal heart rhythm.
Another element is the need for precise information about uterine activity. The external sensor, called a tocodynamometer, only records the frequency and duration of contractions, not their actual intensity. An internal device is necessary if the exact pressure of contractions is needed, especially during labor induction or augmentation.
The procedure requires two physical conditions before insertion: the amniotic membranes must be ruptured, allowing direct access to the uterine cavity, and the cervix must be dilated, usually to at least two to three centimeters, to allow the devices to pass safely.
Attaching the Fetal Scalp Electrode
The Fetal Scalp Electrode (FSE) measures the fetal heart rate directly, bypassing external signal quality issues. Attachment begins with a sterile vaginal examination to confirm prerequisites and identify the fetal presenting part, which is most often the scalp.
The FSE is housed within a sterile, disposable guide tube, inserted through the vagina and the open cervix. The guide tube is advanced until its tip is positioned firmly against the fetal presenting part. The small, spiral wire electrode remains retracted within the tube during insertion to protect maternal tissue.
The clinician rotates the device a half to a full turn once the guide tube is positioned against the scalp. This twisting motion secures the spiral tip of the electrode into the superficial layer of the fetal skin. Resistance confirms secure attachment.
The guide tube is then removed, leaving the electrode attached to the scalp with a thin lead wire extending out. This wire connects to a plate on the mother’s thigh, which links to the electronic fetal monitor to display the direct electrical signal.
Attaching the Intrauterine Pressure Catheter
The second internal device, the Intrauterine Pressure Catheter (IUPC), measures the force of uterine contractions in millimeters of mercury (mmHg). The IUPC is a thin, flexible tube inserted into the uterus alongside the fetal head, designed to sit within the amniotic fluid space.
The catheter is introduced using a plastic introducer, which guides the tube past the cervix. The clinician threads the catheter into the uterine cavity, aiming for a position away from the fetal presenting part and the placenta. Once the catheter is correctly placed, the introducer is removed.
The external end of the IUPC is secured to the mother’s inner thigh with tape. The catheter is then connected to the monitoring system and calibrated to atmospheric pressure. These readings provide a numerical value for contraction intensity, which is a significant advantage over external monitoring.
Safety Considerations and Removal
Internal monitoring is considered a minimally invasive procedure, but it carries potential risks. The most common complication is a minor laceration or abrasion on the fetal scalp, which typically heals without intervention. There is also a slight increase in the risk of infection, such as chorioamnionitis, because the devices create a direct pathway into the uterus.
Rarely, IUPC placement can result in uterine wall perforation or extra-amniotic placement (between the uterine wall and the membranes). The procedure is temporary, and both devices are removed before delivery. The IUPC is simply pulled out, while the FSE is gently unscrewed counter-clockwise from the fetal scalp.