Materials and Methods

We identified 2 composite forms of maternal morbidity on opposite ends of the spectrum of morbidity that can occur during childbirth hospitalization, using methods previously described. The 2 composite morbidities–pelvic and severe–were identified using International Classification of Diseases, Ninth Revision, Clinical Modification diagnosis and procedure codes, supplemented with birth certificate data. Pelvic morbidity comprised episiotomy, third- or fourth-degree laceration, and vulvar or perineal hematoma or other trauma; severe morbidity comprised hysterectomy, unplanned return to operating room, transfer to intensive care unit, maternal death, or length of stay ≥90th percentile for mode of birth with a diagnosis of severe postpartum hemorrhage, maternal sepsis, deep vein thrombosis, pulmonary embolism, uterine rupture, respiratory failure, heart failure, puerperal cerebrovascular accident, severe anesthetic complication, maternal shock, disseminated intravascular coagulation, or renal failure. A complete list of the International Classification of Diseases, Ninth Revision, Clinical Modification and Vital Statistics codes used to determine these composite morbidities is available elsewhere.

The dataset linked California Vital Statistics Records with statewide hospital discharge data for 2005, 2006, and 2007 from the Office of Statewide Health Planning and Development using a probabilistic matching algorithm that produced valid linkage for >96% of records. These were the most recently available linked data at the time of the data request. This linked data set allows for evaluation of demographic characteristics such as race and ethnicity that are not available in hospital discharge data. Births that occurred in military hospitals, birth centers, or at home were excluded because they do not report hospital discharge data. Because multiple births and vaginal births after cesarean represent a small number of births with very different morbidity profiles, we limited our analysis to singleton births and excluded cases of trial of labor after cesarean. Trial of labor after cesarean (resulting in either vaginal or cesarean birth) accounted for <1% of births and is not offered in all facilities in California. The California Perinatal Quality Care Collaborative developed and executed the linkage strategy under a grant from the March of Dimes. Stanford University and University of California, San Francisco Institutional Review Boards approved the study.

We conducted unadjusted and adjusted analyses to evaluate the relationship between time of day and day of the week that birth occurred, maternal demographic and obstetric characteristics, hospital volume, and maternal morbidity during childbirth hospitalization. Maternal age, race, ethnicity, level of education, insurance status, and adequacy of prenatal care were determined from birth certificate data. Adequacy of prenatal care was calculated using the Kotelchuck Index. Maternal comorbidities such as preeclampsia and chronic hypertension could be classified as either risk factors or morbidities. We chose to evaluate preeclampsia and chronic hypertension as risk factors for morbidity as in our previous work. We did not evaluate diabetes as a risk factor in this study because it had minimal effect in our previous morbidity study findings.

Time of birth was identified from the birth certificate. We analyzed time of birth in 1-hour and 4-hour blocks. Four-hour blocks were defined as 0701-1100, 1101-1500, 1501-1900, 1901-2300, 2301-0300, and 0301-0700 to allow analysis of early and late day and night shift, as well as accommodate interpretation of 8- and 12-hour nursing shifts. We used 0701-1100 as the reference value for these analyses. Similarly, day of the week was adjusted to a 24-hour day from 2300-2259 to be inclusive of 8-hour night shift (11 PM-7 AM). We accounted for clustering at the hospital level for all analyses. For unadjusted analyses we used bivariate logistic regression with robust variance estimation. Adjusted analyses were done with mixed effects logistic regression, where individual hospital was a random effect and predictor variables were fixed effects. Adjusted odds ratios (AOR) with 95% confidence intervals (CIs) were estimated using xtmelogit in Stata SE 13 (StataCorp, College Station, TX). For our 2 key outcomes and extremely large sample size, no adjustments were made for multiple comparisons.

Results

The demographic characteristics of the 1,475,593 women in the final cohort are presented in Table 1 .

Pelvic floor morbidity

In unadjusted analyses, the risk for pelvic morbidity by hour was lowest at 8-9 AM, rising in the early afternoon to a relative plateau of 40-57% increased risk from 2 PM-6 AM ( Figure 1 ). The time interval 2301-0300 had the highest odds of pelvic morbidity (odds ratio [OR], 1.53; 95% CI, 1.50–1.56) ( Table 2 ) despite fewer births during this time period ( Figure 1 ). Weekend birth, young maternal age, higher education, Asian race, lack of insurance, and birth in hospitals with <3600 annual births were also associated with increased odds of pelvic floor morbidity. Non-white, non-Asian race, Hispanic ethnicity, multiparity, cesarean birth, hypertensive disorders, and low birthweight were associated with decreased odds of observed pelvic morbidity ( Table 2 ).

Odds ratios for pelvic morbidity by hour and day of birth

Lyndon. Maternal morbidity and time of birth. Am J Obstet Gynecol 2015 .

