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RCT evidence on clinically important outcomes may Applicability: The relevance of the evidence base to an
not be possible because a large number of patients will external population.
need to be recruited to detect rare events, such as
Bias: A systematic error, arising from participant
maternal deaths. Therefore, we additionally propose:
selection or outcome measurement that produces an
• Well-designed, well powered cohort studies erroneous effect estimate.
examining clinical outcomes. These studies should
include a representative and inception cohort of all References
patients with arrested preterm labor. Since
1. Behrman R, Butler A. Preterm Birth: Causes,
observational studies are susceptible to the effects
Consequences, and Prevention. Washington, DC:
of confounding, future observational studies
National Academies Press; 2007.
should measure, report, and adjust for potential
2. Centers for Disease Control and Prevention.
confounders such as fetal fibronectin, cervical
Birthweight and Gestation. Available at:
length/dilation, cerclage, maternal characteristics
www.cdc.gov/nchs/fastats/birthwt.htm. Accessed
(e.g., age, race), level of care and activity, and May 12, 2010.
concomitant medications. Propensity scores based
3. Mathews TJ, MacDorman MF. Infant mortality statistics
on these variables may be considered. Other from the 2006 period linked birth/infant death data set.
considerations about power, multiple comparison Natl Vital Stat Rep 2010 Apr 30;58(17):1-31. PMID:
groups, level of care, reporting of cointerventions, 20815136.
and long-term followup are the same as for RCTs. 4. Nanda K, Cook LA, Gallo MF, et al. Terbutaline pump
maintenance therapy after threatened preterm labor for
• Record linkage studies in which mothers’ prenatal
preventing preterm birth. Cochrane Database Syst Rev
and infants’ NICU and childhood developmental
2002 Dec;(4):
electronic health records are linked may be a more
5. HAYES, Inc. Continuous subcutaneous terbutaline
practical research proposition for the near future
infusion for treatment of preterm labor (Structured
with improvements in quality and accessibility of
abstract). Lansdale, Pa: Hayes, Inc 2006;
electronic patient records. NICU registries in
6. Hayes E, Moroz L, Pizzi L, et al. A cost decision
which prenatal data of mothers are available can
analysis of 4 tocolytic drugs. Am J Obstet Gynecol
be a very valuable source. However, such linkage
2007 Oct;197(4):383-6. PMID: 17904969.
based studies may also be impacted by biases not
7. Agency for Healthcare Research and Quality. Methods
uncommon to cohort study designs, especially
Guide for Effectiveness and Comparative Effectiveness
confounding because of unmeasured or
Reviews [Draft]. AHRQ Publication No. 10(11)-
unrecorded variables with important prognostic EHC063-EF. Available at:
implications. www.effectivehealthcare.ahrq.gov/ehc/products/60/318/
MethodsGuide_Prepublication_Draft_20110824.pdf.
Glossary Rockville, MD: 2011.
8. Santaguida, P and Raina, P. McMaster Quality
Preterm birth: Delivery before completion of the 37th Assessment Scale of Harms (McHarm) for primary
week of gestation. studies: Manual for use of the McHarm. Available at:
http://hiru.mcmaster.ca/epc/mcharm.pdf. Accessed
Tocolytic: An agent that inhibits labor by slowing or December 22, 2010.
halting uterine contractions.
9. Owens DK, Lohr KN, Atkins D, et al. AHRQ series
Strength of evidence: The strength of evidence grading paper 5: grading the strength of a body of evidence
reflects a global assessment of the evidence base. when comparing medical interventions--agency for
healthcare research and quality and the effective health-
Strength of evidence may be designated as insufficient,
care program. J Clin Epidemiol 2010 May;63(5):513-
low, moderate or high based on the domains of study
23. PMID: 19595577.
risk of bias, consistency, directness, and precision.

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