Correspondence

Low Birth Weight in Relation to the Interval between Pregnancies

N Engl J Med 1995; 333:386-387August 10, 1995

Article

To the Editor:

Regarding the article by Rawlings et al. (Jan. 12 issue),1 we wonder whether a short interpregnancy interval might be confounded by factors that were not considered in the analysis. Although all the women studied were drawn from military families with access to high-quality health care, no information was provided about the proportion of mothers who worked and what types of occupations they had. Since it has previously been shown that physical exertion associated with employment in the Army may increase the risk of preterm delivery,2 might a racial difference in employment patterns account to some extent for the increased risk of preterm birth among the black women in the sample?

The large racial difference in the prevalence of low birth weight or preterm delivery among women with interpregnancy intervals of 3 to 5.9 months also deserves comment. It is puzzling to find that this interpregnancy interval has such a disparate effect among black women as compared with white women, when no marked racial difference is apparent at shorter or longer intervals. Nonetheless, the presence of a racial difference associated with even one interpregnancy interval suggests that short intervals themselves cannot wholly explain the racial disparity in pregnancy outcomes.

Also noteworthy is the fact that interpregnancy intervals of less than nine months are associated with poor outcomes among black women, whereas only the shortest intervals (less than three months) are disadvantageous for white women. The authors report in addition that the adverse effect of a short interpregnancy interval was strongest when the preceding pregnancy resulted in a low-birth-weight or preterm infant. These findings are interesting and merit additional discussion.

Gertrud S. Berkowitz, Ph.D.
Maureen Hatch, Ph.D.
Mount Sinai School of Medicine, New York, NY 10029-6574

2 References

1. 1

   Rawlings JS, Rawlings VB, Read JA. Prevalence of low birth weight and preterm delivery in relation to the interval between pregnancies among white and black women. N Engl J Med 1995;332:69-74
   Full Text | Web of Science | Medline

2. 2

   Ramirez G, Grimes RM, Annegers JF, Davis BR, Slater CH. Occupational physical activity and other risk factors for preterm birth among US Army primigravidas. Am J Public Health 1990;80:728-730
   CrossRef | Web of Science | Medline

To the Editor:

Rawlings et al. report that a short interval between pregnancies was associated with a higher risk of low birth weight and preterm delivery among black women than among white women. We sought to replicate this finding with two data sets: the first, a sample of 5230 inner-city women; the second, a subgroup of 14,777 white and black non-Hispanics from a national sample of women who gave birth during 1988.1 To test whether a shorter interpregnancy interval was more adverse for blacks than for whites in terms of low birth weight and preterm delivery, we performed logistic-regression analyses using BMDP's LR2 and a model with three terms: one for race alone, one for interpregnancy interval alone, and one to estimate the interaction of race with interpregnancy interval. Regardless of which interpregnancy interval or sample we used, the direct effects of race and interpregnancy interval were consistently significant, whereas the interaction effect was not significant. For example, in the first sample the risk ratio associated with an interpregnancy interval of less than six months was 1.69, and the risk ratio associated with black race was 2.44. In the second sample, the analysis showed a risk ratio of 1.49 associated with the less-than-six-month period, and a risk ratio of 1.46 for blacks as compared with whites. In neither analysis was there any suggestion of a differential risk of poor birth outcomes for blacks with short intervals between pregnancies; the interaction risk ratios were 1.05 (95 percent confidence interval, 0.55 to 1.98) and 0.94 (95 percent confidence interval, 0.63 to 1.39), respectively. Analyses of the second sample with the use of a nine-month interval, as in the study by Rawlings et al., yielded a risk ratio of 1.34 associated with the less-than-nine-month period as compared with the more-than-nine-month period, and a risk ratio of 1.46 associated with black race, the same as that in the analysis with a six-month interval.

The failure of our analyses to show that a short interval between pregnancies poses any greater risk for blacks than for whites does not necessarily imply that the findings of Rawlings et al. were spurious, nor should our results dampen the search for factors to explain and perhaps mitigate racial disparities in birth outcomes. These findings may alert us to the need to examine the issue further before sounding an alarm about short interpregnancy intervals posing a greater risk for blacks than for whites.

T. Joseph Sheehan, Ph.D.
David I. Gregorio, Ph.D.
University of Connecticut School of Medicine, Farmington, CT 06030-1910

2 References

1. 1

   Public use data tape documentation: 1988 National Maternal and Infant Health Survey. Hyattsville, Md.: National Center for Health Statistics, October 1991.
   

2. 2

   Dixon WJ, et al. BMDP statistical software manual. Vol. 2. Berkeley: University of California Press, 1990:903-19.
   

To the Editor:

It is known that breast-feeding, although not effective as birth control, does have the effect of lengthening the interval between pregnancies.1 For instance, the !Kung people of southern Africa nurse their babies frequently and for several years, with consequent long periods of postpartum amenorrhea and long intervals between pregnancies.2 Similar lengths of postpartum amenorrhea related to nursing have been reported from Zaire,3 Edinburgh,4,5 and Alaska.6 Postpartum amenorrhea is also associated with nursing infants through the night, placing little reliance on supplementary foods, and having hyperprolactinemia.3-5

Is it possible that black women nurse their infants less frequently or use supplements more frequently? If so, one way to increase interpregnancy intervals would be to encourage nursing among black women.

