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Maternal Obesity, Uterine Activity, and the Risk of Spontaneous Preterm Birth

Ehrenberg, Hugh M. MD; Iams, Jay D. MD; Goldenberg, Robert L. MD; Newman, Roger B. MD; Weiner, Steven J. MS; Sibai, Baha M. MD; Caritis, Steve N. MD; Miodovnik, Menachem MD; Dombrowski, Mitchell P. MD

doi: 10.1097/AOG.0b013e318191c818
Original Research

OBJECTIVE: To assess the associations among maternal obesity, uterine contraction frequency, and spontaneous preterm birth in women at risk for spontaneous preterm birth.

METHODS: In a secondary analysis, we analyzed data from 253 women at risk for spontaneous preterm birth (prior spontaneous preterm birth, vaginal bleeding) enrolled in a multicenter observational study of home uterine activity monitoring at 11 centers. All women wore a uterine activity monitor twice daily from 22 weeks through 34 weeks of gestation. Mean and maximal contractions/hour at 22–24, 25–26, 27–28, 29–30, 31–32 weeks, and at or after 33 weeks of gestation were compared between overweight/obese women (a body mass index [BMI] at 22–24 weeks greater than 25 kg/m2) and normal/underweight women (a BMI of 25 kg/m2 or less) at each gestational age interval. Multivariable analysis evaluated the influences of BMI, contractions, fetal fibronectin, and transvaginal cervical length on spontaneous preterm birth before 35 weeks.

RESULTS: Obese/overweight women (n=156) were significantly less likely to experience spontaneous preterm birth before 35 weeks (8.3% compared with 21.7%, P<.01). For each gestational age interval before 32 weeks, obese/overweight women had fewer mean contractions/hour (P<.01 for each) and maximal contractions/hour (P<.01 for each) than normal/underweight women, although their mean cervical lengths (34.3 mm compared with 33.1 mm, P=.25), and fetal fibronectin levels (7.1% compared with 7.2% 50 ng/mL or more, P=.97) were similar at study enrollment. Obese/overweight status was associated with a lower risk of spontaneous preterm birth before 35 weeks after controlling for contraction frequency and other factors evaluated at 22–24 weeks, but not at later periods.

CONCLUSION: Obese/overweight women at risk for spontaneous preterm birth exhibit less uterine activity and less frequent spontaneous preterm birth before 35 weeks of gestation than normal/underweight women.


In women at risk for spontaneous preterm birth, overweight/obese body habitus is associated with significantly fewer uterine contractions throughout pregnancy and a lower risk of spontaneous preterm birth.

* For the other members of the NICHD MFMU who participated in this study, see the Appendix online at

From the 1Case Western Reserve University-MetroHealth Medical Center, Cleveland, Ohio; 2Ohio State University, Columbus, Ohio; 3Drexel University, Philadelphia, Pennsylvania; 4Medical University of South Carolina, Charleston, South Carolina; 5George Washington University Biostatistics Center, Washington, DC; 6University of Tennessee, Memphis, Tennessee; 7University of Pittsburgh, Pittsburgh, Pennsylvania; 8University of Cincinnati, Cincinnati, Ohio; and 9Wayne State University, Detroit, Michigan.

Supported by grants from the National Institute of Child Health and Human Development (HD21410, HD21414, HD21434, HD27860, HD27861, HD27869, HD27883, HD27889, HD27905, HD27915, HD27917, HD19897, HD36801, and HD40544).

The authors thank Francee Johnson, RN, BSN, for protocol development and coordination between clinical research centers, Elizabeth Thom, PhD, for study design, data management, statistical analysis, and manuscript development, and Brian Mercer, MD, for his assistance and guidance with manuscript preparation.

Presented at the Annual Meeting of the Society for Maternal–Fetal Medicine, San Francisco, California, January 31 to February 2, 2007.

Corresponding author: Hugh M. Ehrenberg, MD, Ohio State University Medical Center, 325 E 12th Street, Columbus, Ohio.

Financial Disclosure The authors did not report any potential conflicts of interest.

© 2009 by The American College of Obstetricians and Gynecologists. Published by Wolters Kluwer Health, Inc. All rights reserved.