Placenta Previa: Preponderance of Male Sex at Birth

American Journal of EpidemiologyCopyright O 1999 by The Johns Hopkins University School of Hygiene and Public HealthAll rights reservedVol. 149, No. 9Printed In U.SA,Placenta Previa: Preponderance of Male Sex at BirthKitaw Demissie, 1 ' 2 Mary B. Breckenridge, 1 - 2 Lawrence Joseph, 3 and George G. Rhoads 2 - 4To determine the relation between placenta previa and male sex at birth, the authors conducted two types ofanalysis: 1) a historical cohort analysis of singleton live births in New Jersey hospitals during 1989-1992 (N =447,963); and 2) a meta-analysis of previously published studies on the subject. For the cohort analysis, subjectmother-infant dyads were identified from linked birth certificate and maternal and infant hospital claims data. Theinfant's sex for mothers with an International Classification of Diseases, Ninth Revision, Clinical Modification,code of 641.0-641.1 for placenta previa (n = 2,685) was compared with infant"s sex for mothers without placentaprevia (n = 445,270). For the meta-analysis, seven published articles were located and summary effects werecalculated using both fixed-effect and random-effects models. In the present cohort study, the male:female ratioat birth was significantly higher in women with placenta previa (1.19) than in those without placenta previa (1.05)(p < 0.001). The association of placenta previa with male sex persisted when the analysis was either stratifiedor adjusted for the effects of maternal age, maternal parity, maternal smoking during the index pregnancy,race/ethnicity, the Infant's gestational age, and the Infant's birth weight. The meta-analytic results from the fixedeffectand random-effects models showed a 14% excess of placenta previa when women were carrying a viablemale fetus as compared with a viable female fetus during pregnancy. The results were the same regardless ofwhether the present cohort study was included in the meta-analysis. In conclusion, the evidence obtained fromthese analyses strongly argues for an association between placenta previa and male sex at birth. Themechanism for this association remains to be determined. Am J Epidemiol 1999; 149:824-30.infant, newborn; meta-analysis; placenta praevia; sex ratioThe human sex ratio at birth appears to be influencedby variables such as race, parental age, and external circumstances(e.g., wartime vs. peacetime) (1). A numberof other factors have been considered, including thepsychological status of parents, the time of inseminationwithin the menstrual cycle, parental disease at thetime of conception, inadvertent exposure of the parentsto deleterious chemicals, and hormonal treatment of theparents for subfertility or other conditions (1).Earlier studies carried out in the United Kingdomand the United States (2-5) reported an excess ofReceived for publication February 5,1998, and accepted for publicationAugust 25, 1998.Abbreviations: Cl, confidence interval; OR, odds ratio.1 Department of Family Medicine, University of Medicine andDentistry of New Jersey-Robert Wood Johnson Medical School,New Brunswick, NJ.2 Department of Environmental and Community Medicine,University of Medicine and Dentistry of New Jersey-Robert WoodJohnson Medical School, Piscataway, NJ.3 Division of Clinical Epidemiology, Montreal General Hospital,McGill University, Montreal, Quebec, Canada.4 Department of Obstetrics, Gynecology and ReproductiveSciences, University of Medicine and Dentistry of NewJersey-Robert Wood Johnson Medical School, New Brunswick, NJ.Reprint requests to Dr. Kitaw Demissie, Department of FamilyMedicine, Robert Wood Johnson Medical School, 1 Robert WoodJohnson Place, CN 19, New Brunswick, NJ 08903-0019.male births in pregnancies complicated by placentaprevia. A follow-up study by Alberman and Butler(6), however, failed to replicate this finding. In fact,Alberman and Butler reported an excess of femalerather than male births among cases of placenta previa,though the association did not achieve statisticalsignificance. More than 20 years after the initialreports on this issue, South African (7) and Hungarian(8) investigators reported a preponderance of malesex at birth among pregnancies complicated by placentaprevia.All of the above reports lacked details, were mostlybased on small numbers, and did not control for potentialconfounders. Furthermore, in some of the studies,the sample was unrepresentative. Although thesereports provided promising clues to the causes of placentaprevia, little attention has been given to them.The purposes of this study were 1) to examine therelation between placenta previa and infant sex amongwomen delivering live singleton infants in a larger andmore representative population and 2) to combine theresults of studies that have examined this issue, withand without the present study results, using metaanalyticmethods in order to arrive at a more generalizableconclusion.Downloaded from http://aje.oxfordjournals.org/ by guest on January 5, 2012824

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