title: The greater risk of preterm birth in triplets is mirrored by a more rapid cervical shortening along gestation.
abstract: BACKGROUND
  The risk of preterm birth increases with plurality. However, data are limited about the role of cervical length in triplet pregnancies and how the greater predisposition for preterm birth in triplet pregnancies, compared with twin pregnancies, is reflected by cervical length. Specifically, it is unclear whether the greater predisposition of triplet pregnancies for preterm birth is reflected by a more rapid cervical shortening during gestation compared with twin pregnancies (and therefore a shorter cervical length at any given gestational age), by a greater risk for preterm birth per given cervical length at any given gestational age, or both.
  
  
  OBJECTIVE
  The purpose of the study was to compare the rate of cervical shortening during gestation and the correlation between cervical length and gestational age at birth in asymptomatic women with triplet vs twin pregnancies.
  
  
  METHODS
  This was a retrospective study of women with triplet or twin pregnancies who were who were observed in a tertiary center who underwent serial sonographic measurement of cervical length from 16-32 weeks gestation. Change in cervical length during gestation and relationship of cervical length with gestational age at birth were compared between the triplets and twins groups.
  
  
  RESULTS
  A total of 431 measurements of cervical length from 86 women with triplets was analyzed and compared with 2826 measurements of cervical length from 441 women with twins. The rate of cervical shortening among triplet pregnancies was higher than among twin pregnancies starting from 18 weeks of gestation (slope of regression line, -1.297 vs -0.907; P < .001). Similarly, the proportion of women with cervical length of <25 mm or 15 mm was higher among triplet pregnancies than among twin pregnancies (34.0% vs 21.0% [P < .001] and 16.7% vs 8.4% [P = .001]), respectively. For any given cervical length measured after 22 weeks of gestation, the associated gestational length at birth in triplet pregnancies was lower by 2.7 weeks on average compared with twin pregnancies (P < .001).
  
  
  CONCLUSIONS
  The higher rate of preterm birth in triplet pregnancies, compared with twin pregnancies, is reflected by both a more rapid cervical shortening during gestation and a lower gestational age at birth per any given cervical length.
authors.affiliation: Department of Obstetrics and Gynecology, Lis Hospital for Women, Tel Aviv Sourasky Medical Center, and Tel-Aviv University, Tel Aviv, Israel.
authors.forename: Liran
authors.initials: L
authors.lastname: Hiersch
authors.affiliation: Division of Maternal-Fetal Medicine, Department of Obstetrics and Gynecology, Sunnybrook Health Sciences Centre, University of Toronto, Ontario, Canada.
authors.forename: Hadar
authors.initials: H
authors.lastname: Rosen
authors.affiliation: Division of Maternal-Fetal Medicine, Department of Obstetrics and Gynecology, Sunnybrook Health Sciences Centre, University of Toronto, Ontario, Canada.
authors.forename: Rania
authors.initials: R
authors.lastname: Okby
authors.affiliation: Division of Maternal-Fetal Medicine, Department of Obstetrics and Gynecology, Sunnybrook Health Sciences Centre, University of Toronto, Ontario, Canada.
authors.forename: Howie
authors.initials: H
authors.lastname: Freeman
authors.affiliation: Division of Maternal-Fetal Medicine, Department of Obstetrics and Gynecology, Sunnybrook Health Sciences Centre, University of Toronto, Ontario, Canada.
authors.forename: Jon
authors.initials: J
authors.lastname: Barrett
authors.affiliation: Division of Maternal-Fetal Medicine, Department of Obstetrics and Gynecology, Sunnybrook Health Sciences Centre, University of Toronto, Ontario, Canada. Electronic address: nir.melamed@sunnybrook.ca.
authors.forename: Nir
authors.initials: N
authors.lastname: Melamed
mesh_terms: D000328:Adult; D054791:Cervical Length Measurement; D002584:Cervix Uteri / Q000000981:diagnostic imaging*; D015331:Cohort Studies; D005260:Female; D005865:Gestational Age; D006801:Humans; D011247:Pregnancy; D059286:Pregnancy, Triplet; D059285:Pregnancy, Twin; D047928:Premature Birth / Q000209:etiology*; D012189:Retrospective Studies
keywords: cervical length; change; serial measurement; triplets; twins