Mid-Trimester Cervical Length Screening in Routine Practice: Short-Cervix Prevalence, Preterm Birth and the Shift of the Progesterone Counselling Threshold from 20 to 25 mm in a 15-Year Single-Centre Cohort of 20,168 Singleton Pregnancies

Background/Objectives: Vaginal progesterone reduces preterm birth in women with a short mid-trimester cervix, and the treatment threshold has moved from 15–20 mm to 25 mm; how this change reached everyday practice is rarely documented. We describe cervical length (CL) distribution, short-cervix prevalence, preterm-birth and neonatal outcomes and the evolution of progesterone counselling in routine screening. Methods: Retrospective cohort of singleton pregnancies with a CL measurement at 16+0–24+6 weeks in one Romanian fetal-medicine centre (2011–2026). CL was measured transabdominally and, when <30 mm, transvaginally (FMF criteria). Documented counselling was extracted from the structured report text by keyword search; delivery outcome was available for 20.3% of pregnancies, so outcome analyses describe the outcome-ascertained subgroup. Results: Among 20,168 pregnancies, median CL was 35.8 mm; CL was <25 mm in 1.92% (95% CI 1.74–2.12) and <15 mm in 0.60%, and five times more frequent after a previous late miscarriage or preterm birth. In 4087 pregnancies with known delivery, birth < 34 weeks rose from 1.9% at CL ≥ 35 mm to 26.1% at <15 mm; CL < 25 mm carried an adjusted odds ratio of 9.1 (4.6–17.9) for birth < 34 weeks and birth weight < 1500 g in 8/92 (8.7%) vs. 41/3894 (1.1%), yet identified only 12% of births < 34 weeks (AUC 0.59); 88% (95% CI 79–93%) of births < 34 weeks followed a CL ≥ 25 mm. An explicit progesterone mention at 20–24.9 mm rose from 49.2% in 2011–2016 to 73.3% in 2017–2026 (p = 0.002); a data-driven change-point search located the shift in 2017–2019 (81% of clustered bootstrap samples), as a level change without trend, and the period effect persisted after adjustment for population composition (OR 5.0, 2.3–10.9). Before 2017 the operational threshold was ≈22 mm. Conclusions: A short cervix is rare but carries nine-fold adjusted odds of early preterm birth; screening identifies a small very-high-risk group rather than most preterm births. The structured report captured, in real time, the migration of documented counselling to the full ≤25 mm range between 2017 and 2019; neonatal findings are exploratory.