No. 257-Ultrasonographic Cervical Length Assessment in Predicting Preterm Birth in Singleton Pregnancies

Assessing the multifaceted cervix: examining cervical gland area at cervical length screening to predict spontaneous preterm birth

BACKGROUND

Biologically active cervical glands provide a mucous barrier while influencing the composition and biomechanical strength of the cervical extracellular matrix. Cervical remodeling during ripening may be reflected as loss of the sonographic cervical gland area. As sonographic cervical length remains suboptimal for universal screening, adjunctive evaluation of other facets of the mid-trimester cervix may impart additional screening benefit.

OBJECTIVE

To sonographically assess the cervical gland area at universal cervical length screening for preterm birth.

STUDY DESIGN

We performed a retrospective cohort study of singletons with transvaginal cervical length screening universally performed during anatomic survey between 18 0/7 and 23 6/7 weeks and subsequent live delivery at a single institution in 2018. Uterine anomalies, cerclage, suboptimal imaging, or medically indicated preterm birth were excluded. Ultrasound images were assessed for cervical length and cervical gland area (with quantitative measurements when present). The primary outcome was spontaneous preterm birth <37 weeks. Absent and present gland groups were compared using χ2, Fisher's exact, T-test, and multivariate logistic regression (adjusting for parity and progesterone use, as well as the gestational age, cervical length, and gland absence at screening ultrasound). Gland measurements were evaluated using the Mann-Whitney-U Test and Spearman's correlation.

RESULTS

Among the cohort of 772 patients, absent and present CGA groups were overall similar. Patients were on average 33 years old, ∼20 weeks gestation at screening ultrasound, and overall, 2.5% had history of prior spontaneous preterm birth. The absent gland group was more likely to have been taking progesterone (17% vs 4%, P=.04). Overall rate of preterm birth was 2.6%. However, the 2.3% of patients with absent cervical gland area were significantly more likely to deliver <37 weeks (aOR 23.9, 95% CI 6.4-89, P<.001). Multivariate logistic regression demonstrated better performance of a cervical length screening model for preterm birth prediction with the addition of qualitative gland evaluation (P<.001). Qualitative gland assessment was reproducible (PABAK 0.89), but quantitative gland measurements did not correlate with preterm birth.

CONCLUSION

Qualitative gland absence at mid-gestation cervical length screening was associated with subsequent spontaneous preterm birth, whereas quantitative gland measurements were not. Multifaceted ultrasound screening may be needed to adequately evaluate the multiple biologic functions of the cervix.

Prediction and prevention of preterm birth: Quality assessment and systematic review of clinical practice guidelines using the AGREE II framework

BACKGROUND

Prediction of pregnancies at risk of preterm birth (PTB) may allow targeted prevention strategies.

OBJECTIVES

To assess quality of clinical practice guidelines (CPGs) and identify areas of agreement and contention in prediction and prevention of spontaneous PTB.

SEARCH STRATEGY

We searched for CPGs regarding PTB prediction and prevention in asymptomatic singleton pregnancies without language restriction in January 2024.

SELECTION CRITERIA

CPGs included were published between July 2017 and December 2023 and contained statements intended to direct clinical practice.

DATA COLLECTION AND ANALYSIS

CPG quality was assessed using the AGREE-II tool. Recommendations were extracted and grouped under domains of prediction and prevention, in general populations and high-risk groups.

MAIN RESULTS

We included 37 CPGs from 20 organizations; all were of moderate or high quality overall. There was consensus in prediction of PTB by identification of risk factors and cervical length screening in high-risk pregnancies and prevention of PTB by universal screening and treatment for asymptomatic bacteriuria, screening and treatment for BV in high-risk pregnancies, and use of preventative progesterone and cerclage. Areas of contention or limited consensus were the role of PTB clinics, universal cervical length measurement, biomarkers and cervical pessaries.

CONCLUSIONS

This review identified strengths and limitations of current PTB CPGs, and areas for future research.

Diagnosis or prognosis? An umbrella review of mid-trimester cervical length and spontaneous preterm birth

BACKGROUND

Cervical length is widely used to assess a woman's risk of spontaneous preterm birth (SPTB).

OBJECTIVES

To summarise and critically appraise the evidence from systematic reviews on the prognostic capacity of transvaginal sonographic cervical length in the second trimester in asymptomatic women with singleton or twin pregnancy.

