Placenta Praevia — A Review with Emphasis on the Role of Ultrasound
Authors: Langlois S. Le P. , Miller A.G. 
Published: February 2008

EDITORIAL COMMENT: This paper discusses an important clinical problem, i.e. when does a low‐lying placenta noted on ultrasound merit consideration as a case of placenta praevia, the implication then being that the patient should remain in hospital until delivery. Placenta praevia is a term with clinical implications (maternal risk from haemorrhage, fetal risk from prematurity, need for high Caesarean section rate (about 80%), possibility of placenta accreta especially if the patient has had a previous Caesarean section and the placenta lies over the uterine scar). A low‐lying placenta noted on ultrasound before 20 weeks does not remain so in about 90% of cases but the authors rightly warn that central low‐lying placentas remain low. When does the patient with a low‐lying placenta require hospitalization? In the editor's opinion the answer is when there has been an episode of significant haemorrhage after 20 weeks' gestation, or when there is a major degree of placenta praevia after 32 weeks' gestation, especially if there are other clinical features of praevia (high presenting part, transverse lie).

Although the amount of bleeding is usually proportional to the degree of placenta praevia, there are exceptions; patients with marginal placenta praevia can bleed torrentially, and patients with central placenta praevia may not have an episode of bleeding until full‐term or beyond, especially, ironically, with placenta accreta where bleeding may not occur until the onset of labour — or until the placenta is removed manually after elective repeat Caesarean section. It is also true that major degrees of placenta praevia can cause profound haemorrhage in the second trimester and justify Caesarean section for maternal welfare even when the fetus is not viable — the manoeuvres internal version/bipolar version to ensure vaginal delivery in such patients is seldom indicated in modern obstetrics, unless the patient is in labour and the cervix is almost fully dilated.

These days there is pressure on hospital beds and antenatal patients are less often hospitalized — this is possible largely because tests of fetal and maternal well‐being reassure us that it is safe to allow the mother to rest at home. It is wise to be circumspect about sending home a patient with placenta praevia who has had 1 or more episodes of bleeding — recently this editor did so for the first and last time, when the patient lived near the hospital and was very anxious to go home. The patient was a 32‐year‐old para 2 who had had several episodes of spotting; ultrasound at 32 weeks showed an anterior marginal (grade 1) placenta praevia. She was allowed home at 34.2 weeks; 5 days later she was readmitted with heavy vaginal bleeding in early labour; she was delivered by Caesarean section of a living 2,470 g infant and at operation a major degree of anterior placenta praevia was encountered.

Another common problem is the low‐lying placenta found by sonography in the patient with a threatened abortion. The amount of bleeding and the patient's past history are factors that should influence the difficult decision as to whether or not the patient should remain in hospital. Bleeding frightens patients as well as their doctors — most patients with a placenta praevia who have bled are more than willing to remain in hospital until delivery.

This paper also considers the place of vaginal delivery in patients with placenta praevia. If the patient is not in labour and the pregnancy has reached 37–38 weeks' gestation most would agree that delivery by Caesarean section is indicated. Patients who are in labour with placenta praevia may usually be allowed to deliver vaginally if the presenting part is in the pelvis and bleeding is not copious. In the writer's opinion there is no place for induction of labour, by any method, if the patient is known to have any degree of placenta praevia.

If cases of placenta praevia diagnosed after vaginal delivery by inspection of the secundines (hole in membranes within 4 cm of placental edge) are included, about 20% of patients with a placenta praevia deliver vaginally, usually without an episode ofantepartum haemorrhage — if such patients are included the incidence of placenta praevia is approximately 1%.

Summary: Considerable confusion exists in the literature as a result of the wide range of classification systems for placenta praevia (PP) and low‐lying placenta.

The discrepancy between frequency of low‐lying placentas in the second trimester and PP at term reflects to a certain extent the lack of understanding of the anatomy and physiology of the pregnant uterus. It seems that ‘placental conversion’ is a real phenomenon and is probably due to the differential growth rates of the placenta and uterus. Maternal bladder overdistension and myometrial contractions account for only a small part of the discrepancy.

Diagnostic ultrasound obviously has an important role in placental localization. The role of Magnetic Resonance Imaging remains to be determined. The management of patients with low‐lying placenta diagnosed in the second trimester, and the frequency of repeat scans is determined largely by the management protocol of the attending obstetrician.