Cervical Length Reference Range at 28-34 Weeks in Singleton Pregnancies

Background and Rationale Cervical length, measured by transvaginal ultrasound, plays an important role in obstetric care and can change as gestational age advances, particularly during the third trimester. A single threshold commonly used in the second trimester may not accurately reflect the distribution of cervical lengths at later gestational ages. Local data are lacking for interpreting cervical length measurements from 28 weeks 0 days to 33 weeks 6 days.

This study aims to estimate gestational-age-specific cervical length percentiles in a select group of women with singleton pregnancies at My Duc Hospital. Since eligible participants must have had a cervical length greater than 25 mm during the second trimester, the resulting percentiles will apply specifically to this group and should not be interpreted as reference values for all singleton pregnancies.

Study Objectives The primary objective is to estimate gestational-age-specific percentiles for transvaginal cervical length from 28 weeks 0 days through 33 weeks 6 days in singleton pregnancies that have a documented cervical length greater than 25 mm during the second trimester.

The secondary objectives are to:

1. Describe the frequency of cervical lengths of 25 mm or less during the study period.
2. Explore associations between cervical length and specified maternal or pregnancy characteristics.
3. Describe selected pregnancy and newborn outcomes. Study Design and Participants This is a single-centre, prospective, longitudinal observational cohort study. The investigators plan to enrol 294 pregnant women by identifying potential participants consecutively among those scheduled for antenatal visits during the eligible gestational age period.

At enrollment, participants must meet the following criteria: eligible women must have a singleton pregnancy, be between 28 weeks 0 days and 33 weeks 6 days of gestation, and have a documented cervical length measurement greater than 25 mm obtained between 16 weeks 0 days and 24 weeks 0 days. Gestational age will be established based on a first-trimester or early second-trimester ultrasound. Participants must plan to continue antenatal care and deliver at My Duc Hospital and must provide written informed consent.

Exclusion criteria include the following: pregnancies that are currently multiples or have been reduced from a multiple pregnancy to a singleton; known major fetal anomalies or chromosomal abnormalities at enrollment; fetal death before the first study cervical-length measurement; previous cervical conization, cervical amputation, or other major cervical surgeries; uterine or cervical anomalies likely to affect cervical length measurements; cervical cerclage, cervical pessaries, or other mechanical cervical interventions; and any clinical conditions at enrollment that may prevent reliable cervical-length measurement or follow-up in the study.

Additionally, participation in another interventional study that could influence cervical length, pregnancy management, or pregnancy outcomes will also exclude a participant from this study.

Study Procedures Cervical length will be measured using transvaginal ultrasound following a standardized measurement protocol. The sonographer will obtain a mid-sagittal view that includes the cervical canal and the internal and external cervical os. At least three technically acceptable measurements will be taken during each assessment, and the shortest measurement will be recorded. All ultrasound images will be stored in the hospital imaging system.

Each participant may undergo up to three cervical length assessments during the following gestational age windows:

• 28 weeks 0 days to 29 weeks 6 days
• 30 weeks 0 days to 31 weeks 6 days
• 32 weeks 0 days to 33 weeks 6 days Assessments are expected to occur approximately two weeks apart during scheduled antenatal visits. No separate research-only visit is planned.

If multiple assessments occur within the same gestational age window, the assessment closest to the midpoint of that window will be used for analyses grouped by gestational age. Analyses based on exact gestational age will utilize technically acceptable measurements according to the pre-specified statistical analysis plan.

Participants who enrol after one or more assessment windows or who do not complete all three assessments may still contribute any available technically acceptable measurements. Participants with missing information will remain in the cohort and contribute to analyses for which the required data are available. The number of participants and measurements included in each analysis will be reported.

Quality Assurance All participating sonographers will receive training and adhere to the same measurement and data collection procedures. A random sample of stored ultrasound images will be reviewed by a second trained assessor who will be unaware of the original measurements.

The investigators will evaluate assessor agreement using the intraclass correlation coefficient and by measuring absolute differences between measurements. If the study team does not meet the quality targets specified in the protocol, the investigators will review the measurement process, provide additional training, and increase the proportion of images undergoing independent review.

Clinical Care and Follow-Up The study will not determine or assign any clinical treatments. Treating clinicians will decide on the use of progesterone, hospital admission, antenatal corticosteroids, cerclage, pessary, or other clinical management. Relevant treatments and pregnancy complications that occur after enrollment will be recorded as required by the protocol and will be considered in the interpretation of the findings.