SEARCH STRATEGY

Searches were performed in Medline, Embase, CINAHL and grey literature from 1 January 1995 to 6 July 2021, including keywords 'cervical length', 'preterm birth', 'obstetric labour, premature', 'review' and others, without language restriction.

SELECTION CRITERIA

We included systematic reviews including women who did not receive treatments to reduce SPTB risk.

DATA COLLECTION AND ANALYSIS

From 2472 articles, 14 systematic reviews were included. Summary statistics were independently extracted by two reviewers, tabulated and analysed descriptively. The ROBIS tool was used to evaluate risk of bias of included systematic reviews.

MAIN RESULTS

Twelve reviews performed meta-analyses: two were reported as systematic reviews of prognostic factor studies, ten used diagnostic test accuracy methodology. Ten systematic reviews were at high or unclear risk of bias. Meta-analyses reported up to 80 combinations of cervical length, gestational age at measurement and definition of preterm birth. Cervical length was consistently associated with SPTB, with a likelihood ratio for a positive test of 1.70-142.

CONCLUSIONS

The ability of cervical length to predict SPTB is a prognostic research question; systematic reviews typically analysed diagnostic test accuracy. Individual participant data meta-analysis using prognostic factor research methods is recommended to better quantify how well transvaginal ultrasonographic cervical length can predict SPTB.

Cervical length screening for predicting preterm birth: A comparative review of guidelines

A cervical length (CL) assessment may predict preterm birth (PTB). This study aimed to analyze and compare the recommendations of guidelines on the role of CL in the prediction of PTB. There is no consensus regarding universal screening of asymptomatic women without a history of prior spontaneous PTB (sPTB), using CL. On the other hand, CL assessment is recommended in cases with a history of sPTB due to the high recurrence rate. Finally, there is discrepancy regarding CL assessment in asymptomatic women with multiple pregnancy. Although far from perfect, CL measurement remains the best available method to predict PTB.

Transperineal ultrasound in routine uterine cervix measurement

PURPOSE

Transvaginal ultrasound (TVUS) is used in routine practice to evaluate cervical length (CL). This technique is nevertheless invasive and often viewed as uncomfortable, which is less the case with transperineal ultrasound (TPUS). This study was conducted in light of recent technological improvements in the ultrasound field to evaluate whether TPUS could be used as an alternative to TVUS in CL assessment.

METHODS

This was a prospective single-blind study. Pregnant women requiring CL measurement during their emergency consultation were offered a second assessment by TPUS after an initial TVUS. TPUS was performed by a third-year OBGYN resident, unaware of the CL measurement obtained via TVUS.

RESULTS

Seventy-three women were included. The mean ∂ was 0.59 mm. The interclass Pearson correlation coefficient between the two techniques was 0.8987 (95% CI [0.8429; 0.9353]). None of the tested factors were found to be associated with a difference between TPUS and TVUS CL measurements. ROC curve analysis indicated that a transperineal CL cut-off measurement of 24.9 mm was predictive of a transvaginal CL measurement below 25 mm. This threshold enabled a 95% sensitivity [75.1-99.9%] and a 100% specificity [93.3-100%] for the TPUS CL measurement technique.

CONCLUSION

TPUS should be acknowledged as a reliable alternative to TVUS for CL assessment in routine every day practice.

Cervical length distribution among Brazilian pregnant population and risk factors for short cervix: A multicenter cross-sectional study

Objective

Since there are populational differences and risk factors that influence the cervical length, the aim of the study was to construct a populational curve with measurements of the uterine cervix of pregnant women in the second trimester of pregnancy and to evaluate which variables were related to cervical length (CL) ≤25 mm.

Materials and methods

This was a multicenter cross-sectional study performed at 17 hospitals in several regions of Brazil. From 2015 to 2019, transvaginal ultrasound scan was performed in women with singleton pregnancies at 18 0/7 to 22 6/7 weeks of gestation to measure the CL. We analyzed CL regarding its distribution and the risk factors for CL ≤25 mm using logistic regression.

