Late Decelerations

A deceleration is a decrease in the fetal heart rate below the fetal baseline heart rate. An early deceleration is defined as a waveform with a gradual decrease and return to baseline with time from onset of the deceleration to the lowest point of the deceleration >30 seconds. The nadir of the early deceleration occurs with the peak of a contraction.

A late deceleration also has a waveform with a gradual decrease and return to baseline with time from onset of the deceleration to nadir >30 seconds. However, the late deceleration is "shifted to the right" of the contraction.

The onset of the deceleration occurs after the beginning of the contraction, and the lowest point of the deceleration occurs after the peak of the contraction.

The difference in the timing of early decelerations and late decelerations relative to the contraction may be explained by the underlying mechanism for each of these decelerations.

Early decelerations appear to be caused by vagal discharge produced when the head is compressed by uterine contractions. The onset and depth of early decelerations mirror the shape of the contraction, and tend to be proportional to the strength of the contraction.

Late decelerations occur when a fall in the level of oxygen in the fetal blood triggers chemoreceptors in the fetus to cause reflex constriction of blood vessels in nonvital peripheral areas in order to divert more blood flow to vital organs such as the adrenal glands, heart, and brain. Constriction of peripheral blood vessels causes hypertension that stimulates a baroreceptor mediated vagal response which slows the heart rate. The time consumed in this two step process accounts for the delay in the timing of the deceleration relative to the contraction.

Late decelerations with good variability ("refelex lates") are sometimes caused by excessive uterine contractions or maternal hypotension which may be alleviated by correcting the underlying cause. In conditions with reduced placental exchange such as intrauterine growth restriction (IUGR) measures to improve blood flow and oxygen delivery to the fetus may not be as effective.

If oxygen continues to be limited (hypoxia) to the fetal tissues acidosis can develop as result of increased anaerobic metabolism. Significant levels of acid in the blood (acidemia) can suppress the fetal nervous system leading to decreased variability and direct myocardial depression made evident by shallow late decelerations. If myocardial depression is severe enough, late decelerations may be absent all together

Etiologies of Late Decelerations

Excessive uterine contractions
Maternal hypotension
Maternal hypoxemia (asthma, pneumonia)
Reduced placental exchange as in:

Hypertensive disorders Diabetes IUGR Abruption

1. Cabaniss ML ,Ross MG.Fetal Monitoring Interpretation 2nd ed.Philadelphia, Lippincott Williams & Wilkins, 2009
2. Harris JL, Krueger TR, Parer JT Mechanisms of late decelerations of the fetal heart rate during hypoxia. Am J Obstet Gynecol. 1982 Nov 1;144(5):491-6. PMID:7137235
3. Murata Y, et al.  Fetal heart rate accelerations and late decelerations during the course of intrauterine death in chronically catheterized rhesus monkeys. Am J Obstet Gynecol. 1982 Sep 15;144(2):218-23.PMID:7114133
4. Gaziano EP, Freeman DW Analysis of heart rate patterns preceding fetal death. Obstet Gynecol. 1977 Nov;50(5):578-82. PMID:909663

Review of Three-Tier Fetal Heart Rate Interpretation System

Category I : Normal.
The fetal heart rate tracing shows ALL of the following:

Baseline FHR 110-160 BPM, moderate FHR variability, accelerations may be present or absent, no late or variable decelerations, may have early decelerations.

Strongly predictive of normal acid-base status at the time of observation. Routine care.

Category II : Indeterminate.

The fetal heart rate tracing shows ANY of the following:

Tachycardia, bradycardia without absent variability, minimal variability, absent variability without recurrent decelerations, marked variability, absence of accelerations after stimulation, recurrent variable decelerations with minimal or moderate variability, prolonged deceleration > 2minute but less than 10 minutes, recurrent late decelerations with moderate variability, variable decelerations with other characteristics such as slow return to baseline, and "overshoot".

Not predictive of abnormal fetal acid-base status, but requires continued surveillance and reevaluation.

Category III: Abnormal.

The fetal heart rate tracing shows EITHER of the following:

Sinusoidal pattern OR absent variability with recurrent late decelerations, recurrent variable decelerations, or bradycardia.

Predictive of abnormal fetal-acid base status at the time of observation. Depending on the clinical situation, efforts to expeditiously resolve the underlying cause of the abnormal fetal heart rate pattern should be made.


Case 1
G1P0 . Induction for preeclampsia at 37 weeks.



Case 2
G2P1. Arrived in active labor at 40 weeks


Case 3
G1, P0, 34 weeks gestation admitted to Labor and Delivery with SROM with moderate amount of pink tinged fluid.



Macones GA et al., The 2008 National Institute of Child Health and Human Development Workshop Report on Electronic Fetal MonitoringUpdate on Definitions, Interpretation, and Research Guidelines Obstetrics & Gynecology 2008;112:661-666 PMID:18757666

Home Fetal Monitors- May Give a False Sense of Security

Expectant mothers perceiving a sudden decrease in fetal movements should not rely on fetal heart monitors at home to reassure themselves of their baby's well-being according to Dr. Abhijoy Chakladar of Princess Royal Hospital in West Sussex, England.

Dr. Chakladar reports a case where a mother at 38 weeks, noticed a decrease in her baby's movements, but was reassured by an apparent fetal heartbeat from her monitor. However, the device may have been detecting the mother's heartbeat. A few days later, unable to hear the baby's heartbeat, she went to the hospital where the fetus was discovered to be stillborn.


Abhijoy Chakladar and Hazel Adams Dangers of listening to the fetal heart at home BMJ 2009;339:b4308, doi: 10.1136/bmj.b4308 (Published 5 November 2009)

Antepartum Testing Class

Antepartum Testing Class
9:00 to 12:00 10/29/09

The guidelines proposed by the National Institute of Child Health and Human Development (NICHD) for the interpretation of fetal heart rate tracings are generally applicable to antepartum testing. However, as of this posting, they do not define reactivity.

A non stress test (NST) is considered reactive when two or more fetal heart rate accelerations peak (but do not necessarily remain) at least 15 beats per minute above the baseline and last 15 seconds from baseline to baseline within a 20 minute period with or without fetal movement discernible by the woman.

The following presentation discusses the NICHD guidelines and antepartum testing:
Antepartum Testing (Powerpoint)



The following are 3 of 18 patients ,reviewed during the class, that had been seen for antepartum testing.
(Click on images to enlarge)
Case 1

Case 2

Case 3