How it began
Montevideo units were devised at the University of Uruguay by Roberto Caldeyro-Barcia and Hermógenes Alvarez, an associate professor of obstetrics and gynecology. Working together in the 1940s, the two obtained the first known recording of pressure inside the uterus during labor by placing a small balloon in the uterus. As the uterus contracted it squeezed the balloon, and the compression showed the pressure of the contraction.
Caldeyro-Barcia graduated in medicine in 1947 and kept working with Alvarez. Shortly after that first recording they proposed a unit to express the force of contractions as a number. It is usually dated to 1949 and takes its name from the city where the work was done.
How it works
The calculation covers a ten-minute window. For each contraction in that window, the resting pressure of the uterus is subtracted from the peak pressure of the contraction, measured in millimeters of mercury. The results are added together to give the total in Montevideo units.
A total of about 200 units indicates that labor is progressing as needed. A figure well below that means contractions are not strong enough to open the cervix.
What changed first
The unit gave physicians a common way to compare normal labors with labors that were not progressing. In 1952 Caldeyro-Barcia and Alvarez presented their methods for monitoring uterine pressure and fetal heart rate in England, and the techniques became widely accepted. According to the Venezuelan gynecologist Otto Armas, the unit led to a more rational and calculated use of oxytocin, the drug commonly used to move labor along.
What lasted
The pressure recordings led directly to further findings. In the 1950s the Montevideo group described two patterns of slowed fetal heart rate during contractions, one that recovers at once and one that recovers slowly and can signal a lack of oxygen. Caldeyro-Barcia went on to study the risks of labor induced with oxytocin, and in 1970 became the first director of the Latin American Center of Perinatology and Human Development in Montevideo.
Where it stands today
Montevideo units remain a standard measure of uterine activity in labor, with 200 units still used as the reference for adequate contractions. They are calculated from pressure changes in the amniotic fluid, measured in millimeters of mercury, so they depend on a pressure reading taken inside the uterus.