After completing the cervical-length assessments, participants will continue to receive routine clinical care. Outcomes related to pregnancy and newborn health will be obtained from hospital medical records post-delivery. Follow-up procedures for outcomes assessed after hospital discharge, including neonatal death within 28 days after birth, will adhere to the protocol-defined data collection process.

Outcome Measures The primary outcome of this study is the gestational-age-specific distribution of cervical length from 28 weeks 0 days to 33 weeks 6 days. Cervical length will be recorded in millimeters as the shortest measurement from at least three technically acceptable transvaginal ultrasound assessments taken at each evaluation.

Secondary outcomes include a cervical length of 25 mm or less, gestational age at delivery, spontaneous and medically indicated preterm births, mode of delivery, birth weight, 5-minute Apgar score, admission to the neonatal intensive care unit (NICU), duration of stay, and specified neonatal and perinatal outcomes.

Sample Size The target for enrollment is 294 participants, with an expectation that around 264 will provide analyzable data, allowing for an approximate 10% loss to follow-up or missing key information.

Each participant may provide measurements in up to three gestational age windows; therefore, the total number of measurements will not necessarily equal three times the number of enrolled participants. The number of participants and measurements contributing to each gestational age window will be reported separately.

Statistical Analysis The investigators will summarise cervical length by exact gestational age and across the three predefined gestational-age windows. The primary analysis will estimate gestational-age-specific cervical-length percentiles. The 2.5th and 97.5th percentiles will define the central 95% reference interval for the selected study population. The investigators will also report the 5th, 10th, 50th, 90th, and 95th percentiles, provided the available data allow sufficient precision.

A specified statistical method will be used to estimate smoothed percentile curves across gestational ages. Analyses involving repeated measurements will take into account the correlation between measurements from the same participant. Confidence intervals for percentile estimates will be calculated using bootstrap resampling at the participant level.

The number of participants and measurements contributing at each gestational age or gestational-age window will be reported. Estimates based on sparse data or those with wide confidence intervals will be interpreted cautiously.

Missing cervical-length assessments will not lead to the automatic exclusion of a participant's available measurements. The primary analysis will utilize all technically acceptable measurements available. Reasons for missing assessments will be summarized, including enrollment after the assessment window, early delivery, withdrawal, loss to follow-up, and clinical conditions preventing measurement. Sensitivity analyses will be conducted if missing data or clinical treatments after enrollment significantly affect the estimated percentiles.

Associations between cervical length and specified maternal or pregnancy characteristics may be evaluated in exploratory models that account for repeated measurements. Pregnancy and newborn outcomes will be reported descriptively. Analyses related to pregnancy outcomes will be considered exploratory and will not constitute the creation or validation of a preterm-birth prediction model.

Interpretation and Generalizability The estimated percentiles will describe a selected population of singleton pregnancies, all of which had a documented cervical length greater than 25 mm during the second trimester and were followed at the same hospital. The results should not be generalized to all singleton pregnancies, nor to those with a second-trimester cervical length of 25 mm or less, or to populations receiving care at other hospitals without external validation.

Furthermore, the requirement for a previous cervical-length measurement and the stipulation that participants plan to continue care and deliver at My Duc Hospital may further limit the generalizability of the findings.

Group/Cohort Description This group includes women with a singleton pregnancy who are enrolled between 28 weeks 0 days and 33 weeks 6 days of gestation, and who have a documented cervical length measurement greater than 25 mm taken between 16 weeks 0 days and 24 weeks 0 days of gestation.

Participants may undergo up to three prospective transvaginal cervical length assessments during the following gestational age windows: 28 weeks 0 days to 29 weeks 6 days, 30 weeks 0 days to 31 weeks 6 days, and 32 weeks 0 days to 33 weeks 6 days. The number of measurements contributed by each participant will depend on the participant's gestational age at enrollment, attendance at scheduled antenatal visits, and the availability of technically acceptable measurements.

Participants will continue to receive routine clinical care, as the study will not assign or change any medical treatment. The investigators will collect pregnancy- and newborn-related outcomes throughout the protocol-defined follow-up period.

Intervention/Exposure This study involves no intervention. This is a prospective observational cohort study, and transvaginal cervical length assessment is being used solely as a measurement, not as a therapeutic intervention.