Risk of Preterm Labor: Likelihood Ratios

Introduction: These tests are for prediction of likelihood of going into preterm labor. In other words, these patients are asymptomatic. For patients who are symptomatic, see Preterm Labor.

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Prevalence

Population / Calculator	Prevalence	Comments / Study / Link
Women in the U.S.	12%	approximately... from March of Dimes numbers
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The likelihood ratios of findings for Risk of Preterm Labor are listed below. See the left navigation bar to change the display.

Pre-Test Probability (Prevalence): %.	Post-Test Probability (Predictive Value): %.	Switch to display mode.
Note that calculating probabilities from more than one** finding is inherently inaccurate because findings are not independent. (For example, using two positive findings that share a common pathogenesis is likely to overestimate the true probability.)

Good Positive and Negative Likelihood Ratios
Finding	Result	+LR	-LR
[ + ] Cervical Length	+ 0 -	3.5	0.4
shortest cervical length q2wk up to 24 weeks is <25mm, for birth <35 wk in women with h/o spon preterm birth Study: JAMA 2001 Sep 19;286(11):1340-8. PMID 11560539

Good Positive Likelihood Ratio
Finding	Result	+LR	-LR
[ + ] Cervical Length	+ 0 -	9.5	0.8
<25mm at 16-19wk, for birth <35wk in women with h/o spon preterm birth Study: JAMA 2001 Sep 19;286(11):1340-8. PMID 11560539