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ORIGINAL RESEARCH |
From the College of Nursing, Texas Woman's University and the Department of Obstetrics and Gynecology, UT Southwestern Medical Center, Dallas, Texas.
Address reprint requests to: James M. Alexander, MD, Department of Obstetrics and Gynecology, University of Texas Southwestern Medical Center at Dallas, 5323 Harry Hines Boulevard, Dallas, TX 75235-9032; e-mail: james.alexander{at}utsouthwestern.edu.
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ABSTRACT |
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MATERIALS AND METHODS: The timing of birth of women who delivered in the low-risk labor unit at Parkland Hospital, Dallas, Texas, between January 1, 2000, to December 31, 2000, was analyzed. Women admitted to this unit were between 360/7 and 416/7 weeks of gestation, were in spontaneous labor, and had a singleton gestation. Women with contraindications to labor, significant medical problems, a known fetal anomaly, and stillbirths were excluded from analysis. The frequency of birth was analyzed in relation to the time of day, day of week, and month of the year.
RESULTS: Low-risk women (n = 6,608) met the study criteria and were included in the analysis. No association was found between the day of the week and the frequency of births (P = .31). Births were most common between the hours of 1 to 2 pm and least common between the hours of 10:00 to 12:00 hours (Central Standard Time, P = .04). Births were more common in the fall, September through November, and least common in the winter, December through February. Daylight Saving Time did not affect these results.
CONCLUSION: Birth after the spontaneous onset of labor is most common in the early afternoon, and most births occur in the fall. There is no natural association between spontaneous birth in low-risk women and the day of the week.
LEVEL OF EVIDENCE: II-2
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MATERIALS AND METHODS |
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Labor in our low-risk pregnancy unit is managed with a standardized management protocol. Briefly, active labor is diagnosed in the presence of at least 1 of the following: 1) palpable uterine contractions every 3 minutes and cervical dilation of 3 or more centimeters, 2) uterine contractions that result in cervical dilation in 2 hours, or 3) rupture of membranes and palpable contractions every 3 minutes. Pelvic exams are performed approximately every 2 hours. If the cervix does not dilate within the first 2 hours of admission, an amniotomy is performed and labor progress is evaluated at the next 2-hour evaluation. If labor does not progress, an intrauterine pressure catheter is placed to assess uterine activity. If hypotonic contractions are present in the setting of no further cervical progress, oxytocin is administered. Uterine activity of 200 to 250 Montevideo units is expected for 2 to 4 hours before the diagnosis of dystocia. Therefore, it would not be expected for a cesarean for dystocia to be performed for at least 6 to 8 hours from admission. Staffing of the labor unit included 24-hour supervision by obstetrical faculty members from the Department of Obstetrics and Gynecology at the University of Texas Southwestern Medical School. Other staff members included 3 or 4 certified nurse-midwives per shift, directly supervised by 1 on-site second-year house officer in obstetrics and gynecology who reported to 1 on-site fourth-year house officer.
The number of births per day of the week and the number of births per month of the year were analyzed. The time of day was analyzed in 1- and 3-hour intervals with analysis performed in Central Standard Time. The effect of daylight saving on the time of day was analyzed separately. The months were grouped into 4 seasons. In Dallas, Texas, the coldest 3 months of the year are December, January, and February,11 with the warmest 3 months being June, July, and August; thus, our analysis of seasonal variation was December through February, March through May, June through August, and September through November.
The goodness of fit 
2 test was used to examine whether hours of the day, days of the week, or seasons of the year had a uniform distribution pattern. An analysis of variance was used to examine the equality of the frequency of events per time period. In the event of statistical significance, the Student–Newman–Keuls approach was used to examine the pairwise comparisons. Pearson 2 tests were used for comparisons of categorical data. A Newman–Keuls approach was used to evaluate multiple comparisons for 2 as well.
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RESULTS |
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The mean number of births per hour was 1.42, with the most births occurring between 13:00 and 13:59 hours and the least number of births occurring between 22:00 and 22:59 hours, as shown in Figure 1. Table 2 shows the distribution of births in relation to the time of day broken into 3-hour intervals. Sixteen percent of women required oxytocin augmentation, and 3.8% underwent cesarean delivery. Women whose labor required these interventions were more likely to deliver between the hours of 11:00 and 13:59, whereas women who required no intervention in labor and had a vaginal delivery were more likely to deliver between the hours of 08:00 and 10:59 (P < .001). Parity also had an effect on the timing of spontaneous birth. Multiparous women were more likely to deliver between the hours of 08:00 and 10:59 (P = .02) when compared with nulliparous women. Parity did not affect the time period during which the least number of births occurred (22:00 to 22:59, both groups). There was no effect of maternal age or race on the timing of spontaneous birth (P = .93, .95, respectively). Table 3 shows the number of births in relation to the season of the year. The greatest number of deliveries occurred in the fall, September through November, and the fewest deliveries occurred in the spring, March through May.
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Analysis of variance was used to examine the effect of the time of year (season) on the number of births and the day of week. Although the actual numbers of births per day of the week changes as the seasons change, these changes are randomly distributed with no trend toward any particular day (P = .34). Similarly, the day of the week had no effect on the time of birth (P = .34). Finally, daylight saving time had no effect on the time of day of birth (P = .32).
