BACKGROUND:
The definition of normal labor progression influences all birthing women because it defines thresholds for interventions. Contemporary models highlight the diversity of normal progression but suffer from endpoint definition issues and exclusion bias. It is unclear whether a precise definition of the onset of active labor, based on cervical dilation, is necessary. A dynamic, individualized model describing cervical dilation curves is needed.

OBJECTIVES:
To develop a model that describes individual cervical dilation curves during the active first stage of labor.

METHODS:
A population-based cohort study was conducted at a large hospital in Norway, including 62,583 women with live singleton deliveries (≥22 weeks), in breech or vertex positions. Cervical dilation data were extracted from the partographs. The cervical dilation measures were used to develop a new model for individual labor curves. The model has three main components: (1) a modified Gompertz curve, (2) an individual, random parameter describing the location of the time scale, and (3) an individual, random parameter describing labor progression speed.

RESULTS:
The primary difference between individuals was the speed of labor progression, not the pattern, regardless of parity. The model predicts both future and past labor progression during the active first stage of labor, based on one or more cervical dilation measurements. The model does not require a specific dilation that defines labor onset, but calculates a parameter that describes location of the time scale. The model produced a sigmoid pattern that aligns with seminal research but contrasts with most contemporary labor curves.

CONCLUSIONS:
Labor progression, defined as cervical dilation over time, follows a consistent sigmoid pattern across individuals and parities. This study introduces an individualized, dynamic model for labor progression, offering a refined understanding of cervical dilation during labor.

KEY MESSAGE:
Labor curves defined by cervical dilation over time follow a sigmoid pattern, irrespective of parity. Individual labor progression can be modeled. Physiological - labour progress (including three-minute presentation competition)