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Obstetrics & Gynecology 2003;102:1262-1268
© 2003 by The American College of Obstetricians and Gynecologists
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ORIGINAL RESEARCH

Racial Disparity in Meconium-Stained Amniotic Fluid and Meconium Aspiration Syndrome in the United States, 1989–2000

Sudhir Sriram, MBBS, MRCP, Stephen N. Wall, MD, MPH, Babak Khoshnood, MD, MPH, Jaideep K. Singh, MD, Hui-Lung Hsieh, MD and Kwang-Sun Lee, MD

From the Neonatology Section and Center for Perinatal Epidemiology, University of Chicago Children’s Hospital, Pritzker School of Medicine, University of Chicago, Chicago, Illinois.

Address reprint requests to: Sudhir Sriram, MBBS, MRCP, University of Chicago Children’s Hospital, Department of Pediatrics, MC 6060, 5841 S. Maryland Avenue, Chicago, IL 60637; E-mail: ssriram{at}peds.bsd.uchicago.edu.


   ABSTRACT
TOP
ABSTRACT
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
OBJECTIVE: To estimate the prevalence of meconium-stained amniotic fluid and meconium aspiration syndrome, as well as the differences in case fatality from meconium aspiration syndrome, between non-Hispanic black and non-Hispanic white infants.

METHODS: We studied non-Hispanic black and non-Hispanic white live births with weights greater than 2.5 kg and gestational ages greater than 35 weeks, using the linked US birth and infant death cohorts for three periods: 1989–1991, 1995–1997, and 1998–2000. We used logistic regression to estimate the risks of meconium-stained amniotic fluid and meconium aspiration syndrome and to estimate the case fatality of meconium aspiration syndrome by maternal race, birth weight, period, and pregnancy complications.

RESULTS: Risk of meconium-stained amniotic fluid was 80% higher in non-Hispanic blacks when compared with non-Hispanic whites (birth weight–adjusted odds ratio [OR], 1.81, 95% confidence interval [CI] 1.80, 1.82). The prevalence of pregnancy complications did not explain this racial disparity. Risk of meconium aspiration syndrome in non-Hispanic blacks was 67% higher when compared with non-Hispanic whites (birth weight–adjusted OR 1.67, 95% CI 1.64, 1.70). The case fatality rate of meconium aspiration syndrome was similar between non-Hispanic blacks and non-Hispanic whites in the three periods, with rates of 15.5, 15.2, and 11.2 per 1000 in non-Hispanic blacks and 13.5, 11.2, and 10.1 per 1000 in non-Hispanic whites in 1989–1991, 1995–1997, and 1998–2000, respectively.

CONCLUSION: Our results suggest that when compared with non-Hispanic whites, non-Hispanic blacks are at significantly greater risk for meconium-stained amniotic fluid and meconium aspiration syndrome but not for meconium aspiration syndrome case fatality.

Passage of meconium in utero in most cases is the result of a physiologic event. Infrequently, it can occur as a result of fetal hypoxia or acidosis.1–3 Regardless of its cause, once passed in utero, it might result in meconium-stained amniotic fluid and sometimes lead to meconium aspiration syndrome. Meconium aspiration syndrome represents a wide spectrum of clinical disease, ranging from mild transient respiratory distress to severe parenchymal lung disease and persistent pulmonary hypertension with high mortality.4–8

To date, only a few epidemiologic studies9,10 have been carried out on meconium-stained amniotic fluid and meconium aspiration syndrome. These studies have been limited to a regional or selected population and varied in their estimates of the impact of racial differences on the prevalence of meconium-stained amniotic fluid and meconium aspiration syndrome and its resultant mortality. With advances in obstetric and neonatal care, prevalence and case fatality of meconium aspiration syndrome would have improved over time.

The objectives of this study were to estimate the prevalence of meconium-stained amniotic fluid and meconium aspiration syndrome, as well as the differences in case fatality from meconium aspiration syndrome, between non-Hispanic black and non-Hispanic white infants with birth weights greater than 2.5 kg and gestational ages greater than 35 weeks in the entire US birth cohort for three periods: 1989–1991, 1995–1997, and 1998–2000.