Table 2

Pelvic morbidity, by risk factors (1,475,593)

Predictor variable	Unadjusted odds ratio a (95% confidence interval)	Adjusted odds ratio b (95% confidence interval)
Maternal age, y
<18	1.85 (1.81–1.90)	1.12 (1.09–1.15)
18–25	1	1
26–35	0.70 (0.69–0.70)	1.05 (1.03–1.06)
>35	0.52 (0.51–0.52)	1.17 (1.14–1.19)
Race
White	1	1
Black	0.66 (0.64–0.67)	0.61 (0.59–0.63)
American Indian/Alaskan Native	0.79 (0.73–0.85)	0.76 (0.69–0.83)
Asian	1.76 (1.74–1.79)	1.68 (1.65–1.71)
Hawaiian/Pacific Islander	0.81 (0.75–0.87)	0.80 (0.74–0.88)
Other/multirace	0.93 (0.90–0.96)	0.83 (0.80–0.86)
Ethnicity
Hispanic	0.73 (0.72–0.74)	0.83 (0.82–0.85)
Non-Hispanic	1	1
Education
Less than high school	1	1
High school	1.09 (1.08–1.11)	1.03 (1.01–1.04)
Some college	1.29 (1.28–1.31)	1.12 (1.10–1.14)
Some postgraduate	1.58 (1.55–1.61)	1.26 (1.23–1.29)
Payor
None/self	1.13 (1.10–1.16)	1.08 (1.05–1.12)
Medi-Cal/government	0.89 (0.88–0.90)	0.98 (0.97–1.00)
Private	1	1
Parity–mode of birth
Primiparous vaginal	1	1
Multiparous vaginal	0.21 (0.21–0.21)	0.202 (0.200–0.204)
Primiparous primary cesarean	0.003 (0.003–0.003)	0.003 (0.003–0.003)
Multiparous primary cesarean	0.002 (0.002–0.003)	0.002 (0.002–0.002)
Repeat cesarean	0.001 (0.001–0.001)	0.001 (0.001–0.001)
Prenatal care c
Inadequate	0.94 (0.93–0.95)	0.98 (0.97–1.00)
Adequate	1	1
Comorbidities
Preeclampsia
Yes	0.75 (0.73–0.76)	0.97 (0.95–1.00)
No	1	1
Chronic hypertension
Yes	0.41 (0.38–0.44)	0.80 (0.74–0.86)
No	1	1
Infant birthweight, g
<2500	0.54 (0.53–0.55)	0.55 (0.53–0.56)
2500–3999	1	1
≥4000	0.93 (0.92–0.95)	1.61 (1.58–1.64)
Hour of birth
0701–1100	1	1
1101–1500	1.25 (1.23–1.27)	1.07 (1.06–1.09)
1501–1900	1.50 (1.48–1.52)	1.08 (1.06–1.10)
1901–2300	1.47 (1.45–1.50)	1.03 (1.01–1.05)
2301–0300	1.53 (1.50–1.56)	0.97 (0.95–0.98)
0301–0700	1.50 (1.48–1.53)	0.95 (0.94–0.97)
Day of birth d
Monday	1	1
Tuesday	1.03 (1.01–1.04)	1.02 (1.00–1.04)
Wednesday	1.01 (0.99–1.03)	1.00 (0.98–1.02)
Thursday	1.02 (1.00–1.03)	1.01 (0.99–1.02)
Friday	1.00 (0.98–1.02)	1.01 (0.99–1.03)
Saturday	1.20 (1.18–1.22)	1.00 (0.98–1.02)
Sunday	1.25 (1.23–1.27)	1.01 (0.99–1.03)
Hospital volume, births/y
<1200	1.11 (1.03–1.19)	1.17 (1.07–1.27)
1200–2399	1.18 (1.13–1.23)	1.23 (1.17–1.29)
2400–3599	1.13 (1.10–1.16)	1.15 (1.11–1.19)
≥3600	1	1

Lyndon. Maternal morbidity and time of birth. Am J Obstet Gynecol 2015 .

a Bivariate logistic regression with robust variance estimation, clustered by hospital

b Mixed effects logistic regression model with individual hospital as random effect and predictor variables as fixed effects

c Kotelchuck Adequacy of Prenatal Care Utilization Index

d Day of week adjusted to 24-h day from 2300-2259 to be inclusive of 8-hour night shift from 11PM-7AM.

In the adjusted analysis, the effect of time of birth was substantially reduced by controlling for other variables and more closely followed the pattern of births per hour ( Figure 1 ). The risk of pelvic morbidity was increased 3-8% from 1101-2300, while time of birth between 2301-0700 was associated with a 3-5% decreased risk of pelvic floor morbidity. Weekday was not significant in the adjusted model. Maternal age <18 and >35 years, higher education, Asian race, lack of insurance, birthweight >4000 g, and birth at lower-volume hospitals were associated with increased odds of pelvic morbidity. The strongest independent risk factor was Asian race (AOR, 1.68; 95% CI, 1.65–1.71).

