Editing Cardiotocography (section)

===Periodic or episodic decelerations===
{{anchor|deceleration}}
Periodic refers to decelerations that are associated with contractions; episodic refers to those not associated with contractions. There are four types of decelerations as defined by the NICHD nomenclature, all of which are visually assessed.<ref name=nic/>
*Early decelerations: a result of increased [[vagal tone]] due to compression of the fetal head during contractions. Monitoring usually shows a symmetrical, gradual decrease and return to baseline of FHR, which is associated with a uterine contraction. A 'gradual' deceleration has a time from onset to nadir of 30 seconds or more. Early decelerations begin and end at approximately the same time as contractions, and the low point of the fetal heart rate occurs at the peak of the contraction.<ref name=Callahan2013>{{cite book|last1=Callahan|first1=Tamara|last2=Caughey|first2=Aaron B.|title=Blueprints Obstetrics & Gynecology|publisher=Lippincott Williams & Wilkins|location=Baltimore, MD|date=January 2013|isbn=978-1-4511-1702-8|pages=43–47}}</ref>
*Late decelerations: a result of [[placental insufficiency]], which can result in [[fetal distress]]. Monitoring usually shows symmetrical gradual decrease and return to baseline of the fetal heart rate in association with a uterine contraction. A 'gradual' deceleration has an onset to nadir of 30 seconds or more. In contrast to early deceleration, the low point of fetal heart rate occurs after the peak of the contraction, and returns to baseline after the contraction is complete.<ref name=Callahan2013/>
*Variable decelerations: generally a result of [[umbilical cord compression]], and contractions may further compress a cord when it is wrapped around the neck or under the shoulder of the fetus. They are defined as abrupt decreases in fetal heart rate, with less than 30 seconds from the beginning of the decrease to the nadir of heart rate. The decrease in FHR is at least 15 beats per minute, lasting at least 15 seconds but less than 2 minutes in duration.<ref name=Callahan2013/> When variable decelerations are associated with uterine contractions, their onset, depth, and duration commonly vary with successive uterine contractions.{{citation needed|date=September 2016}}
*Prolonged deceleration: a decrease in FHR from baseline of at least 15{{nbsp}}bpm, lasting at least 2 minutes but less than 10 minutes. A deceleration of at least 10 minutes is a baseline change.{{citation needed|date=September 2016}}

Additionally, decelerations can be ''recurrent'' or ''intermittent'' based on their frequency (more or less than 50% of the time) within a 20-minute window.<ref name=nic/>