   MATERIALS AND METHODS
TOP
ABSTRACT
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
We used the linked US birth and infant death cohorts from 1989–1991, 1995–1997, and 1998–2000. Annual data sets consist of live birth files linked to infant death files. The percentage of linked birth and death records is approximately 98% in all data sets. Natality and mortality files produced annually by the National Center for Health Statistics include statistical data from birth and death certificates provided to the National Center for Health Statistics by the states. Based on the manual of the International Classification of Diseases and Causes of Death (ICD-9), both meconium-stained amniotic fluid and meconium aspiration syndrome were classified on birth and death records. The data were published in CD-ROM format by the National Center for Health Statistics.11 For our study, we used the most recent available data sets for linked infant birth and death files of the US birth cohorts for the years 1989–1991, 1995–1997, and 1998–2000. Meconium-stained amniotic fluid and meconium aspiration syndrome commonly occur in term infants (greater than 37 weeks’ gestation).12 Birth weights greater than 2.5 kg approximately correspond to the 50th percentile of 35 weeks’ gestation, so we selected infants with birth weights greater than 2.5 kg. There were 25,228,271 infants with birth weights greater than 2.5 kg. Of these, we excluded 171,210 (3.5%) non-Hispanic black infants and 949,241(4.6%) non-Hispanic white infants with unknown status of meconium-stained amniotic fluid and meconium aspiration syndrome. Our study population was then 4,564,976 non-Hispanic black infants and 19,542,744 non-Hispanic white infants with birth weights greater than 2.5 kg. We also grouped our study population by gestational age: preterm (35–37 weeks), term (38–40 weeks), and postterm (greater than 42 weeks).

Prevalence of meconium-stained amniotic fluid and meconium aspiration syndrome, as well as the case fatality of meconium aspiration syndrome, were compared between non-Hispanic blacks and non-Hispanic whites during the three periods. Within these groups, prevalence and case fatality were also compared between birth-weight groups (from 2.5 to 4.9 kg in 500-g increments) and gestational age groups (from 35 to greater than 42 weeks in 2-week increments). Prevalence of meconium-stained amniotic fluid was also examined in births with and without maternal complications, such as fetal distress, prolonged labor, chronic or pregnancy-induced hypertension, placenta previa, abruptio placenta, and unclassified third-trimester vaginal bleeding, all of which were defined according to ICD-9 classification. Logistic regression was used to estimate the risk of meconium-stained amniotic fluid, meconium aspiration syndrome, and neonatal death due to meconium aspiration syndrome by maternal race, birth weight, period, and maternal complications, with Stata statistical software (Stata Corp., College Station, TX).


   RESULTS
TOP
ABSTRACT
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
For both non-Hispanic blacks and non-Hispanic whites, the prevalence of meconium-stained amniotic fluid showed a small but significant decline across the periods 1989–1991, 1995–1997, and 1998–2000. The prevalence was 89, 86, and 81 per 1000 live births in non-Hispanic blacks and 55, 52, and 49 per 1000 live births in non-Hispanic whites, respectively (P < .001) (Table 1Go). However, the prevalence of meconium-stained amniotic fluid in non-Hispanic blacks was higher than in non-Hispanic whites. Within each racial group, the prevalence of meconium-stained amniotic fluid was progressively higher with an increase in birth weight. The prevalence of meconium-stained amniotic fluid was significantly higher in non-Hispanic blacks in each birth weight group when compared with non-Hispanic whites. When adjusted for birth weight, the prevalence of meconium-stained amniotic fluid was found to be about 80% higher in non-Hispanic blacks when compared with non-Hispanic whites in each period (adjusted odds ratio [OR] 1.80, 95% confidence interval [CI] 1.79, 1.81 in 1989–1991; OR 1.83, 95% CI 1.81, 1.84 in 1995–1997; and OR 1.82, 95% CI 1.81, 1.83 in 1998–2000).


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  		Table 1. Prevalence of Meconium-Stained Amniotic Fluid in US Birth Cohorts, 1989–91, 1995–1997, and 1995–2000
 
The prevalence of meconium-stained amniotic fluid was higher with maternal complications such as fetal distress, prolonged labor, chronic or pregnancy-induced hypertension, placenta previa, abruptio placenta, and other third-trimester vaginal bleeding. The prevalence of meconium-stained amniotic fluid was 99.5 per 1000 in the group with one or more of these complications and 59.4 per 1000 in those without any of these complications. Over the three periods, approximately 9.0% of non-Hispanic black births had at least one maternal complication, compared with 8.4% in non-Hispanic white births (Table 2Go). However, this difference in the prevalence of maternal complications between non-Hispanic blacks and non-Hispanic whites did not explain their disparity in prevalence of meconium-stained amniotic fluid. Controlling for the presence of maternal complications, the risk of meconium-stained amniotic fluid in non-Hispanic blacks remained higher than in non-Hispanic whites (unadjusted OR 1.79, 95% CI 1.78, 1.80; adjusted OR 1.79, 95% CI 1.78, 1.80).