Results

The percentage of CL ≤ 25mm was 6.67%. Shorter cervices, when measured using both straight and curve techniques, showed similar results: range 21.0–25.0 mm in straight versus 22.6–26.0 mm in curve measurement for the 5^th percentile. However, the difference between the two techniques became more pronounced after the 75^th percentile (range 41.0–42.0 mm straight x 43.6–45.0 mm in curve measurement). The risk factors identified for short cervix were low body mass index (BMI) (OR: 1.81 CI: 1.16–2.82), higher education (OR: 1.39 CI: 1.10–1.75) and personal history ([one prior miscarriage OR: 1.41 CI: 1.11–1.78 and ≥2 prior miscarriages OR: 1.67 CI: 1.24–2.25], preterm birth [OR: 1.70 CI: 1.12–2.59], previous low birth weight <2500 g [OR: 1.70 CI: 1.15–2.50], cervical surgery [OR: 4.33 CI: 2.58–7.27]). By contrast, obesity (OR: 0.64 CI: 0.51–0.82), living with a partner (OR: 0.76 CI: 0.61–0.95) and previous pregnancy (OR: 0.46 CI: 0.37–0.57) decreased the risk of short cervix.

Conclusions

The CL distribution showed a relatively low percentage of cervix ≤25 mm. There may be populational differences in the CL distribution and this as well as the risk factors for short CL need to be considered when adopting a screening strategy for short cervix.

Depolarization imaging for fast and non-invasive monitoring of cervical microstructure remodeling in vivo during pregnancy

The cervix plays a crucial role in conception, maintenance of pregnancy, and childbirth. The mechanical properties of a pregnant woman's cervix change dramatically during gestation due to a remodeling of its microstructure, necessary for delivery. However, external factors can accelerate this process and lead to prematurity, the primary cause of perinatal mortality worldwide, due to the inefficiency of existing diagnostic methods. This study shows that polarized light is a powerful tool to probe the cervical microstructure during pregnancy. A wide-field multispectral polarimetric imaging system was fabricated to explore in vivo the cervix of full-term pregnant women. The polarimetric properties of the cervix change significantly with pregnancy progression. In particular, a set of several depolarization parameters (intrinsic and extrinsic) showed a strong linear correlation with gestational age in the red part of the visible spectral range. This trend can be attributed, among other things, to a decrease in collagen density and an increase in hydration of cervical connective tissue. Wide field depolarization imaging is a very promising tool for rapid and non-invasive analysis of cervical tissue in vivo to monitor the steady progression of pregnancy, providing the practitioner with useful information to improve the detection of preterm birth.

Nomogram Incorporating Multimodal Transvaginal Ultrasound Assessment at 20 to 24 Weeks' Gestation for Predicting Spontaneous Preterm Delivery in Low-Risk Women

Background

The majority of women who experience spontaneous preterm delivery (SPTD) have low-risk, asymptomatic pregnancies with a cervical length (CL) ≥25mm and no clear risk factors. Despite the fact that cervical elastography is a potential tool for predicting SPTD, there is currently no feasible solution to make a reliable prediction for preventing SPTD.

Objective

The aim of this study was to construct a nomogram including multimodal transvaginal ultrasound parameters during the second trimester to predict SPTD in low-risk women.

Methods

This multi-center study enrolled 1260 women with singleton pregnancies between 20 and 24 weeks’ gestation. CL and cervical elastography data were obtained when they were undergoing the second-trimester anomaly scan. Univariate and multivariate Logistic regression were utilized to screen predictors independently related to SPTD from the maternal characteristics and multimodal ultrasound data. Then construct a nomogram to determine the likelihood of SPTD in pregnant women.

Results

A total of 66 pregnancies in the training cohort (7.8%, 66/842) and 37 pregnancies (8.9%, 37/418) in the validation cohort ended in SPTD. Age, uterine curettage, CL, and strain in the anterior lip of internal os were the independent predictors of SPTD (P < 0.001, < 0.001, = 0.007, and < 0.001, respectively). These predictors constituted a nomogram to predict the probability of SPTD for a pregnant woman in her second trimester. It showed good discrimination (C-index = 0.898 and 0.839), calibration (P = 0.258 and 0.115), and yielded net benefits both in the training and validation cohorts.

Conclusion

The nomogram including data of multimodal transvaginal ultrasound at 20 to 24 weeks’ gestation is expected to identify women with SPTD in the low-risk, asymptomatic population.

