Materials and Methods

The course of labor and method of delivery were compared between the CPP and the CSL. The CPP was a prospective study of 54,390 births to 48,197 women recruited from 1959 to 1965 (last delivery in 1966). Women were enrolled at 12 university clinical centers across the United States, and detailed information on demographics, medical history, socioeconomic status, and behavior were collected via interviews. A physical examination and blood sample were also performed at the first visit. The women were followed up throughout pregnancy, and repeat interviews and physical examinations were performed. Upon admission to labor and delivery, a trained observer recorded the cervical examinations as well as all labor and delivery, postpartum, and neonatal events. A senior obstetrician also completed a summary of the pregnancy and labor and delivery. Children were followed up to age 7 years.

The CSL was a retrospective cohort study conducted by the Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, and included women giving birth between 2002 and 2008, with the majority (87%) between 2005 and 2007. There were 228,668 deliveries from 12 clinical centers and 19 hospitals representing 9 American College of Obstetrics and Gynecology districts. Institutional review board approval was obtained by all participating institutions.

Data were extracted from the patient electronic medical records including demographic data, medical history, and labor and delivery information as well as obstetrical, postpartum, and neonatal outcomes. Data from the neonatal intensive care unit were collected and linked to the newborn record. Maternal and newborn discharge International Classification of Diseases , ninth revision, codes were also collected for each delivery. Data were transferred in electronic format from each site and were mapped to common categories for each predefined variable at the data coordinating center. Investigators at each site completed surveys on hospital and physician characteristics. Data inquiries, cleaning, and logic checking were performed. Validation studies were performed for 4 key variables and the electronic medical records were found to be an accurate representation of the medical charts.

There were 54,304 observations in the CPP obstetrical data file, and we limited our analysis to each individual woman’s first enrolled pregnancy (n = 44,669) to avoid an intraperson correlation. Only women who presented in spontaneous labor were included, leaving 39,491 for analysis. In the CSL database, there were 228,668 deliveries, and we included 208,695 observations from each individual woman’s first enrolled pregnancy. Only women who presented in spontaneous labor and had a singleton gestation were included, for a total of 98,359 women. There were a total of 137,850 women in the combined CPP plus CSL dataset.

Statistical Methods

The maternal, labor, and neonatal characteristics of women by study were compared using χ ² and the Wilcoxon rank-sum test where applicable ( Table 1 ). To compare continuous demographic or neonatal characteristics and calculate the P values ( Tables 2-4 ), we used a linear regression with a term for study (CPP or CSL). For binary variables, we used linear probability models (PROC GENMOD with binomial distribution and identity link in SAS; SAS Institute, Cary, NC) to obtain the adjusted mean differences and associated P values. When linear probability models failed to converge, we used instead a logistic regression to produce the corresponding P values. Labor progression transition times from 1 cm to another were estimated by fitting an interval censored regression (using PROC LIFEREG of SAS), as previously described by Zhang et al.

TABLE 4

Comparison between CPP and CSL in multiparous women

Parameter	CPP (n = 16,277)	CSL (n = 27,312)	Adjusted median difference a	Adjusted P value
Maternal age, y (mean ± SD)	28.0 ± 5.7	30.1 ± 5.3		< .001
Delivery BMI, kg/m 2	27.1 ± 4.5	30.4 ± 5.1		< .001
Admission characteristics
Gestational age at delivery, wks	39.1 ± 3.5	38.4 ± 2.2		< .001
Dilation on admission
Median (10th, 90th percentiles)	4 (1.5, 8)	4 (2, 8)
Mean ± SD	4.2 ± 2.4	4.7 ± 2.4	0.8 b	< .001
Effacement on admission
Median (10th, 90th percentiles)	75 (25, 100)	80 (50, 100)
Mean ± SD	68 ± 28	80 ± 20	12 b	< .001
Spontaneous ROM prior to admission, %	13	38	19	< .001
Labor characteristics
Epidural, %	4	48	30	< .001
Oxytocin augmentation, %	9	27	9	< .001
First-stage durations, h (median, 95th)
From 5 cm to C/C	1.0 (8.6)	2.8 (15.5)	2.0	< .001
From 6 cm to C/C	0.5 (5.1)	1.3 (8.9)	0.9	< .001
Second-stage durations, h
With spontaneous delivery (median, 95th percentiles)	0.15 (0.60)	0.20 (1.0)	0.05	< .001
With assisted delivery (median, 95th percentiles)	0.35 (1.18)	0.42 (2.11)	0.12	< .001
Forceps, %	18	0.5	–15	< .001
Vacuum, %	0.1	1.7	2	< .001
Forceps or vacuum, %	18	2.5	–14	< .001
Episiotomy, %	36	7	–25	< .001
Third- or fourth-degree laceration, %	1	0.3	–1	.1
Intrapartum cesarean delivery, %	2	7	3	< .001
Neonatal characteristics
Birthweight, g (mean ± SD)	3176 ± 625	3269 ± 577
Apgar score at 1 min (mean)	8.0 ± 1.9	8.2 ± 1.3
<4, %	5	2
4-6	12	4
≥7	83	94
Apgar score at 1 min <7, %	17	6	–11	< .001
Apgar score at 5 min (mean)	9.0 ± 1.3	8.9 ± 0.8
<4, %	1.5	0.6
4-6	2.8	0.8
≥7	95.7	98.6
Apgar score at 5 min <7, %	4	1	–3	< .001

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