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  		Table 2. Prevalence of Births With Maternal Complications* and Prevalence of Meconium-Stained Amniotic Fluid in Births With and Without Maternal Complications in US Birth Cohorts, 1989–1991, 1995–1997, and 1998–2000
 
As in meconium-stained amniotic fluid, there was a decline in the prevalence of meconium aspiration syndrome, from 3.8 to 3.0 per 1000 live births in non-Hispanic blacks (P < .001) and from 2.5 to 1.7 per 1000 live births in non-Hispanic whites (P < .001) (Table 3Go). As in meconium-stained amniotic fluid, the prevalence of meconium aspiration syndrome was progressively higher with an increase in birth weight. Similarly, for each birth weight group this prevalence was higher in non-Hispanic blacks than in non-Hispanic whites. The birth weight–adjusted risk of meconium aspiration syndrome in non-Hispanic blacks was 62% higher (adjusted OR 1.62, 95% CI 1.58, 1.67) during 1989–1991; 59% higher (adjusted OR 1.59, 95% CI 1.54, 1.65) during 1995–1997; and 85% higher (adjusted OR 1.85, 95% CI 1.78, 1.91) during 1998–2000 than in the respective non-Hispanic white groups.


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  		Table 3. Prevalence of Meconium Aspiration Syndrome in US Birth Cohorts, 1989–1991, 1995–1997, and 1998–2000
 
Although the prevalence of meconium-stained amniotic fluid and meconium aspiration syndrome was significantly higher in non-Hispanic blacks, the case fatality rate from meconium aspiration syndrome was not significantly different between these two racial groups throughout the three periods, with rates of 15.5, 15.2, and 11.2 in non-Hispanic blacks and 13.5, 11.2, and 10.1 in non-Hispanic whites per 1000 (Table 4Go). After adjustment for birth weight, the risk of neonatal death from meconium aspiration syndrome in non-Hispanic blacks was not different from non-Hispanic whites: adjusted OR 1.06, 95% CI 0.83, 1.35 during 1989–1991; adjusted OR 1.28, 95% CI 0.96, 1.69 during 1995–1997; and adjusted OR 1.12, 95% CI 0.80, 1.56 during 1998–2000.


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  		Table 4. Neonatal Mortality With and Without Meconium Aspiration Syndrome in US Birth Cohorts, 1989–1991, 1995–1997, and 1998–2000
 
For all three periods, overall neonatal mortality of infants with birth weights greater than 2.5 kg was higher in non-Hispanic black infants compared with non-Hispanic white infants. These rates were 1.5 and 1.1 per 1000 live births for non-Hispanic blacks and for non-Hispanic whites, respectively, during 1989–1991 (P < .001). During the periods 1995–1997 and 1998–2000, these rates remained the same. They were 1.1 and 0.9 in non-Hispanic blacks and in non-Hispanic whites, respectively, and thus the disparity persisted (P < .001) (Table 4Go).

Compared with non-Hispanic whites, the risk of neonatal death in non-Hispanic blacks from all causes was 39% higher in the early period (P < .001), 23% higher in the middle period, and 30% higher in the late period (P < .001) (Table 5Go). Because the prevalence of meconium aspiration syndrome was higher in non-Hispanic blacks, we attempted to quantify a contribution of meconium aspiration syndrome mortality to this disparity. Controlling for the differences in birth weight distribution, disparity in this risk was narrowed. However, a further adjustment for meconium aspiration syndrome hardly made any additional reduction in this risk, which indicates that the disparity in overall mortality between these two racial groups cannot be attributable to the difference in prevalence of meconium aspiration syndrome (Table 5Go).


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  		Table 5. Odds of Neonatal Death in Infants With Birth Weights Greater Than 2.5 kg in US Birth Cohorts, 1989–1991, 1995–1997, and 1998–2000
 
We also examined the prevalence of meconium-stained amniotic fluid, meconium aspiration syndrome, and meconium aspiration syndrome–specific case fatality by gestational age. The results were similar to those observed when analyzed by birth weight.


