Cervical Length Percentile Calculator

The cervix is normally a firm, closed cylinder that shortens and softens only as term approaches, and premature loss of that length is among the earliest measurable steps on the pathway to spontaneous preterm birth. Transvaginal ultrasound measures the endocervical canal from internal os to external os with the probe in the anterior fornix under minimal pressure, since pressure artificially lengthens the image; three measurements are taken over roughly five minutes and the shortest is recorded. Transabdominal scanning overestimates by about 5 to 10 mm and is inadequate for screening. Median length is near 40 mm at 16 to 20 weeks, 35 mm at 24 weeks, and 28 mm by 32 weeks, so any percentile must be read against gestational age rather than a single number. Risk rises steeply rather than linearly as the cervix shortens. Around 25 mm at 24 weeks, roughly the tenth percentile, the risk of delivery before 34 weeks is on the order of 10 to 15 percent; near 15 mm it is 25 to 40 percent; below 10 mm it exceeds half. The measurement predicts risk in populations far better than it predicts timing in an individual, with positive predictive values of only about 25 to 60 percent depending on threshold and population. Twins run 3 to 5 mm shorter at the same gestational age, and interventions do not transfer between groups: vaginal progesterone has support in singletons with a short cervix, while cerclage has not shown benefit in twins and may cause harm.

Assessment of the cervix in pregnancy was digital until transvaginal probes of the late 1980s made the internal os reliably visible. The defining study came from Jay Iams and the National Institute of Child Health and Human Development Maternal-Fetal Medicine Units Network, published in the New England Journal of Medicine in 1996: cervical length was measured in 2,915 women at 24 weeks, the distribution proved roughly normal with a median near 35 mm, and the risk of preterm delivery rose continuously as length fell, with 25 mm marking approximately the tenth percentile. That paper converted the cervix from a qualitative examination finding into a graded risk variable. Mechanical treatment predated the measurement: V. N. Shirodkar described his cerclage in 1955 and Ian McDonald his simpler purse-string suture in 1957, both for women with recurrent mid-trimester loss. Pharmacological prevention arrived after the ultrasound work, with Eduardo Fonseca and colleagues reporting in the New England Journal of Medicine in 2007 that vaginal progesterone reduced preterm birth in women with a short cervix, and the PREGNANT trial led by Sonia Hassan confirming the effect in 2011. Meta-analyses by Roberto Romero and colleagues, including an individual-patient-data analysis published in 2018, consolidated the evidence, and ACOG Practice Bulletin 234, issued in 2021, set out current screening and treatment recommendations.