Bruce Williams, M.D.
Youthdale Treatment Centres, Toronto, ON M5B 1T8, Canada

6 References

1. 1

   Reproductive function during lactation. In: Lawrence RA. Breastfeeding: a guide for the medical profession. 2nd ed. St. Louis: Mosby, 1985:429-42.
   

2. 2

   Konner M, Worthman C. Nursing frequency, gonadal function, and birth spacing among !Kung hunter-gatherers. Science 1980;207:788-791
   CrossRef | Web of Science | Medline

3. 3

   Delvoye P, Demaegd M, Uwayitu-Nyampeta RC. Serum prolactin, gonadotropins, and estradiol in menstruating and amenorrheic mothers during two years' lactation. Am J Obstet Gynecol 1978;130:635-639
   Web of Science | Medline

4. 4

   Howie PW, McNeilly AS, Houston MJ, Cook A, Boyle H. Fertility after childbirth: infant feeding patterns, basal PRL levels and post-partum ovulation. Clin Endocrinol (Oxf) 1982;17:315-322
   CrossRef | Web of Science | Medline

5. 5

   Howie PW, McNeilly AS, Houston MJ, Cook A, Boyle H. Fertility after childbirth: post-partum ovulation and menstruation in bottle and breast feeding mothers. Clin Endocrinol (Oxf) 1982;17:323-332
   CrossRef | Web of Science | Medline

6. 6

   Berman ML, Hanson K, Hellman IL. Effect of breast-feeding on postpartum menstruation, ovulation, and pregnancy in Alaskan Eskimos. Am J Obstet Gynecol 1972;114:524-534
   Web of Science | Medline

Author/Editor Response

The authors reply:

To the Editor: Drs. Berkowitz and Hatch have correctly observed that active-duty status of a woman in the military has been shown to be a risk factor for adverse pregnancy outcome. In our study population, 26.2 percent of the black women and 10.4 percent of the white women were on active duty. This may have been a factor in the greater prevalence of preterm delivery and low birth weight among black women. However, it does not explain the race-specific differences in pregnancy outcomes in relation to the interval between pregnancies. Within each racial group, the prevalence of active-duty status did not differ between women with intervals of less than three, six, or nine months and those with longer intervals, respectively (Table 1Table 1Relation between Interpregnancy Interval and Active-Duty Status among White and Black Women.).

It was not our intention to suggest that the wide racial disparity in pregnancy outcomes in the United States can be attributed solely to interracial differences in the prevalence or influence of short interpregnancy intervals. In our study population, pregnancy outcomes were similar for white and black women with intervals of nine months or longer. However, this was a population with many health advantages over the general American public. These advantages served to limit the potentially confounding effects of obstetrical risk factors that are often more prevalent among black women in the American population as a whole. One risk factor that was not uncommon in our study population was a history of poor outcome in the preceding pregnancy. This risk factor appeared to accentuate the relation between short interpregnancy intervals and poor outcome among the white women.

We are interested in the work of Drs. Sheehan and Gregorio on the relation of pregnancy outcome to interpregnancy interval and race with multivariate analysis used in populations larger than that investigated in our study. It is difficult to interpret the results of their analysis in the absence of comparisons that confirm that there were not large differences in the prevalence of other well-recognized obstetrical risk factors between the racial groups and between women with short interpregnancy intervals and those with longer intervals.

We agree with the observation of Dr. Williams that breast-feeding has been associated with longer interpregnancy intervals. In our delivery population, breast-feeding at the time of hospital discharge was more common among white women (64.3 percent) than among black women (36.2 percent).

James S. Rawlings, M.D.
Virginia B. Rawlings, R.D., M.S.P.H.
John A. Read, M.D.
Madigan Army Medical Center, Tacoma, WA 98431

Citing Articles (3)

Citing Articles

1. 1

   Dyan M. Simon, Shilpa Vyas, Nikhil G. Prachand, Richard J. David, James W. Collins. (2006) Relation of Maternal Low Birth Weight to Infant Growth Retardation and Prematurity. Maternal and Child Health Journal 10:4, 321-327
   CrossRef

2. 2

   Janet Rich-Edwards, Nancy Krieger, Joseph Majzoub, Sally Zierler, Ellice Lieberman, Matthew Gillman. (2001) Maternal experiences of racism and violence as predictors of preterm birth: rationale and study design. Paediatric and Perinatal Epidemiology 15:s2, 124-135
   CrossRef

3. 3

   David, Richard J., Collins, James W. Jr., . (1997) Differing Birth Weight among Infants of U.S.-Born Blacks, African-Born Blacks, and U.S.-Born Whites. New England Journal of Medicine 337:17, 1209-1214
   Full Text