A nurse is monitoring a laboring client with an external electronic fetal monitor and notes the fetal heart rate decelerates after the contraction begins, with the lowest point of the deceleration occurring after the peak of the contraction. What is the nurse's priority action?

The pattern described — fetal heart rate deceleration that begins after the contraction starts and reaches its nadir after the peak of the contraction — is a late deceleration. Late decelerations are caused by uteroplacental insufficiency and indicate that the fetus is receiving inadequate oxygen during contractions. The priority nursing intervention is to reposition the client to the left lateral position, which relieves aortocaval compression and improves uteroplacental blood flow, combined with increasing the intravenous fluid rate to optimize maternal circulating volume and placental perfusion. Additional interventions include administering supplemental oxygen and discontinuing oxytocin if infusing. Preparing the client for immediate cesarean delivery is premature before conservative interventions have been attempted; cesarean becomes necessary only when the pattern is non-reassuring despite all corrective measures. Administering oxygen at 10 L/min via non-rebreather mask may be part of the intervention bundle, but repositioning and IV fluid increases address the root cause more directly and are prioritized first. Documenting the finding as an early deceleration is incorrect because the timing described is consistent with late, not early, decelerations; early decelerations mirror contractions and are benign, caused by fetal head compression.