Being told your placenta is low-lying at the anatomy scan can feel alarming. But in the vast majority of cases, the placenta 'migrates' upward as the uterus grows — and by 32–34 weeks, the issue has resolved.
How Placental Migration Works
The placenta does not literally move. What happens is that the lower uterine segment — the part closest to the cervix — grows disproportionately in the second trimester, pulling the lower edge of the placenta upward relative to the cervix. This apparent upward movement is called migration.
| Placenta Position at 20 Weeks | Likelihood of Resolving by 36 Weeks |
|---|---|
| Posterior, low-lying | ~98–99% |
| Anterior, low-lying | ~90–95% |
| Praevia (covering os) at 20 weeks | ~70–90% |
| Complete praevia (true grade IV) | Unlikely to resolve fully |
Grades of Placenta Praevia
Modern classification uses the distance from the internal cervical os:
- Low-lying: placental edge within 2 cm of the os but not covering it
- Praevia: placental edge at or covering the os
- Complete (major) praevia: placenta covers the os completely
- Distance 2 cm or more from os at 32–34 weeks: vaginal birth is generally safe
Risks of Placenta Praevia
- Antepartum haemorrhage (painless bright red bleeding)
- Requirement for C-section delivery
- Risk of placenta accreta (abnormal deep attachment) — especially with prior C-section
- Neonatal anaemia if there is significant maternal blood loss
Management
A low-lying placenta at 20 weeks is monitored with a repeat scan at 32–34 weeks. If it has migrated away from the os, no further action is needed. If it persists as praevia, pelvic rest (no penetrative sex), hospitalisation near term, and planned C-section are standard management.
Emergency
If you have been told you have a low-lying placenta and you experience vaginal bleeding during pregnancy — even light bleeding — go to the hospital immediately.