Prevention of Preterm Birth with Progesterone

Gestational age at birth is a critical factor for perinatal and adulthood outcomes, and even for transgenerational conditions' effects. Preterm birth (PTB) (prematurity) is still the main determinant for infant mortality and morbidity leading cause of infant morbidity and mortality. Unfortunately, preterm birth (PTB) is a relevant public health issue worldwide and the global PTB rate is around 11%. The premature activation of labor is underlined by complex mechanisms, with a multifactorial origin influenced by numerous known and probably unknown triggers. The possible mechanisms involved in a too early labor activation have been partially explained, and involve chemokines, receptors, and imbalanced inflammatory paths. Strategies for the early detection and prevention of this obstetric condition were proposed in clinical settings with interesting results. Progesterone has been demonstrated to have a key role in PTB prevention, showing several positive effects, such as lower prostaglandin synthesis, the inhibition of cervical stromal degradation, modulating the inflammatory response, reducing gap junction formation, and decreasing myometrial activation. The available scientific knowledge, data and recommendations address multiple current areas of debate regarding the use of progesterone in multifetal gestation, including different formulations, doses and routes of administration and its safety profile in pregnancy.

Prediction and Prevention of Spontaneous Preterm Birth: ACOG Practice Bulletin, Number 234

Preterm birth is among the most complex and important challenges in obstetrics. Despite decades of research and clinical advancement, approximately 1 in 10 newborns in the United States is born prematurely. These newborns account for approximately three-quarters of perinatal mortality and more than one half of long-term neonatal morbidity, at significant social and economic cost (1-3). Because preterm birth is the common endpoint for multiple pathophysiologic processes, detailed classification schemes for preterm birth phenotype and etiology have been proposed (4, 5). In general, approximately one half of preterm births follow spontaneous preterm labor, about a quarter follow preterm prelabor rupture of membranes (PPROM), and the remaining quarter of preterm births are intentional, medically indicated by maternal or fetal complications. There are pronounced racial disparities in the preterm birth rate in the United States. The purpose of this document is to describe the risk factors, screening methods, and treatments for preventing spontaneous preterm birth, and to review the evidence supporting their roles in clinical practice. This Practice Bulletin has been updated to include information on increasing rates of preterm birth in the United States, disparities in preterm birth rates, and approaches to screening and prevention strategies for patients at risk for spontaneous preterm birth.

The perceptions of pregnant women about cervical length screening for preterm birth prevention

BACKGROUND

The Western Australian Preterm Birth Prevention Initiative was officially launched in November 2014. It demonstrated an initial decrease in the preterm birth rate in Western Australia. One of the key points of this initiative is the routine ultrasound measurement of the cervical length in mid-pregnancy.

AIM

The aim of this study was to evaluate the perceptions and attitudes of Western Australian women regarding transvaginal cervical length (TVCL) screening for preterm birth prevention.

MATERIALS AND METHODS

Self-administered questionnaires were completed by pregnant women before and after their mid-trimester obstetric ultrasound. Maternal demographical and medical data were collected in addition to opinions of TVCL screening.

RESULTS

A total of 598 women participated with the maternal age range representative of the Western Australian obstetric population. There was a high rate of acceptance of TVCL imaging, with 2% of the 149 women declining a transvaginal ultrasound (TVU) offered in this study, 1.5% refusing a TVU in the past and 5.7% reporting that they would have a concern in having a TVU in the future. Women stated that they either had declined TVCL in the past (11%) or would decline in the future (30.7%) as they wanted to discuss the procedure with their clinician first, had been advised not to have a TVU by their clinician (11% in the past and 47.7% in the future) or felt uncomfortable with the operator (22% at a previous TVU and 34.4% in the future).

CONCLUSION

Pregnant women participating in this study had a high acceptance of TVCL screening for preterm birth prevention.

It is about time. The advantage of transabdominal cervical length screening

PURPOSE

Sonographic cervical length (CL) measurement has become a standard practice for identifying women at increased risk for preterm birth. We aimed to assess the time needed for CL measurement during fetal anatomy scan using either transabdominal or transvaginal ultrasound, and to provide a predictive value of the transabdominal CL measurements for reassuring transvaginal CL measurements of more than 25 mm.

METHODS

In a prospective study we measured CL transabdominally, and transvaginally in all the anatomy scans performed at 18-25 weeks' gestation, from January 2017 until January 2018. CL and the duration of the examination were compared between the two approaches. Adjustments were made for, body mass index, previous cesarean sections and parity.

