Description
Placenta praevia is a condition in pregnancy where the placenta implants in the lower uterine segment, sometimes covering the cervix entirely or partially. This placement can obstruct the cervical canal, which is critical during the delivery process. It is classified into several types based on the degree of cervical coverage: complete, partial, marginal, and low-lying.
Pathogenesis
Placenta praevia occurs when the placenta attaches to the lower part of the uterus, an area that does not typically provide sufficient space or nutritional support compared to the upper part. Factors contributing to the development of placenta praevia include a history of surgery on the uterus, including cesarean sections, induced abortions, or conditions that increase the endometrial surface area such as multiple gestations or maternal age.
Epidemiology, Risk Factors & Associations
- Occurs in about 1 in 200 pregnancies.
- Risk factors include prior placenta praevia, prior cesarean section, multiple gestation, increased maternal age, increased parity, large placentas (multiple gestations, erythroblastosis), smoking, assisted conception and previous manual removal of placenta.
- Women with placenta praevia are at increased risk for placenta accreta, especially if they have had previous uterine surgery.
Clinical Features
- Painless, bright red vaginal bleeding in the second or third trimester is the hallmark.
- The bleeding can be intermittent and varies in severity, sometimes becoming life-threatening.
- It may be associated with contractions, or none at all.
Complications
- Major haemorrhage during labour, delivery, or in the postpartum period, requiring blood transfusion or hysterectomy.
- Preterm labour and delivery due to bleeding.
- Increased risk for perinatal morbidity and mortality.
- Maternal morbidity associated with placenta accreta, increta, or percreta in subsequent pregnancies.
Pathological Features
Histopathology
- Macroscopic: Examination typically shows a placenta located at the lower uterine segment, potentially covering the cervix.
- Microscopic: No specific microscopic features distinguish it from a normally positioned placenta.
Radiological Features
Ultrasound
- Transvaginal ultrasound is the gold standard for detailed assessment near the cervix, being safer and more accurate in the third trimester. It should be performed with an empty bladder to ensure the best visualisation and to avoid artificially elevating the placenta due to a distended bladder.
- Transabdominal ultrasound can be limited by factors such as maternal obesity, anterior placental position, or a full bladder.
- Colour Doppler: May be used to assess blood flow and differentiate between placental tissues and other structures, providing crucial information for planning delivery.
Grading
- Grade I – Low-lying Placenta:
- The placenta is positioned in the lower uterine segment.
- The lower edge of the placenta is near but does not reach the internal cervical os, specifically located 0.5 to 2.0 cm away from the os.
- This grade often requires follow-up imaging to determine if migration away from the cervix occurs as the pregnancy progresses.
- Grade II – Marginal Praevia:
- The placental tissue reaches the margin of the internal cervical os but does not cover it.
- Marginal praevia involves the edge of the placenta being exactly at the internal os but not extending over it.
- Close monitoring is essential, as with slight changes in the lower uterine segment’s dilation or contraction, bleeding risks can increase.
- Grade III – Partial Praevia:
- The placenta partially covers the internal cervical os.
- This partial coverage can complicate vaginal delivery, depending on the extent of the coverage and the clinical scenario, including the presence of bleeding or labor signs.
- Grade IV – Complete Praevia:
- The placenta completely covers the internal cervical os, obstructing the birth canal.
- This condition usually necessitates delivery via cesarean section because vaginal delivery would be obstructed by the placenta, posing significant risks to both mother and baby.
Diagnosis
Diagnosis is primarily based on ultrasound findings during routine prenatal care or following an episode of vaginal bleeding in the second or third trimester. It is confirmed via transabdominal or more accurately by transvaginal ultrasound, which provides a clear view of the lower uterine segment and cervical os.
Differential Diagnosis
Clinical-based
- Vasa praevia: Vessels traverse the foetal membranes across the cervical os.
- Abruptio placentae: Premature separation of a normally positioned placenta, typically painful bleeding.
- Uterine rupture: Rare but life-threatening event with a history of uterine surgery.
Management
- Conservative: If diagnosed early in pregnancy, many cases of placenta praevia resolve as the uterus expands.
- Active Management: Includes hospitalisation and monitoring for acute bleeds, steroid administration to enhance fetal lung maturity if preterm delivery is anticipated, and planned cesarean delivery, typically after 36 weeks gestation or earlier if significant bleeding occurs.
- Postpartum: Careful monitoring for postpartum hemorrhage.