Placental Abnormalities and Third-Trimester Bleeding - Management of Third Trimester Haemorrhage

Sep 20, 2026

Comprehensive obstetric overview of placental abnormalities associated with third-trimester hemorrhage, focusing on placenta previa, placenta accreta, and vasa previa. Provides definitions, epidemiology, risk factors, and clinical presentation of placenta previa, including timing patterns of bleeding and links to maternal transfusion and adverse perinatal outcomes. Details diagnostic strategy with transabdominal/transvaginal ultrasound and color Doppler, plus accretism and cervical-placental distance evaluation. Outlines management including follow-up imaging schedules and approaches for asymptomatic versus symptomatic cases.

LanguageEnglish

CategoryHealthcare

FormatPDF

Pages25

Reading Time1 h 3 min

AccessFree to download

Placental Abnormalities

Placenta Previa

Clinical Findings

Diagnosis

Management

PLACENTAL ABNORMALITIES (PLACENTA PREVIA, PLACENTA ACCRETA AND VASA PREVIA) MANAGEMENT OF THIRD TRIMESTER HAEMORRHAGE Marta López, Narcís Masoller, Nora Izquierdo, Mariona Genero, Johanna Parra 1. PLACENTA PREVIA The term placenta previa (PP) refers to an abnormal placenta overlying the endocervical os. Placenta previa affects 0.25-0.5% of singleton pregnancies (approximately 4/1000 births) . The risk is higher in the case of previous caesarean sections and increases proportionally with an increasing number of prior caesarean sections. There is also an increased risk in other cases of uterine surgery, such as myomectomy, history of curettage or manual removal of a retained placenta. Other risk factors for placenta previa have been described: maternal age, smoking, black and Asian race, multiparity, multiple gestations, assisted reproductive technology, placenta previa in previous pregnancies, cocaine use and history of embolisation. PP accounts for 20% of third-trimester bleedings. It is a common cause of maternal transfusion and obstetric hysterectomy, and it is associated with significant perinatal morbidity and mortality. Women with PP are almost 10 times more likely to have bleeding during the third trimester. Fetal complications are mainly associated with prematurity. The most common presentation of PP is an incidental finding during the routine secondtrimester ultrasound. Currently, it is diagnosed in 1-6% of pregnancies, but the vast majority of these will not have a placenta lining the internal os in the third trimester (90% resolve before delivery) . In recent years, the terms ‘partial’and ‘marginal’ placenta previa have been deleted. All placentas that overlie the internal cervical os (to any degree) are termed previa and those that extend into the lower uterine segment without covering the os are termed low-lying. This term is usually applied when the placental edge is within 2 cm from the internal cervical os (Figure 1) . Figure 1. A) Placenta previa. B) Low-lying placenta. 1.1 CLINICAL FINDINGS The main symptom of PP is abundant isolated metrorrhagia of bright red blood of maternal origin, associated with uterine contractions in 10-30% of cases. Approximately 30% of patients will have bleeding before 30 weeks, 30% between 30 and 36 weeks and 30% after 36 weeks. Only 10% of patients with PP will remain asymptomatic throughout pregnancy. Although the exact mechanism of bleeding is unknown, it is thought to be associated with the occurrence of uterine contractions, even if subclinical. 1.2 DIAGNOSIS The placental location should be determined during the routine second and third-trimester scans. If PP is suspected, or in the case of bilobed placentas or twin gestations, a transvaginal ultrasound can help confirm or exclude the diagnosis of PP. Ultrasound assessment of placenta previa should include: • Transabdominal ultrasound to evaluate placental territory, insertion of the umbilical cord and ruling out of the presence of haematomas. • Transvaginal ultrasound (performed with partially filled bladder) to evaluate the lower uterine segment: anterior, posterior and lateral (transverse view) . Sometimes it is necessary to push back the fetal presentation. • Colour Doppler ultrasound to rule out vasa previa. • Evaluation of typical signs of placental accretism, especially if there is a history of previous caesarean section. • Evaluation the precise relationship between lower placental edge and internal cervical os (with empty bladder) and cervical length measurement. The distance that the placenta overlies or is away from the os should be reported. 1.3 MANAGEMENT The relationship between the cervix and the placenta changes over time with the placenta typically moving away from the cervix. The optimal time for subsequent imaging in pregnancies with PP has not been well established. It is reasonable to perform a follow-up ultrasound at approximately 28 weeks in pregnancies thought

FAQs

What is placenta previa and how does it affect pregnancy risk?

Placenta previa refers to an abnormal placenta overlying the endocervical os. It affects about 0.25–0.5% of singleton pregnancies and is more common with prior caesarean sections, other uterine surgery, and multiple risk factors.

How is placenta previa diagnosed during pregnancy?

Placental location is assessed during routine second- and third-trimester scans. If suspected, transvaginal ultrasound can confirm or exclude the diagnosis, and ultrasound evaluation should include transabdominal/transvaginal views, color Doppler to rule out vasa previa, signs of accretism, and precise measurement of placental edge distance to the internal cervical os.

How do management and follow-up differ between asymptomatic and symptomatic placenta previa?

Asymptomatic cases with low-lying placenta ≥10 mm from the os generally do not need third-trimester follow-up because migration is likely. If diagnosed at 28 weeks, stable cases may be monitored monthly, while symptomatic patients with bleeding may be managed with observation or hospital admission depending on severity and clinical history.

Sep 20, 2026

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