The electronic fetal heart monitoring tracing in this vignette depicts variable decelerations, which are abrupt departures from baseline at intervals unrelated to the timing of uterine contractions. Variable decelerations are directly caused by compression of the umbilical cord.
Umbilical cord compression, the direct cause of variable decelerations, can occur more frequently in the setting of oligohydramnios. Oligohydramnios, in turn, can occur due to rupture of membranes or fetal congenital anomaly such as renal agenesis or another urinary tract abnormality. Initial treatment options of recurrent variable decelerations include shifting maternal position or, in cases of ruptured membranes, amnioinfusion.
Sweha et al. report that variable decelerations are commonly seen in patients who experienced premature rupture of membranes and decreased amniotic fluid volume. Variable decelerations are frequently accompanied by shoulder accelerations prior to the initiation of the deceleration; this is thought to be due to the initial pressure on the cord occluding the umbilical vein.
Hasegawa et al. investigate fetal heart rate patterns associated with various umbilical cord abnormalities. Atypical variable decelerations are associated with velamentous cord insertions and hypercoiling of the umbilical cord.
Figure A depicts variable decelerations.
Illustration A shows early decelerations; note how they coincide with the timing of the uterine contractions.
Illustration B depicts late decelerations; note how they lag behind the occurrence of uterine contractions.
Illustration C depicts variable decelerations with annotation (red circles), note how they are abrupt, jagged and variable in their occurrence to uterine contractions (green circles).
Answer 1: Uteroplacental insufficiency is the cause of late decelerations, which are characterized by departures from baseline that occur after uterine contractions.
Answer 2: Oligohydramnios is an indirect cause of variable decelerations, mediated through the increased likelihood of cord compression in the setting of decreased amniotic fluid.
Answer 3: Fetal head compression is the cause of early decelerations, which are characterized by decreases in fetal heart rate that coincide with uterine contractions.
Answer 5: Spontaneous rupture of membranes is not a direct cause of variable decelerations; it may be an indirect cause due to ensuing oligohydramnios and ensuing cord compression.
Sweha A, Hacker TW, Nuovo J. Interpretation of the electronic fetal heart rate during labor. Am Fam Physician. 1999 May 1;59(9):2487-500.
PMID:10323356 (Link to Abstract)
Hasegawa J, Matsuoka R, Ichizuka K, Kotani M, Nakamura M, Mikoshiba T, Sekizawa A, Okai T. Atypical variable deceleration in the first stage of labor is a characteristic fetal heart-rate pattern for velamentous cord insertion and hypercoiled cord. J Obstet Gynaecol Res. 2009 Feb;35(1):35-9.
PMID:19215545 (Link to Abstract)