Early Decelerations

Early decelerations follow the shape of the contraction, but inverted. The heart rate begins to dip as the contraction starts. The bottom of the dip is at the peak of the contraction, and it returns back to baseline at the end of the contraction. It looks somewhat like a U-shape. The descent and return are also gradual, and they rarely dip below 40 beats per min. below baseline.

It is thought that these are caused by the cervix overriding the front fontanel of the baby's head. This causes an altered cerebral blood flow, with a subsequent slowing of the FHR due to what is called the Vagal reflex. It usually occurs between 4 to 6 cm of dilation, and is not an indication of a decrease in oxygen.

References:

Obstetrics: Normal and Problem Pregnancies, 5th ed. Gabbe et al.

Fanaroff and Martin's Neonatal-Perinatal Medicine, 8th ed. Martin et al.