Placenta praevia and low-lying placenta

The referral says "grade 2 anterior praevia" and you are about to make three mistakes at once: that grading system no longer exists, the scan was transabdominal, and it was done at 20 weeks. Getting placental position right is not pedantry — it decides where this woman gives birth, how many units of blood are in the fridge, and whether she keeps her uterus.^[1][18]

Overview and definition — the two-term system

Ultrasound reports used to grade praevia from I to IV and call it marginal, partial or complete. Those words are retired because they described a static picture of a dynamic organ and because they never told the operator what mattered: how far the edge is from the os.^[1][2]

• Placenta praevia — the placenta lies directly over the internal cervical os.^[1][2]
• Low-lying placenta — the placental edge is within 20 mm of the internal os but does not cover it.^[1][2]
• Neither — the edge is more than 20 mm from the os, and the placenta is simply low in the uterus, which is not a diagnosis.^[1]

The commonest technical error is anatomical, not mechanical: mistaking the uterine isthmus for the cervix. A prospective multicentre study showed that deliberately distinguishing the two improves diagnostic accuracy, because measuring from the wrong landmark moves the placenta a whole classification.^[18]

References20ShowHide

1. [1]Jauniaux E, Bhide A, Hussein AM, et al. Placenta Praevia and Placenta Accreta Spectrum: Diagnosis and Management: Green-Top Guideline No. 27a BJOG, 2026.PMID 42374711
2. [2]Jauniaux E, Alfirevic Z, Bhide AG, et al. Placenta Praevia and Placenta Accreta: Diagnosis and Management: Green-top Guideline No. 27a BJOG, 2019.PMID 30260097
3. [3]Silver RM Abnormal Placentation: Placenta Previa, Vasa Previa, and Placenta Accreta Obstet Gynecol, 2015.PMID 26244528
4. [4]Cresswell JA, Ronsmans C, Calvert C, Filippi V Prevalence of placenta praevia by world region: a systematic review and meta-analysis Trop Med Int Health, 2013.PMID 23551357
5. [5]Treffers A, Reynoldson O, Beckmann M Risk Factors for Antepartum Haemorrhage in Women With Placenta Praevia Aust N Z J Obstet Gynaecol, 2025.PMID 39744804
6. [6]Neilson JP Interventions for suspected placenta praevia Cochrane Database Syst Rev, 2003.PMID 12804418
7. [7]Wing DA, Paul RH, Millar LK Management of the symptomatic placenta previa: a randomized, controlled trial of inpatient versus outpatient expectant management Am J Obstet Gynecol, 1996.PMID 8885726
8. [8]Hessami K, Mitts M, Zargarzadeh N, et al. Ultrasonographic cervical length assessment in pregnancies with placenta previa and risk of perinatal adverse outcomes: a systematic review and meta-analysis Am J Obstet Gynecol MFM, 2024.PMID 37778698
9. [9]Balayla J, Wo BL, Bédard MJ A late-preterm, early-term stratified analysis of neonatal outcomes by gestational age in placenta previa: defining the optimal timing for delivery J Matern Fetal Neonatal Med, 2015.PMID 25283850
10. [10]McGoldrick E, Stewart F, Parker R, Dalziel SR Antenatal corticosteroids for accelerating fetal lung maturation for women at risk of preterm birth Cochrane Database Syst Rev, 2020.PMID 33368142
11. [11]Gyamfi-Bannerman C, Thom EA, Blackwell SC, et al. Antenatal Betamethasone for Women at Risk for Late Preterm Delivery N Engl J Med, 2016.PMID 26842679
12. [12]Jauniaux E, Bhide A, Kennedy A, et al. FIGO consensus guidelines on placenta accreta spectrum disorders: Prenatal diagnosis and screening Int J Gynaecol Obstet, 2018.PMID 29405319
13. [13]Jauniaux E, Chantraine F, Silver RM, Langhoff-Roos J FIGO consensus guidelines on placenta accreta spectrum disorders: Epidemiology Int J Gynaecol Obstet, 2018.PMID 29405321
14. [14]Silver RM, Landon MB, Rouse DJ, et al. Maternal morbidity associated with multiple repeat cesarean deliveries Obstet Gynecol, 2006.PMID 16738145
15. [15]Bonanni G, Lopez MC, Tarchi SM, et al. Postpartum Hemorrhage in Patients With a Low-Lying Placenta: A Systematic Review and Meta-analysis Obstet Gynecol, 2025.PMID 40472373
16. [16]Zhang L, Bi S, Chen L, et al. Prophylactic tranexamic acid for the prevention of postpartum haemorrhage in women with placenta praevia: multicentre, double blind, randomised, placebo controlled, phase 3 trial BMJ, 2026.PMID 42128454
17. [17]Morfaw F, Fundoh M, Bartoszko J, Mbuagbaw L, Thabane L Using tocolysis in pregnant women with symptomatic placenta praevia does not significantly improve prenatal, perinatal, neonatal and maternal outcomes: a systematic review and meta-analysis Syst Rev, 2018.PMID 30591076
18. [18]Hasegawa J, Kawabata I, Takeda Y, et al. Improving the Accuracy of Diagnosing Placenta Previa on Transvaginal Ultrasound by Distinguishing between the Uterine Isthmus and Cervix: A Prospective Multicenter Observational Study Fetal Diagn Ther, 2017.PMID 27174433
19. [19]Allen L, Jauniaux E, Hobson S, Papillon-Smith J, Belfort MA FIGO consensus guidelines on placenta accreta spectrum disorders: Nonconservative surgical management Int J Gynaecol Obstet, 2018.PMID 29405317
20. [20]Oyelese Y, Javinani A, Gudanowski B, et al. Vasa previa in singleton pregnancies: diagnosis and clinical management based on an international expert consensus Am J Obstet Gynecol, 2024.PMID 38494071