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DISCUSSION |
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Our finding that term births after the spontaneous onset of labor peak in the early afternoon supports previous reports of a circadian pattern of birth.1–5,8–10,12 Anderka et al,1 in a study of 242,276 low-risk term singleton births in Massachusetts, reported a 24-hour cycle in spontaneous birth, with a peak incidence between 11:00 and 13:00. Similar to our study, they reported an effect of parity with nulliparous women being more likely to deliver midday, as opposed to multiparous women who delivered in the early morning hours. Consistent with this is the observation by Cooperstock et al12 that the onset of labor is most typically at night. In their study of more than 18,000 singleton pregnancies, the onset of labor peaked at 01:45 in both term and preterm births. The mechanisms underlying the variations in the timing of birth are not well understood but may be influenced by the maternal and fetal endocrine system. Walsh et al,13 in a study of primates, showed a correlation between uterine activity and maternal and fetal steroid levels with fetal dehydroepiandrosterone sulfate and progesterone levels paralleling uterine activity in late gestation. These fetal steroid levels followed a circadian rhythm. The authors hypothesized that interaction between the maternal and fetal endocrine system may limit parturition, or at least its initiation to certain times of the day.
Seasonal patterns to human birth have been observed in virtually all populations studied, with more births occurring during certain times of the year. The peak birth rate appears to be determined primarily by location and the climatic conditions of the population studied. In northern Europe, for example, births peak in the spring with a nadir in the fall and winter months.14 In contrast, we found the fewest number of births during the spring, suggesting that women were least likely to conceive during the hottest months. This is consistent with other studies of birthing patterns in the southern United States. One explanation for this pattern of births in warmer climates may be related to male factors. Levine et al6 studied the effect of warm weather on semen quality of outdoor workers in San Antonio, Texas. They found that semen quality decreases in the summer and concluded that this correlated with decreased fertility in the hottest months of the year and a corresponding decreased number of births during the spring.
Our findings are compared with recently reported National Vital Statistics in Table 4. Nationally, there is a significant variation in the weekday and weekend birth rates not seen in our study population.10 In contrast to our report, the National Vital Statistics data are a summary of all pregnancies including those women with significant medical complications and prior cesareans. Consequently, inductions of labor and scheduled repeat cesareans would be expected to be more prevalent and likely explains the difference between our results and the national data. Our inability to find an association between the day of the week and the number of births in spontaneously laboring, uncomplicated pregnancies is not be unexpected. A given day does not in and of itself reflect a naturally occurring event. The time of day, however, as well as the season of the year is determined by naturally occurring phenomenon that appear to have effects on both conception rates and the onset of labor through circadian rhythms and biological clocks.
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Footnotes |
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10.1097/01.AOG.0000118309.70035.63
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REFERENCES |
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2. James WH. Seasonal variation in human births. J Biosoc Sci 1990;22:113–9.[Medline]
3. Glattre E, Bjerkedal T. The 24-hour rhythmicity of birth: a populational study. Acta Obstet Gynecol Scand 1983;62:31–6.[Medline]
4. Cunningham FG, Gant NF, Leveno KJ, Gilstrap LC III, Hauth JC, Wenstrom KD. Williams obstetrics. 21st ed. New York (NY): McGraw-Hill; 2001.
5. Bronson FH. Seasonal variation in human reproduction: environmental factors [review]. Q Rev Biol 1995;70:141–64.[Medline]
6. Levine RJ, Mathew RM, Chenault CB, Brown MH, Hurtt ME, Bentley KS, et al. Differences in the quality of semen in outdoor workers during summer and winter. N Engl J Med 1990;323:12–6.[Abstract]
7. Wellings K, MacDowall W, Catchpole M, Goodrich J. Seasonal variations in sexual activity and their implications for sexual health promotion. J R Soc Med 1999;92:60–4.[Abstract]
8. Warren CW, Gwinn ML, Rubin GL. Seasonal variation in conception and various pregnancy outcomes. Soc Biol 1986;33:116–26.[Medline]
9. Martin JA, Hamilton BE, Ventura SJ, Menacker F, Park MM. Births: final data for 2000. Natl Vital Stat Rep 2002;50:1–101.[Medline]
10. Curtin SC, Park MM. Trends in the attendant, place, and timing of births, and in the use of obstetric interventions: United States, 1989–97. Natl Vital Stat Rep 1999;47:1–12.[Medline]
11. National Weather Service Website, Dallas/Fort Worth Climatology. Available at http://www.srh.noaa.gov/fwd/ntexclima.html. Retrieved January 19, 2004
12. Cooperstock M, England JE, Wolfe RA. Circadian incidence of labor onset hour in preterm birth and chorioamnionitis. Obstet Gynecol 1987;70:852–5.[Medline]
13. Walsh SW, Ducsay CA, Novy MJ. Circadian hormonal interactions among the mother, fetus, and amniotic fluid. Am J Obstet Gynecol 1984;150:745–53.[Medline]
14. Lam DA, Miron JA. Global patterns of seasonal variation in human fertility. Ann NY Acad Sci 1994;709:9–28.[Abstract]
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