   DISCUSSION
TOP
ABSTRACT
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
In our study population, the overall prevalence of meconium-stained amniotic fluid was 60 per 1000 live births. In recent reports,13–18 the prevalence of meconium-stained amniotic fluid ranged from 5.6% to 24% (median 14%). In our study, there was a significant difference between non-Hispanic blacks and non-Hispanic whites in the prevalence of meconium-stained amniotic fluid. It was 80% higher in non-Hispanic blacks than in non-Hispanic whites during all three periods. This difference is similar to the observation made by Alexander et al,9 who examined term infants in a university hospital from 1982 to 1990 and found a 50% increased prevalence of meconium-stained amniotic fluid in non-Hispanic blacks when compared with non-Hispanic whites.

One possible explanation for this racial difference in the prevalence of meconium-stained amniotic fluid in our study might have been the racial disparity in the prevalence of maternal complications associated with meconium-stained amniotic fluid. We ascertained the commonest maternal complications: fetal distress, prolonged labor, pregnancy-induced hypertension, chronic hypertension, placenta previa, abruptio placenta, and unclassified vaginal bleeding in the last trimester. However, adjustment for the difference in prevalence of these maternal complications did not reduce the higher risk of meconium-stained amniotic fluid in non-Hispanic blacks. Other maternal complications, such as cardiopulmonary disease and hematologic conditions, would have not impacted the disparity in the prevalence of meconium-stained amniotic fluid between these two racial groups because they are infrequent causes of meconium-stained amniotic fluid.

In our study, the overall prevalence of meconium aspiration syndrome was 2.5 per 1000 live births. In both non-Hispanic blacks and non-Hispanic whites, this prevalence declined over the three periods. However, there was a significant difference in the prevalence of meconium aspiration syndrome between non-Hispanic blacks and non-Hispanic whites. Racial disparity in the prevalence of meconium aspiration syndrome followed the pattern of meconium-stained amniotic fluid. The higher prevalence of meconium aspiration syndrome in non-Hispanic blacks was entirely attributable to the prevalence of meconium-stained amniotic fluid. In fact, if the rate of meconium-stained amniotic fluid were the same between the two groups, the prevalence of meconium aspiration syndrome would be 5% lower in non-Hispanic backs than in non-Hispanic whites.

Wiswell et al10 found no significant difference in the prevalence of meconium aspiration syndrome between non-Hispanic blacks and non-Hispanic whites when they studied 175,000 infants born in seven US Army Medical Centers. Their study was limited to a military population. The risk of meconium aspiration syndrome in this population might be different from that of the US population as a whole. In our study, the prevalence of meconium aspiration syndrome gradually declined over time. Wiswell et al10 noted a similar decline in meconium aspiration syndrome in the earlier period, between 1973 and 1987. They attributed this decline to the continuous improvement of delivery room care for infants born with meconium-stained amniotic fluid.

In our study, non-Hispanic black infants were more likely to die from meconium aspiration syndrome, compared with non-Hispanic white infants in all three periods. This is primarily because of the higher prevalence of meconium-stained amniotic fluid and, in turn, meconium aspiration syndrome in non-Hispanic blacks. However, once infants had meconium aspiration syndrome, case fatality was not different between these two racial groups in all three periods.

A word of caution is proper for interpreting the results of our analysis. The validity and accuracy of meconium-stained amniotic fluid and meconium aspiration syndrome reported on the US linked birth and infant death data are not known. It is quite possible that meconium-stained amniotic fluid and meconium aspiration syndrome were underreported in this data set. Hence, our figures might have underestimated the true prevalence of meconium-stained amniotic fluid and meconium aspiration syndrome. However, it is unlikely that this under-reporting is biased favoring one racial group over the other.

In conclusion, our results suggest that, compared with non-Hispanic whites, non-Hispanic blacks are at a significantly greater risk for meconium-stained amniotic fluid and meconium aspiration syndrome but not for meconium aspiration syndrome case fatality. Further investigation on pathophysiologic mechanism(s) leading to meconium-stained amniotic fluid and meconium aspiration syndrome would contribute to the future prevention of this serious perinatal complication.