Severe morbidity

In unadjusted analyses, there were fluctuations in risk for severe morbidity of approximately ±10% throughout the hours of 7 AM-7 PM ( Figure 2 ). These fluctuations were not statistically significant. Unadjusted risk for severe morbidity spiked at about 12 AM and remained elevated by approximately 20% until about 4 AM when the risk began to decline toward baseline ( Figure 2 ). Overall, the odds for severe morbidity were elevated for births between 2301-0700 (AOR, 2301–0300 = 1.2; 95% CI, 1.12–1.30; AOR<0301–0700 = 1.15; 95% CI, 1.06–1.24) ( Table 3 ). Friday or Saturday birth was also associated with severe morbidity, as was maternal age (<18 and >35 years), and black and American Indian/Alaskan Native race ( Table 3 ). Birthweight, cesarean birth, hypertensive disorders, and lack of private insurance also conferred increased odds of severe morbidity. Hospital volume, Hispanic ethnicity, education level, and adequacy of prenatal care were not associated with severe morbidity.

Odds ratios for severe morbidity by hour and day of birth

Lyndon. Maternal morbidity and time of birth. Am J Obstet Gynecol 2015 .

Table 3

Severe morbidity, by risk factors (n = 1,475,593)

Predictor variable	Unadjusted odds ratio a (95% confidence interval)	Adjusted odds ratio b (95% confidence interval)
Maternal age, y
<18	1.45 (1.29–1.62)	1.15 (1.02–1.29)
18–25	1	1
26–35	1.06 (1.01–1.12)	1.22 (1.15–1.29)
>35	1.76 (1.65–1.87)	1.88 (1.76–2.02)
Race
White	1	1
Black	1.53 (1.41–1.66)	1.39 (1.27–1.53)
American Indian/Alaskan Native	1.34 (1.02–1.76)	1.32 (1.00–1.74)
Asian	1.08 (1.00–1.16)	1.13 (1.04–1.22)
Hawaiian/Pacific Islander	1.22 (0.92–1.62)	1.22 (0.91–1.62)
Other/multirace	1.07 (0.94–1.22)	1.05 (0.92–1.20)
Ethnicity
Hispanic	0.97 (0.92–1.02)	1.10 (1.03–1.17)
Non-Hispanic	1	1
Education
Less than high school	1	1
High school	0.94 (0.89–1.00)	0.93 (0.87–0.99)
Some college	0.94 (0.89–1.00)	0.90 (0.84–0.96)
Some postgraduate	0.84 (0.77–0.92)	0.80 (0.72–0.88)
Payor
None/self	1.19 (1.04–1.38)	1.14 (0.99–1.32)
Medi-Cal/government/other	1.16 (1.10–1.23)	1.20 (1.11–1.28)
Private	1	1
Parity–mode of birth
Primiparous vaginal	1	1
Multiparous vaginal	0.56 (0.53–0.59)	0.53 (0.50–0.56)
Primiparous primary cesarean	1.46 (1.36–1.55)	1.16 (1.09–1.24)
Multiparous primary cesarean	1.73 (1.59–1.88)	1.20 (1.10–1.31)
Repeat cesarean	1.35 (1.26–1.44)	1.18 (1.10–1.27)
Prenatal care c
Inadequate	1.00 (0.95–1.06)	1.08 (1.02–1.14)
Adequate	1	1
Comorbidities
Preeclampsia
Yes	3.71 (3.52–3.92)	2.55 (2.41–2.71)
No	1	1
Chronic hypertension
Yes	3.50 (3.12–3.93)	1.33 (1.18–1.50)
No	1	1
Infant birthweight, g
<2500	4.15 (3.92–4.39)	2.91 (2.75–3.09)
2500–3999	1	1
≥4000	1.24 (1.15–1.33)	1.21 (1.12–1.31)
Hour of birth
0701–1100	1	1
1101–1500	1.02 (0.95–1.09)	1.03 (0.97–1.11)
1501–1900	0.97 (0.90–1.04)	0.99 (0.92–1.06)
1901–2300	0.97 (0.90–1.05)	0.98 (0.91–1.06)
2301–0300	1.20 (1.12–1.30)	1.31 (1.21–1.41)
0301–0700	1.15 (1.06–1.24)	1.30 (1.20–1.41)
Day of birth d
Monday	1	1
Tuesday	1.03 (0.95–1.11)	1.01 (0.93–1.09)
Wednesday	1.08 (0.99–1.17)	1.04 (0.96–1.13)
Thursday	1.05 (0.97–1.13)	1.02 (0.94–1.10)
Friday	1.09 (1.01–1.18)	1.06 (0.98–1.15)
Saturday	1.10 (1.01–1.20)	1.09 (1.00–1.18)
Sunday	1.01 (0.92–1.11)	1.03 (0.94–1.13)
Hospital volume, births/y
<1200	1.05 (0.86–1.29)	1.12 (0.92–1.37)
1200–2399	1.08 (0.90–1.28)	1.10 (0.93–1.30)
2400–3599	1.01 (0.87–1.17)	1.02 (0.88–1.18)
≥3600	1	1

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