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Uterine distention as a factor in birth timing: retrospective nationwide cohort study in Sweden
  1. Jonas Bacelis1,
  2. Julius Juodakis2,
  3. Kristina M Adams Waldorf1,3,4,
  4. Verena Sengpiel1,
  5. Louis J Muglia5,6,
  6. Ge Zhang5,6,
  7. Bo Jacobsson1,7
  1. 1 Department of Obstetrics and Gynecology, Sahlgrenska University Hospital Östra, Gothenburg, Sweden
  2. 2 Department of Obstetrics and Gynecology, Institute of Clinical Sciences, Sahlgrenska Academy, University of Gothenburg, Gothenburg, Sweden
  3. 3 Department of Obstetrics and Gynecology, University of Washington, Seattle, Washington, USA
  4. 4 Department of Global Health, University of Washington, Seattle, Washington, USA
  5. 5 Human Genetics Division, Cincinnati Children’s Hospital Medical Center, Cincinnati, Ohio, USA
  6. 6 Center for Prevention of Preterm Birth, Cincinnati Children’s Hospital Medical Center, Cincinnati, Ohio, USA
  7. 7 Department of Genetics and Bioinformatics, Area of Health Data and Digitalisation, Norwegian Institute of Public Health, Oslo, Norway
  1. Correspondence to Mr Jonas Bacelis; jonas.bacelis{at}


Objectives To determine whether uterine distention is associated with human pregnancy duration in a non-invasive observational setting.

Design Retrospective cohort study modelling uterine distention by interaction between maternal height and uterine load.

Setting The study is based on the 1990–2013 population data from all delivery units in Sweden.

Participants Uncomplicated first pregnancies of healthy Nordic-born mothers with spontaneous onset of labour. Pregnancies were classified as twin (n=2846) or singleton (n=527 868). Singleton pregnancies were further classified as carrying a large for gestational age fetus (LGA, n=24 286) or small for gestational age fetus (SGA, n=33 780).

Outcome measures Statistical interaction between maternal height and uterine load categories (twin vs singleton pregnancies, and LGA vs SGA singleton pregnancies), where the outcome is pregnancy duration.

Results In all models, statistically significant interaction was found. Mothers carrying twins had 2.9 times larger positive linear effect of maternal height on gestational age than mothers carrying singletons (interaction p=5e−14). Similarly, the effect of maternal height was strongly modulated by the fetal growth rate in singleton pregnancies: the effect size of maternal height on gestational age in LGA pregnancies was 2.1 times larger than that in SGA pregnancies (interaction p<1e−11). Preterm birth OR was 1.4 when the mother was short, and 2.8 when the fetus was extremely large for its gestational age; however, when both risk factors were present together, the OR for preterm birth was larger than expected, 10.2 (interaction p<0.0005).

Conclusions Across all classes, maternal height was significantly associated with child’s gestational age at birth. Interestingly, in short-statured women with large uterine load (twins, LGA), spontaneous delivery occurred much earlier than expected. The interaction between maternal height, uterine load size and gestational age at birth strongly suggests the effect of uterine distention imposed by fetal growth on birth timing.

  • gestational age at birth
  • large for gestational age
  • small for gestational age
  • preterm delivery
  • twins
  • uterine distention

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  • GZ and BJ contributed equally.

  • Contributors JB and GZ: study design and data analysis and interpretation; BJ and VS: data acquisition; JB and JJ: data cleaning and figures; KMAW: literature search; JB and KMAW: writing; BJ, JJ, LJM, VS and GZ: critical revision. All authors approved the final version to be submitted for publication.

  • Funding This work was supported by Swedish government grants to researchers in the public health sector (ALFGBG-717501, ALFGBG-507701, ALFGBG-426411), The Swedish Research Council (2015-02559), March of Dimes Foundation (21-FY16-121) and the Burroughs Wellcome Fund Preterm Birth Research Grant (10172896).

  • Competing interests None declared.

  • Patient consent Not required.

  • Ethics approval The work was approved by the Regional Ethical Review Board in the Western Health Care Region of Sweden (Dnr:968-14).

  • Provenance and peer review Not commissioned; externally peer reviewed.

  • Data sharing statement Researchers can apply for data access by contacting The Swedish National Board of Health and Welfare.

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