Am J Perinatol 2023; 40(01): 062-067
DOI: 10.1055/s-0041-1729161
Original Article

Trends in Spontaneous and Medically Indicated Preterm Birth in Twins versus Singletons: A California Cohort 2007 to 2011

Amen Ness
1   Department of Obstetrics and Gynecology, St. Elizabeth’s Medical Center, Boston, Massacheusetts
Jonathan A. Mayo
2   Department of Pediatrics, Stanford University, Stanford, California
Yasser Y. El-Sayed
3   Department of Obstetrics and Gynecology, Stanford University, Stanford, California
Maurice L. Druzin
3   Department of Obstetrics and Gynecology, Stanford University, Stanford, California
David K. Stevenson
2   Department of Pediatrics, Stanford University, Stanford, California
2   Department of Pediatrics, Stanford University, Stanford, California
› Author Affiliations
Funding This work was supported by the March of Dimes Prematurity Research Center at Stanford University School of Medicine.


Objective The study aimed to describe preterm birth (PTB) rates, subtypes, and risk factors in twins compared with singletons to better understand reasons for the decline in PTB rate between 2007 and 2011.

Study Design This was a retrospective population-based analysis using the California linked birth certificates and maternal-infant hospital discharge records from 2007 to 2011. The main outcomes were overall, spontaneous (following spontaneous labor or preterm premature rupture of membranes), and medically indicated PTB at various gestational age categories: <37, <32, and 34 to 36 weeks in twins and singletons.

Results Among the 2,290,973 singletons and 28,937 twin live births pairs included, overall PTB <37 weeks decreased by 8.46% (6.77–6.20%) in singletons and 7.17% (55.31–51.35%) in twins during the study period. In singletons, this was primarily due to a 24.91% decrease in medically indicated PTB with almost no change in spontaneous PTB, whereas in twins indicated PTB declined 7.02% and spontaneous PTB by 7.39%.

Conclusion Recent declines in PTB in singletons appear to be largely due to declines in indicated PTB, whereas both spontaneous and indicated PTB declined in twins.

Key Points

  • The declines in PTB noted between 2006 and 2014 occurred in both singleton and twins.

  • Declines were mostly in medically indicated PTB.

  • Interventions proposed as causing the declines in singletons would not apply to twins.


This work was previously presented at the Society for Maternal-Fetal Medicine 37th Annual Meeting: The Pregnancy Meeting in Las Vegas, NV from January 23 to 28, 2017. During the period 2007 to 2011, declines in PTB rates in California occurred in both singletons and twins but differed by PTB phenotype.

Publication History

Received: 10 June 2020

Accepted: 02 March 2021

Article published online:
02 May 2021

© 2021. Thieme. All rights reserved.

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