RESULTS

Four hundred sixty-eight patients participated in the study. The time needed for the transabdominal CL measurement was significantly lower compare with the transvaginal CL measurement (0.46 ± 0.3 min versus 6.9 ± 1.38 min, p < 0.001). The CL measured transabdominally was significantly shorter than the CL measured transvaginally (36.7 ± 6.7 mm versus 41.9 ± 7.3 mm, respectively p < 0.001). All women with transabdominal CL measurements above 36 mm (64.2% of patients) had a transvaginal CL of more than 25 mm.

CONCLUSION

Transvaginal measurement of CL is a time-consuming procedure that prolong fetal anomaly scan by 25%. Transabdominal CL measurement of 36 mm is reassuring CL above 25 mm. Transabdominal ultrasound should be used as an initial tool for CL screening. Transvaginal measurements of CL should be reserved for high-risk women, for women with difficulties in demonstrating the cervix abdominally, and for women with transabdominal measurements of less than 36 mm.

Cervical Length as a Predictor of Latency to Labour in Twin Pregnancies Complicated by Preterm Pre-Labour Rupture of Membranes: A Retrospective Study

OBJECTIVE

Preterm pre-labour rupture of membranes (PPROM), one of the leading causes of preterm delivery, represents a serious economic and psychosocial health care burden. The unpredictability of the interval between PPROM and labour, also known as the latency period, can be unsettling for patients and clinicians alike. In singleton pregnancies, the literature supports the use of ultrasound-determined cervical length to estimate the latency period.

METHODS

With this retrospective cohort study, we sought to determine whether a shorter cervical length (≤25 vs. >25 mm) is associated with a shorter latency period in twin pregnancies complicated by PPROM. At McMaster University Medical Centre, a tertiary care centre, we used the Better Outcomes Registry Network (BoRN) database to identify patient records that met our criteria for inclusion over a 5-year period.

RESULTS

Forty-three records were included in our analysis. On average, shorter cervical length was statistically significantly associated with a 6-day shorter latency period (P = 0.035). Confounders were controlled for using ANCOVA statistical analysis. A moderate, positive correlation between cervical length and latency period was also identified (r = 0.4).

CONCLUSION

Understanding that a shorter cervical length is associated with a shorter latency period can assist clinicians in optimizing maternal counselling and patient care, including triaging transfers to tertiary care centres.

Cervical length at 31-34 weeks of gestation: transvaginal vs. transperineal ultrasonographic approach

Objectives To investigate the correlation between transperineal (TP) and transvaginal (TV) ultrasonography in the measurement of cervical length (CL) in the third trimester of pregnancy. Methods In this prospective study, CL measurements were conducted in women between 31 and 34 weeks of gestation by both the TP and TV approaches. A comparison of the measurements was made between the two techniques. The Pearson correlation coefficient, the paired samples t-test and Bland-Altman plot were used. Results In total, 240 women participated in the study. The CL was successfully measured transvaginally in all cases and transperineally in 229 (95.4%) of the cases. The mean TV CL was 32.8 ± 8.2 mm and the mean TP CL was 32.5 ± 8.1 mm. In the 229 cases with paired measurements, the Pearson correlation coefficient was 0.964. No significant differences in mean CL were identified between the two methods (t = 1.805; P = 0.072). In the Bland-Altman plot, the difference between the paired means was 0.26 mm and the 95% tolerance interval for any given paired observation (TV CL-TP CL) was -4.05 to 4.57 mm. Conclusion The findings of this study suggest that at 31-34 weeks of gestation the cervix can be visualized adequately by both the TV and TP sonography in about 95% of cases. The TP CL measurements demonstrate a close correlation with the TV measurements. TP ultrasound is a feasible alternative, especially in cases where the use of the vaginal transducer should be minimized or is not acceptable by women.

Effect of QUiPP prediction algorithm on treatment decisions in women with a previous preterm birth: a prospective cohort study

OBJECTIVE

The QUiPP algorithm combines cervical length, quantitative fetal fibronectin (qfFN) and medical history to quantify risk of preterm birth. We assessed the utility of QUiPP to inform preterm birth prevention treatment decisions.

DESIGN

A prospective cohort study with a subsequent impact assessment using the QUiPP risk of birth before 34 weeks' gestation.

SETTING

A UK tertiary referral hospital.