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  		APPENDIX A Risk of Meconium-Stained Amniotic Fluid for Non-Hispanic Blacks by Gestational Age for Three Periods in the United States
 

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  		APPENDIX B Risk of Meconium Aspiration Syndrome for Non-Hispanic Blacks by Gestational Age for Three Periods in the United States
 

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  		APPENDIX C Risk of Meconium-Stained Amniotic Fluid for Non-Hispanic Blacks by Birth Weight for Three Periods in the United States
 

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  		APPENDIX D Risk of Meconium Aspiration Syndrome for Non-Hispanic Blacks by Birth Weight for Three Periods in the United States
 

   Footnotes
 
doi:10.1016/j.obstetgynecol.2003.07.006

Received March 19, 2003. Received in revised form July 17, 2003. Accepted July 31, 2003.


   REFERENCES
TOP
ABSTRACT
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
1. Murdina MT, Desmond MD, Moore J, Lindley JE, Brown CA. Meconium staining of the amniotic fluid: A marker of fetal hypoxia. Obstet Gynecol 1957;9:91–103.[Free Full Text]

2. Fujikura T, Kilionsky B. The significance of meconium staining. Am J Obstet Gynecol 1975;121:45–50.[Medline]

3. Miller FC, Sacks DA, Yeh SY, Paul RH, Schifrin BS, Martin CB Jr, et al. Significance of meconium during labor. Am J Obstet Gynecol 1975;122:573–80.[Medline]

4. Drummond WH, Peckham GJ, Fox WW. The clinical profile of newborn with persistent pulmonary hypertension. Clin Pediatrics 1977;16:335–4.

5. Murphey JD, Vawter GF, Reid LM. Pulmonary vascular disease in fetal meconium aspiration. J Pediatr 1984;104:758–62.[Medline]

6. Coltari TM, Bryne DL, Bates SA. Meconium aspiration syndrome: 6 year retrospective study. Br J Obstet Gynaecol 1989;96:411–4.[Medline]

7. Rossi EM, Phillipson EH, Williams TG, Kalahan SC. Meconium aspiration syndrome: Intrapartum and neonatal attributes. Am J Obstet and Gynecol 1989;161:1106–10.[Medline]

8. Goetzman BW. Meconium aspiration. Am J Dis Child 1992;146:1282–3.[Medline]

9. Alexander GR, Husley TC, Robillard PY, De Caunes F, Papiernik E. Determinants of meconium-stained amniotic fluid in term pregnancies. J Perinatol 1994;14:259–63.[Medline]

10. Wiswell TE, Tuggle JM, Turner BS. Meconium aspiration syndrome: Have we made a difference? Pediatrics 1990;85:715–21.[Abstract/Free Full Text]

11. National Center for Health Statistics. 1989–91, 1995–97, and 1998–2000 birth cohort linked birth and infant death data set [CD-ROM series 20, nos 5, 6, 7, 12, 14, 15, 18, 20, 21]. Hyattsville, Maryland: Centers for Disease Control and Prevention, Department of Health and Human Services, 1996–2003.

12. Ostrea EM, Naqui N. Influence of gestational age on the ability of fetus to pass meconium in utero: Clinical applications. Acta Obstet Gynaecol Scand 1982;61:257–77.

13. Berkus MD, Langer O, Samueloff A, Xenakis EM, Field NT, Ridgway LE. Meconium-stained amniotic fluid: Increased risk for adverse neonatal outcome. Obstet Gynecol 1994;84:115–20.[Abstract/Free Full Text]

14. Yoder BA. Meconium-stained amniotic fluid and respiratory complications: Impact of selective tracheal suction. Obstet Gynecol 1994;83:77–84.[Abstract/Free Full Text]

15. Peng TC, Gutcer GR, Van Dorsten JP. A selective aggressive approach to the neonate exposed to meconium-stained amniotic fluid. Am J Obstet Gynecol 1996;175:296–301.[Medline]

16. Ramin KD, Leveno KJ, Kelly MA, Carmody TJ. Amniotic fluid meconium: A fetal environmental hazard. Obstet Gynecol 1996;87:181–4.[Abstract]

17. Narang A, Nair PM, Bhakoo ON, Vashisht K. Management of meconium stained amniotic fluid: A team approach. Indian Pediatr 1993;30:9–13.[Medline]

18. Gupta V, Bhatia BD, Mishra OP. Meconium stained amniotic fluid: Antenatal, intrapartum and neonatal attributes. Indian Pediatr 1996;33:293–7.[Medline]




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