We used data from the Consortium on Safe Labor, a multicenter retrospective observational study that abstracted detailed labor and delivery information from electronic medical records in 12 clinical centers (with 19 hospitals) across 9 American College of Obstetricians and Gynecologists (ACOG) U.S. districts from 2002 to 2008. 87% of births occurred in 2005 – 2007. Detailed description of the study was provided elsewhere.
5 Briefly, participating institutions extracted detailed information on maternal demographic characteristics, medical history, reproductive and prenatal history, labor and delivery summary, postpartum and newborn information. Information from the neonatal intensive care unit (NICU) was linked to the newborn records. Data on labor progression (repeated, time-stamped cervical dilation, station and effacement) were extracted from the electronic labor database. To make our study population reflect the overall U.S. obstetric population and to minimize the impact of the various number of births from different institutions, we assigned a weight to each subject based on ACOG district, maternal race/ethnicity (non-Hispanic white, non-Hispanic black, Hispanic and others), parity (nulliparas vs. multiparas) and plurality (singleton vs. multiple gestation). We first calculated the probability of each delivery with these four factors according to the 2004 National Natality data. Then, based on the number of subjects each hospital contributed to the database, we assigned a weight to each subject .
5 We applied the weight to the current analysis. This project was approved by the Institutional Review Boards of all participating institutions.
There were a total of 228,668 deliveries in the database. A total of 62,415 parturients were selected.
Figure 1 depicts the sample selection process for the current analysis. Women were grouped by parity (0, 1, 2+). We used a repeated-measures analysis with 8
th degree polynomial model to construct average labor curves by parity.
6 In this analysis, the starting point was set at the first time when the dilation reached 10 cm (time = 0) and the time was calculated backwards (e.g., 60 minutes before the complete dilation, -60 minutes). After the labor curve models had been computed, the x-axis (time) was reverted to a positive value, i.e., instead of being -12 → 0 hours, it became 0 → 12 hours.
To estimate duration of labor, we used an interval-censored regression
7 to estimate the distribution of times for progression from one integer centimeter of dilation to the next (called “traverse time”) with an assumption that the labor data are log-normally distributed.
8 (link) The median and 95
th percentiles were calculated. Because multiparous women tended to be admitted at a more advanced stage labor than nulliparous women, many multiparous women did not have information on cervical dilation prior to 4 cm. Therefore, the labor curve for multiparous women started at 5 cm rather than at 4 cm as for nulliparous women.
Finally, to address the clinical experience wherein a woman is first observed at a given dilation and then measured periodically, we calculated cumulative duration of labor from admission to any given dilation up to the first 10 cm in nulliparas. The same interval censored regression approach was used. We provide the estimates according to the dilation at admission (2.0 or 2.5 cm, 3.0 or 3.5 cm, 4.0 or 4.5 cm, 5.0 or 5.5 cm) because women admitted at different dilation levels may have different patterns of labor progression. We then plotted the 95
th percentiles of the duration of labor from admission as a partogram. All statistical analyses were performed using SAS version 9.1 (PROC MIXED for the repeated-measures analysis and PROC LIFEREG for interval censored regression). Since the objective of this paper is to describe labor patterns and estimate duration of labor without comparing among various groups, no statistical tests were performed.
Zhang J., Landy H.J., Branch D.W., Burkman R., Haberman S., Gregory K.D., Hatjis C.G., Ramirez M.M., Bailit J.L., Gonzalez-Quintero V.H., Hibbard J.U., Hoffman M.K., Kominiarek M., Learman L.A., Van Veldhuisen P., Troendle J, & Reddy U.M. (2010). Contemporary Patterns of Spontaneous Labor With Normal Neonatal Outcomes. Obstetrics and gynecology, 116(6), 1281-1287.