SAMPLE

In all, 119 women with previous spontaneous preterm birth (sPTB) or preterm premature rupture of membranes (PPROM) before 34 weeks' gestation.

METHODS

Cervical length and qfFN were measured at 19⁺⁰ to 23⁺⁰  weeks' gestation. Clinical management was based on history and cervical length. After birth, clinicians were unblinded to qfFN results and QUiPP analysis was undertaken.

MAIN OUTCOME MEASURES

Predictive statistics of QUiPP algorithm using 10% risk of sPTB before 34⁺⁰  weeks as treatment threshold.

RESULTS

Fifteen of 119 women (13%) had PPROM or sPTB before 34 weeks. Of these, 53% (8/15) had QUiPP risk of sPTB before 34⁺⁰  weeks above 10%. Applying this treatment threshold in practice would have doubled our treatment rate (20 versus 42%). QUIPP threshold of 10% had positive likelihood ratio (LR) of 1.3 (95% CI 0.76-2.18), and negative LR of 0.8 (95% CI 0.45-1.40) for predicting sPTB before 34⁺⁰  weeks.

CONCLUSIONS

Use of the QUiPP algorithm in this population may lead to substantial increase in interventions without evidence that currently available treatment options are beneficial for this particular group.

TWEETABLE ABSTRACT

Independent study finds that the QUiPP algorithm could lead to substantial increases in treatment without evidence of benefit.

Factors associated with the timing of the first prenatal ultrasound in Canada

Background

The aim of this study was to investigate the factors associated with the timing of the first prenatal ultrasound in Canada.

Methods

This was a secondary data analysis of the Maternity Experiences Survey, a cross-sectional survey covering different aspects of pregnancy, labour, birth and the post-partum period. Bivariate and multivariate multinomial logistic regressions were performed to assess the relationship between timing of first prenatal ultrasound and different independent variables.

Results

68.4% of Canadian women received an optimally timed first prenatal ultrasound, 27.4% received early ultrasounds and 4.3% received late ultrasound. The highest prevalence of early ultrasound was in Ontario (33.3%) and the lowest was in Manitoba (13.3%). The highest prevalence of late ultrasound was found in Manitoba (12.1%) and the lowest was in British Columbia and Ontario (3.5% each). The highest prevalence of optimal timing of first prenatal ultrasound was in Quebec (77%) and the lowest was in Ontario (63.2%). Factors influencing the timing of ultrasound included: Early – maternal age < 20 (adjusted OR = 0.54, 95%CI:0.34–0.84), alcohol use during pregnancy (adjusted OR = 0.69, 95%CI:0.53–0.90), history of premature birth (adjusted OR = 1.41, 95%CI:1.06–1.89), multiparity (adjusted OR = 0.67, 95%CI:0.57–0.78), born outside of Canada (adjusted OR = 0.82, 95%CI:0.67–0.99), prenatal care in Newfoundland and Labrador (adjusted OR = 1.66, 95%CI:1.20–1.30), Nova Scotia (adjusted OR = 1.68, 95%CI:1.25–2.28), Ontario (adjusted OR = 2.16, 95%CI:1.76–2.65), Saskatchewan (adjusted OR = 1.50, 95%CI:1.05–2.14), Alberta (adjusted OR = 1.37, 95%CI:1.05–1.77) British Columbia (adjusted OR = 1.90, 95%CI:1.45–2.50) and Manitoba (adjusted OR = 0.66, 95%CI:0.45–0.98) Late – unintended pregnancy (adjusted OR = 1.89, 95%CI:1.38–2.59), born outside of Canada (adjusted OR = 1.75, 95%CI:1.14–2.68), prenatal care in Manitoba (adjusted OR = 2.88, 95%CI:1.64–5.05) and the Territories (adjusted OR = 4.50, 95%CI:2.27–8.93). An interaction between history of miscarriage and having ‘other’ prenatal care provider significantly affected timing of ultrasound (adjusted OR = 0.31, 95%CI:0.14–0.66).

Conclusion

Only 68% of Canadian women received an optimally timed prenatal ultrasound which was influenced by several factors including province of prenatal care, maternal age and country of birth, and an interaction effect between prenatal care provider and history of miscarriage. These findings establish a baseline of factors influencing the timing of prenatal ultrasound in Canada, which can be built upon by future studies.