Symptomatic · log-normal survival
Risk of spontaneous preterm birth
Color: <1% low · 1–4.9% mild · 5–9.9% moderate · ≥10% high · N/A past threshold
A unified spontaneous preterm birth (sPTB) risk calculator covering two distinct clinical populations. Select Symptomatic for women presenting with threatened preterm labour symptoms (23+0–34+6 weeks) or Asymptomatic for high-risk women in a surveillance clinic (18+0–36+6 weeks). The tool calculates conditional probabilities of delivery before 30, 34, and 37 weeks, and within 1, 2, and 4 weeks of testing.
A 2×3 colour-coded risk grid: rows = gestational thresholds (<30, <34, <37 weeks) and time windows (within 1, 2, 4 weeks); columns = the selected test method. Cells are colour-coded: <1% low · 1–4.9% mild · 5–9.9% moderate · ≥10% high. Cells already past a gestational threshold are shown as N/A. Colour bands are intended as a reading aid, not clinical decision thresholds.
Symptomatic — Log-normal survival model (Carter et al. 2020). Available methods: CL + qfFN, CL only, qfFN only. History term used by the equation: twin pregnancy, previous cervical surgery, previous PPROM, or previous preterm birth ≤ 36+6. Outputs: risk <30, <34, <37 weeks; within 1, 2, 4 weeks.
Asymptomatic — Log-logistic survival model (Watson et al. 2020). Available methods: CL + qfFN, CL only, qfFN only. Risk factors in the equations: twin pregnancy, previous sPTB, previous late miscarriage. Outputs: risk <30, <34, <37 weeks; within 1, 2, 4 weeks.
The calculators estimate conditional probability from the day of testing, assuming the pregnancy has continued to that date. Cells labelled <30, <34, <37 weeks give the probability of spontaneous preterm birth before that gestational threshold. Cells labelled within 1, 2, or 4 weeks give the probability of delivery within that interval. Cells already past a gestational threshold are marked N/A.
Symptomatic — Implemented from Appendix S1 of Carter et al. 2020. Uses Method 4 (CL + qfFN), Method 5 (CL only), and Method 6 (qfFN only). Asymptomatic — Implemented from Appendix S1 of Watson et al. 2020. Uses Methods 1–3 for CL + qfFN, CL only, and qfFN only. Neither model is valid for triplets or higher-order multiples.