Materials and Methods

This is a retrospective cohort study of women undergoing routine CL assessment between 18-23 6/7 weeks of gestation from December 2010 through January 2014 at Northwestern Memorial Hospital in Chicago, IL. Women were included in the cohort if they were at least 18 years of age, had a singleton gestation, and had available delivery records. CLs were measured transvaginally by staff sonographers who were educated in the context of multicenter trials. All ultrasounds were read by an attending sonologist. If multiple CL ultrasounds were performed on an individual patient, we incorporated the measurement taken closest to 20 weeks of gestation for analysis. A short cervix was defined as ≤2.5 cm. During the study period, a transvaginal CL assessment was a routine part of the fetal anatomic survey.

Medical records of all women who underwent transvaginal CL screening were identified and reviewed. Demographic characteristics and baseline clinical data including maternal age, race/ethnicity, body mass index at delivery, tobacco use, mode of conception, and obstetric history were abstracted. Any history of cervical dysplasia was a prompted field on the obstetric admission history and physical electronic medical record during the study period. This field is typically filled out based on information present in the patient’s prenatal records. Dysplasia was defined as any lesion requiring colposcopic evaluation or any notation of an abnormal Pap smear if no other documentation was available. Women were stratified into 3 groups according to their history of cervical dysplasia (ie, no dysplasia, dysplasia alone, or dysplasia with excisional procedure). Gestational age at delivery was recorded and a PTB was defined as a gestational age at delivery <37 weeks. Charts of patients with missing data were rereviewed and, if the variable continued to be missing, the individual variable was omitted from analysis.

Women were stratified by dysplasia and cervical excision procedure history, and groups compared using χ ² or analysis of variance analyses, as appropriate. Variables that significantly differed by exposure ( P < .05) were eligible for entry into a multivariable logistic regression equation that was constructed using backward elimination.

The first regression excluded the variable “short cervix” to assess whether either prior dysplasia alone or a prior excisional procedure were associated with PTB. “Short cervix” was then added to the model to determine whether dysplasia with or without an excisional procedure remained independently associated with PTB. Interaction terms were created between short cervix and either prior dysplasia alone or prior excisional procedure and were evaluated in the regression model as well.

Analyses were performed using Stata, version 11.1 (StataCorp, College Station, TX). All tests were 2-tailed and a P < .05 was used to define statistical significance. Approval for this study was obtained from the Northwestern University Institutional Review Board with a waiver of informed consent.

Results

Of the 18,528 women who met inclusion criteria, 14,149 (76.4%) had no history of dysplasia, 3023 (16.3%) had a history of dysplasia without an excisional procedure, and 1356 (7.3%) had a prior cervical excisional procedure. Women with prior dysplasia (with or without a prior excisional procedure) were older, were more likely to be non-Hispanic white, and were more likely to have a prior PTB ( Table 1 ). Women with a prior excisional procedure were more likely to have conceived by assisted reproduction.

The mean CLs for women without prior dysplasia, with prior dysplasia alone, or with a prior excisional procedure were significantly different, with the shortest mean length in women with a prior excisional procedure ( Table 2 ). Similarly women with a prior excisional procedure had the highest frequency of a short cervix. The mean gestational age at delivery was different between the different exposure groups; women with a prior excisional procedure had the earliest gestational age at delivery. Correspondingly, women with a prior excisional procedure had the highest frequency of PTB.

After adjusting for potential confounding factors, a prior excisional procedure (adjusted odds ratio [aOR], 1.38; 95% confidence interval [CI], 1.11–1.73), but not prior dysplasia alone (aOR, 1.02; 95% CI, 0.86–1.22), was associated with PTB ( Table 3 ). This relationship persisted after adjusting for the presence of a short cervix (prior excisional procedure aOR, 1.31; 95% CI, 1.04–1.64; prior dysplasia alone aOR, 1.02; 95% CI, 0.85–1.21). In a third regression, the interaction terms between short cervix and prior dysplasia alone or prior excisional procedure were nonsignificant demonstrating that the presence of a short cervix conveyed similar risk for PTB regardless of a woman’s dysplasia or excisional procedure history. These results did not significantly change when using 2.0 mm as the cut-off to define a short cervix (data not shown).

Table 3

Multivariable analyses for the outcome of preterm birth

Variable	Regression 1 a		Regression 2 b		Regression 3 c
	aOR	95% CI	aOR	95% CI	aOR	95% CI
Non-Hispanic black race	1.90	1.61–2.24	1.87	1.53–2.14	1.80	1.53–2.14
IVF conception	2.38	1.87–3.03	2.34	1.83–2.98	2.33	1.83–2.98
Nulliparous	1.56	1.35–1.81	1.57	1.36–1.82	1.57	1.36–1.81
Prior preterm birth	4.91	4.01–6.01	4.85	1.95–5.95	4.86	3.96–5.96
Dysplasia history
None	1.00	(referent)	1.00	(referent)
Dysplasia alone	1.02	0.86–1.22	1.02	0.85–1.21	1.04	0.87–1.24
Excisional procedure	1.38	1.11–1.73	1.31	1.04–1.64	1.29	1.01–1.63
CL ≤2.5 cm	—	—	5.60	3.85–8.14	5.90	3.72–9.36
Interaction CL ≤2.5 cm + prior dysplasia	—	—	—	—	0.56	0.19–1.65
Interaction CL ≤2.5 cm + excisional procedure	—	—	—	—	1.26	0.47–3.37

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