The normal heart rate for a baby during labor is 110 to 160 beats per minute (bpm). Fetal heart rate monitoring tracks this rate, providing caregivers with insight into the baby’s well-being. A temporary decrease is termed a deceleration, while a sustained drop below 110 bpm is known as fetal bradycardia. These heart rate changes are a physiological response to the stress of labor, and understanding their cause helps determine if the baby is adapting appropriately to birth.
Causes Linked to Reduced Uterine Blood Flow
A late deceleration is a concerning pattern of fetal heart rate drop directly linked to uteroplacental insufficiency, where the placenta cannot meet the fetus’s oxygen demands during a uterine contraction. The drop is termed “late” because it begins after the contraction has started and reaches its lowest point, or nadir, after the contraction has peaked.
The core mechanism involves a reduction in the fetal oxygen level, which triggers a reflex response to conserve energy. When oxygen supply decreases, chemoreceptors in the baby’s body initiate a response that shunts blood away from non-essential areas toward the brain and heart. This response includes a parasympathetic signal via the vagus nerve, which acts to slow the heart rate and reduce the heart’s oxygen consumption.
Several maternal conditions can interfere with blood flow to the uterus, leading to this pattern. Maternal hypotension (low blood pressure) can significantly reduce the pressure pushing blood into the placental circulation. This is sometimes seen following the administration of epidural anesthesia, which can cause a drop in systemic blood pressure.
Uterine hyperstimulation, characterized by contractions that are too frequent, strong, or long, prevents the uterus from having a sufficient rest period. Without this adequate break, oxygen transfer is continuously compromised, as the uterine arteries cannot replenish the oxygen supply to the placenta. Conditions such as placental abruption (premature separation from the uterine wall) or placenta previa (placenta covering the cervix) also drastically reduce the surface area available for oxygen exchange, causing poor blood flow.
Causes Linked to Umbilical Cord Compression
A distinct type of heart rate drop, the variable deceleration, is caused by physical pressure on the umbilical cord, which temporarily restricts blood flow. These drops are characterized by their abrupt onset and recovery, often appearing as V, U, or W shapes on the monitoring strip. Their timing is “variable” because it does not have a consistent relationship with the timing of the uterine contraction.
Cord compression first constricts the softer umbilical vein, which carries oxygenated blood to the baby, momentarily decreasing the baby’s blood return to the heart. This triggers a slight drop in heart rate. If the compression continues, the umbilical arteries, which carry waste products away, are also squeezed, causing an increase in the baby’s peripheral blood pressure.
This sudden increase in blood pressure is detected by baroreceptors, which then stimulate the vagus nerve, resulting in a rapid and more pronounced drop in the heart rate. The heart rate quickly returns to normal as the contraction passes and the pressure on the cord is relieved. This mechanical interruption of blood flow is the most common cause of all deceleration types during labor.
A decrease in the volume of amniotic fluid (oligohydramnios) increases the likelihood of cord compression because there is less fluid cushioning the cord from the contracting uterus. Similarly, a nuchal cord, where the umbilical cord is wrapped around the baby’s neck or body, can lead to compression as the baby descends. The most severe form is a cord prolapse, where the cord slips into the birth canal ahead of the baby and is compressed by the baby’s head.
Causes Linked to Head Compression and Other Factors
A fetal heart rate drop known as an early deceleration is primarily caused by pressure on the baby’s skull as it moves through the birth canal. These drops are a normal physiological event of labor. They are characterized by a smooth, symmetrical decrease and return to the baseline rate, perfectly mirroring the shape of the uterine contraction.
The mechanism involves the vagus nerve, which passes near the fetal head. When the skull is compressed by the contracting uterus or the maternal pelvis, the resulting pressure stimulates the vagus nerve. This stimulation triggers a reflex that temporarily slows the heart rate. Since the drop is directly tied to the physical pressure coinciding with the contraction, it begins and ends with the contraction and indicates that the baby’s central nervous system is intact.
While mechanical factors and blood flow issues account for the majority of decelerations, other factors can cause a sustained drop in the baseline fetal heart rate. Severe maternal infection, such as chorioamnionitis, can trigger a fetal inflammatory response that affects the baby’s cardiac function, sometimes leading to a lower heart rate or an irregular rhythm.
Pre-existing fetal cardiac conditions, like congenital heart block, can also manifest as a low heart rate that is independent of contractions. Congenital heart block occurs when the electrical signals between the upper and lower chambers of the heart are disrupted, often due to maternal autoimmune antibodies that cross the placenta. This electrical problem directly limits the heart’s ability to maintain a normal beat, resulting in a persistently slow rate.