Placenta praevia !
In placenta praevia the placenta lies low in the uterus, over or near the cervix's opening; it typically causes painless bright red bleeding later in pregnancy.
Updated 2026-10-29
Structures involved
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In histology
In embryology
- Week 1: cleavage, morula, blastocyst and implantation
- Abnormal placentation: previa and the accreta spectrum
- Ionizing radiation, tobacco and cocaine
In the clinical exam
In physiology
Why it happens
What it is
A placenta implanted in the lower uterine segment that covers or lies very close to the internal cervical os. It classically causes painless bright red vaginal bleeding in the second half of pregnancy.
Who gets it
More common after previous caesarean sections, a pattern that matters in Latin America, where caesarean rates are high. Many low placentas seen in mid-pregnancy move away from the cervix as the lower segment grows.
Causes
- Implantation in the lower uterine segment, often where the upper uterus is scarred or poorly vascularised
- Larger placental surface area (multiple pregnancy)
Risk factors
- Previous caesarean section or other uterine surgery
- Previous placenta previa
- High parity and older maternal age
- Multiple pregnancy
- Smoking
- Assisted reproduction
Pathophysiology
As the lower uterine segment stretches and the cervix begins to efface late in pregnancy, the placenta overlying it separates partly from its bed and maternal sinuses bleed. Because the bleeding comes from the maternal side and the uterus is not irritated, it is usually painless. The thin lower segment also contracts poorly after delivery.
Classification
- Placenta previa: the placenta covers the internal os
- Low-lying placenta: its edge lies close to the internal os without covering it (commonly within 2 cm)
- Older terms: complete, partial and marginal previa
- Placenta accreta spectrum may coexist, especially after caesarean
Symptoms and signs
- Sudden painless bright red vaginal bleeding, often after 28 weeks
- A smaller warning (sentinel) bleed may come first
- Soft, non-tender uterus
- Abnormal fetal lie or high presenting part
- Often found on a routine ultrasound before any bleeding
Diagnosis
- Clinical
- No digital vaginal examination until the placental location is known, because it can provoke severe bleeding. A gentle speculum exam may assess the amount and source of bleeding.
- Tests
- Full blood count, blood group and Rh, crossmatch and coagulation studies when bleeding; fetal heart monitoring.
- Imaging
- Transvaginal ultrasound is safe and the most accurate way to locate the placenta. A low placenta at the mid-pregnancy scan is re-checked in the third trimester. Ultrasound or MRI assess for accreta spectrum.
Differential diagnosis
- Placental abruption
- Vasa previa
- Bloody show of labour
- Cervical lesions (ectropion, polyp, cervicitis, cancer)
- Uterine rupture
Goal of treatment
Keep mother and baby stable, prolong pregnancy toward term when it is safe, and plan a caesarean birth with blood available.
Treatment
- Conservative
- Stable, not bleeding: expectant care, pelvic rest (no intercourse or vaginal examinations) and a plan for rapid access to the hospital.
- Acute
- Active bleeding: hospital admission, IV access, blood products as needed and continuous fetal monitoring.
- Medical
- Antenatal corticosteroids when preterm birth is likely; anti-D immunoglobulin for Rh-negative women.
- Surgical
- Caesarean delivery, planned in the late preterm or early term period, or urgent if bleeding is heavy or the fetus is compromised; hysterectomy may be needed with accreta.
Complications
- Severe antepartum and postpartum haemorrhage
- Placenta accreta spectrum and caesarean hysterectomy
- Preterm birth
- Need for blood transfusion
- Malpresentation
Prognosis
With prenatal diagnosis and planned delivery, maternal death is rare and most babies do well; the main fetal risk is prematurity. The risk is higher when accreta spectrum is present.
Prevention
- Avoiding caesarean sections without a medical reason
- Routine placental localisation at the mid-pregnancy scan
Test yourself
Think of the answer, then tap the question to see it.
What is the classic presentation of placenta previa?
Painless bright red vaginal bleeding in the second half of pregnancy with a soft, non-tender uterus.
Which examination is contraindicated until the placental location is known?
Digital vaginal examination, because it can cause severe bleeding.
What is the best test to locate the placenta?
Transvaginal ultrasound, which is safe in placenta previa.
Which serious condition is linked to placenta previa after a previous caesarean?
Placenta accreta spectrum, with a high risk of haemorrhage and hysterectomy.
Related conditions
Educational information, not medical advice. For any symptom, see a health professional.
Sources: Placenta Previa (StatPearls (NCBI Bookshelf), 2023).
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Representative model, for educational purposes only.
