Normal Labour Mechanisms and Diagnosis

Vocabulary and key concepts regarding the definitions, stages, mechanics, and cardinal movements of normal labor and delivery.

Normal Labour

Regular/rhythmic, painful uterine contractions which progressively increase in frequency, intensity, and duration accompanied by effacement and progressive dilatation of the cervix and descent of the presenting part of the fetus, leading to the expulsion of the products of conception.

True Labor

Labor characterized by regular contractions that increase in frequency, intensity, and duration, produce cervical change, and involve pain usually beginning in the back and radiating to the front in a band.

False Labor

Less regular contractions that do not progressively increase in frequency and intensity, show minimal cervical change, and may be relieved by rest, position change, or hydration.

Braxton-Hicks

Uterine contractions not associated with cervical change that are shorter in duration, less intense, located over the lower abdomen and groin, and resolve with ambulation.

First Stage of Labor

The interval between the onset of labor and full cervical dilatation.

Latent Phase

The period between the onset of labor and the point at which a change in slope of the rate of cervical dilatation is noted, typically at $4\,cm$.

Active Phase

The period characterized by a greater rate of cervical dilatation, usually beginning around $5\,cm$ and continuing to full dilatation.

Second Stage of Labor

The interval between full cervical dilatation and the delivery of the fetus.

Third Stage of Labor

The interval between the delivery of the fetus and the delivery of the placenta and membranes, with a maximum duration of $30$ minutes.

Fourth Stage of Labor

The period from the delivery of the placenta and membranes to $1$ hour thereafter.

The Powers

Forces generated by the uterine musculature, often measured in Montevideo units.

Mild Contractions

Uterine contractions that last less than $20$ seconds.

Moderate Contractions

Uterine contractions that last between $20$ and $40$ seconds.

Strong Contractions

Uterine contractions that last more than $40$ seconds.

Lie

The relationship of the longitudinal axis of the fetus relative to the longitudinal axis of the uterus (e.g., longitudinal, transverse, or oblique).

Presentation

The fetal part that overlies the pelvic inlet and occupies the lower pole of the uterus, such as cephalic, breech, or shoulder.

Malpresentation

Any fetal presentation that is not cephalic with the occiput leading.

Attitude

The position of the fetal head with regard to the fetal spine, involving the degree of flexion or extension.

Denominator

An arbitrary chosen fixed bony point on the presenting part (e.g., occiput, mentum, or sacrum) used to determine fetal position.

Position

The relationship of the denominator to the different quadrants of the maternal pelvis.

Naegele's obliquity

A term for anterior asynclitism where the anterior parietal bone presents.

Litzmann's obliquity

A term for posterior asynclitism where the posterior parietal bone presents.

Station

A measure of the descent of the presenting part of the fetus through the birth canal.

Engagement

The passage of the widest diameter of the presenting part to a level below the plane of the pelvic inlet, referred to as $0$ station.

Flexion

A cardinal movement occurring passively as the head descends, allowing the smallest diameter of the fetal head to pass through the pelvis.

Internal Rotation

The cardinal movement where the presenting part rotates from its original position (usually transverse) to an anteroposterior position.

Restitution

The external rotation of the fetal head to return to the correct anatomic position in relation to the fetal torso after the head is delivered.

Brandt-Andrews Maneuver

A method to deliver the placenta where the abdominal hand secures the uterine fundus to prevent inversion while the other hand exerts sustained downward controlled cord traction.

Crede maneuver

A maneuver where the umbilical cord is fixed with the lower hand while the uterine fundus is secured and sustained upward traction is applied using the abdominal hand.

Marginal insertion

A placental abnormality where the umbilical cord inserts into the edge of the placenta.

Membranous insertion

A placental abnormality where fetal vessels course through the membranes prior to attaching to the placental disk.