Placenta praevia

Placenta praevia is the partial or total implantation of the placenta into the lower uterine segment. The word ‘praevia’ is derived from

Box 22.2 Key points in the clinical history of antepartum haemorrhage

• Timing of onset of bleeding in relation to presentation

• An estimate of the amount of observed blood loss

• Presence and characterization of pain

• Previous episodes of bleeding in pregnancy

• Changes in fetal movements

• Medication history and history of substance misuse (especially cocaine)

• Smoking

• Cervical smear history

• Symptom of maternal compromise two Latin words ‘prae (which means before) and ‘via (which means way) (11) and literally implies that placenta praevia is when the placenta lies in the way of childbirth.

Grading of placenta praevia

Placenta praevia has been traditionally graded as I to IV with the higher grades corresponding to its increasing encroachment of the lower uterine segment down to covering the internal cervical os. In clinical practice, however, placenta praevia is managed as either minor (grades I and II) or major praevia (grades III and IV) in align­ment with the anticipated risks of haemorrhage, likelihood of pre­term delivery, and determination of mode and timing of delivery Figure 22.1 (16).

Grading scheme

I Placenta extends into the lower uterine segment but does not reach the internal os.

II The lowermost edge of the placenta reaches the internal os but does not cover it.

III The placenta reaches and partially covers the internal os.

IV The placenta completely (symmetrically) covers the internal os.

Risk factors

There are a number of risk factors associated with the development of placenta praevia; the strongest are a previous history of placenta praevia or a previous caesarean section, with increasing risk associ­ated with multiple repeat caesarean sections (13, 16-21). A previous prelabour caesarean section is associated with a higher risk of pla­centa praevia more than a previous intrapartum caesarean section or vaginal birth (18). Box 22.3 summarizes the risk factors associ­ated with placenta praevia.

Pathophysiology

There is no clear unifying hypothesis for the development of pla­centa praevia, however, a purported hypothesis suggests that trophoblastic implantation in the upper part of the uterine cavity is inhibited due to the presence of poorly vascularized endometrium resulting from previous pregnancy or uterine surgery leading to im­plantation in the lower uterine cavity (15). Also, a large placental surface area, as observed in multiple pregnancy, increases the like­lihood of the placenta extending into the lower uterine segment. In established placenta praevia, the progressive stretching of the lower uterine segment and changes in cervical morphology as preg­nancy advances result in subtle disruption, leading to the observed symptom of bleeding. Physical disruption of the intervillous space

Figure 22.1 Placenta praevia (symmetrical)—illustration of major placenta praevia.

Box 22.3 Risk factors for placenta praevia

• Previous placenta praevia (adjusted odds ratio (OR) 9.7)

• Previous caesarean section (relative risk 2.6; 95% confidence interval (CI) 2.3-3.0 with background risk of 0.5%):

— One previous caesarean section: OR 2.2 (95% CI 1.4-3.4 with a background risk of 1%)

— Two previous caesarean sections: OR 4.1 (95% CI 1.9-8.8)

— Three previous caesarean sections: OR 22.4 (95% CI 6.4-78.3)

• Previous termination of pregnancy

• Multiparity

• Fertility treatment

• Smoking

• Multiple pregnancy

• Advanced maternal age (age >40 years)

• Defective endometrium due to the presence or history of:

— uterine scar/previous intrauterine surgery

— endometritis

— manual removal of placenta

— curettage

— submucous fibroids

• Male fetus

can occur with digital examination or intercourse. The bleeding in placenta praevia is maternal in origin, unless there is a coexisting vasa praevia.

Clinical presentation

Placenta praevia classically presents with painless unprovoked or provoked vaginal bleeding, commonly in the second half of preg­nancy. This ‘classic’ presentation occurs in about 80% of cases while the rest will present with bleeding associated with painful contrac­tions, thus mimicking placenta abruption (22) and up 10% of cases of confirmed placenta praevia presenting acutely with bleeding will have coexisting placenta abruption. About a third of patients will present with their first (warning) bleed before 30 weeks’ gestation and most patients with placenta praevia will have a bleed by 36 weeks and only 10% of cases of confirmed placenta praevia will re­main asymptomatic until delivery (23).

In most cases the bleeding is self- limiting, and usually settles by the time of initial assessment or shortly afterwards. Subsequent bleeding after the ‘warning’ bleed is more likely to be more significant as the gestation advances with progressive stretching of the lower segment. Uncommonly, previously undiagnosed placenta praevia can present with significant life-threatening haemorrhage, thus supporting the importance of maternity service preparedness for such cases, where prompt resuscitation, and availability of blood transfusion facilities are key lifesaving procedures (24).

Ultrasound diagnosis of placenta praevia

The practice of a routine mid-trimester detailed fetal anomaly scan which includes placenta localization can identify a low-lying pla­centa in asymptomatic patients. The transvaginal approach is the preferred modality for confirming a low-lying placenta identified on transabdominal ultrasound. Transvaginal ultrasound has been shown to be safe, acceptable with superior views, and will reclas­sify up to 25% of placentas diagnosed as low lying by abdominal ultrasound (25, 26). Transvaginal ultrasound also enables a more accurate measurement of the lower edge of the placenta from the in­ternal cervical os (27). For those patients identified by transvaginal ultrasound with a low-lying placenta in the first half of pregnancy, a follow-up scan should be performed around 32-34 weeks, and in those with a persistent low-lying placenta, a further scan at 36 weeks is recommended prior to delivery.

There should be no hesitation in undertaking transvaginal ultra­sound to determine placenta localization in stable symptomatic pa­tients because the position of the probe is 2-3 cm from the cervix and the angle between the cervix and the vaginal probe is suffi­cient to prevent the probe from inadvertently slipping into the cer­vical os (32). Figure 22.2 shows a low-lying placenta diagnosed by ultrasound scan.