3.3 Late Decelerations

The Delayed Dip

A late deceleration shares the gradual shape of an early deceleration — NICHD defines onset to nadir as 30 seconds or more — and it is also associated with a uterine contraction. The single critical difference is timing. In a late deceleration, the onset, nadir, and recovery are all shifted to the right (delayed) relative to the contraction. The nadir occurs after the peak of the contraction, and the FHR frequently does not return to baseline until well after the contraction has ended. If you draw a vertical line down from the contraction peak, it lands on the descending limb of the deceleration, not the bottom.

Visually, a late deceleration looks like it is "chasing" the contraction. Because both early and late decelerations are smooth and gradual, shape alone cannot separate them — only the timing of the nadir can. This is the most frequently tested discrimination on the C-EFM exam, so train yourself to find the contraction peak first, then ask where the nadir falls.

Why It Happens: Uteroplacental Insufficiency

Late decelerations are caused by uteroplacental insufficiency — inadequate oxygen transfer across the placenta. Maternal blood normally perfuses the intervillous space, where oxygen diffuses from maternal blood to fetal blood. During every contraction, perfusion of the intervillous space is transiently reduced because the contracting myometrium compresses the spiral arteries. A fetus with healthy placental reserve tolerates this dip without difficulty. But when placental reserve is compromised, the contraction-driven fall in oxygen reaches a critical threshold and the fetus becomes transiently hypoxemic.

That transient hypoxemia is sensed by fetal chemoreceptors, which trigger a reflex slowing of the heart; with worsening hypoxia, a direct myocardial depressant effect contributes as well. Crucially, this response takes time to build, so the deceleration appears after the oxygen deficit accumulates — which is precisely why the dip is delayed relative to the contraction. The mechanism explains the timing.

Common Causes of Uteroplacental Insufficiency

• Maternal hypotension — frequently after epidural or spinal anesthesia (a high-yield exam link).
• Uterine tachysystole — more than 5 contractions per 10 minutes leaves too little recovery time.
• Preeclampsia / chronic hypertension — impaired spiral-artery remodeling.
• Placental abruption — loss of functional placental surface.
• Post-term pregnancy and intrauterine growth restriction (IUGR) — reduced placental reserve.
• Maternal diabetes, anemia, or hypoxia — reduced oxygen-carrying capacity.

Clinical Significance

Late decelerations are non-reassuring. A single isolated late deceleration may be transient, but recurrent late decelerations — defined as occurring with 50% or more of contractions in a 20-minute window — indicate ongoing uteroplacental compromise. The most dangerous finding is recurrent late decelerations with minimal or absent variability: the loss of variability tells you the fetal autonomic system can no longer compensate, and this combination is strongly associated with fetal metabolic acidemia and defines a Category III tracing.

Initial management is intrauterine resuscitation aimed at restoring placental oxygen delivery: lateral repositioning (relieves aortocaval compression and improves uterine perfusion), an IV fluid bolus (corrects hypotension and hypovolemia), discontinuing oxytocin (reduces tachysystole), correcting maternal hypotension (fluids and, if needed, a vasopressor), and supplemental oxygen when indicated. If recurrent late decelerations persist despite resuscitation, the team prepares to expedite delivery.

A key exam nuance is what variability tells you alongside the late decelerations. Recurrent late decelerations with moderate variability are still non-reassuring and require resuscitation, but moderate variability indicates the fetus is compensating and is reassuring about current acid-base status. It is the loss of variability — minimal or absent — accompanying recurrent late decelerations that escalates the tracing to Category III and signals decompensation. Always read the late deceleration and the variability together, never in isolation.

Early vs. Late: Timing Comparison

Worked example: A nullipara at 40 weeks receives an epidural; ten minutes later her blood pressure falls from 120/74 to 88/52. The FHR baseline is 145 with moderate variability. With each of the next several contractions, the nurse sees a smooth FHR dip whose nadir falls roughly 30 seconds after the contraction peak, recovering after the contraction ends; this pattern accompanies 4 of the last 6 contractions. Interpretation: recurrent late decelerations (more than 50% of contractions) from uteroplacental insufficiency, precipitated by post-epidural hypotension. Correct actions: turn the patient lateral, give a rapid IV bolus, stop oxytocin, and treat the hypotension (consider a vasopressor). Because variability is still moderate, the fetus is compensating — but recurrence demands immediate intrauterine resuscitation.