Placenta Previa

Placenta previa is a placenta that lies over or reaches the internal cervical os, found on ultrasound from 16 to 20 weeks, and a placenta whose edge lies less than 20 mm from the os without reaching it is called low lying. Many women have a low placenta at the 20 week scan, but most move away as the lower segment grows. It matters because it causes painless antepartum haemorrhage that can be heavy and recurrent, and it needs caesarean birth.

• Risk factors: previous caesarean section (risk rises with each), previous previa, multiple pregnancy, maternal age above 35 years, smoking, previous curettage, assisted conception, high parity.
• The risk of placenta accreta spectrum rises with each previous caesarean section.
• Women with previa and bleeding need admission until birth in many cases.

The classic presentation is sudden painless bright red vaginal bleeding after 28 weeks, often at rest, with a soft non tender uterus. Malpresentation (breech, transverse or oblique lie) and a high presenting part are common.

• Fetal heart rate usually normal unless maternal shock.
• First bleed is rarely fatal and often self limiting.
• Differential diagnosis: placental abruption, vasa previa, cervical polyp or ectropion, cervical cancer, show in labour, genital tract trauma, uterine rupture.
• Complications: haemorrhage and shock, anaemia, preterm birth, placenta accreta spectrum, caesarean hysterectomy, postpartum haemorrhage, malpresentation, fetal growth restriction, stillbirth, maternal death.

The approach is ultrasound localisation, with no digital vaginal examination.

• Transabdominal then transvaginal ultrasound (safe) to measure the distance from the placental edge to the internal os; rescan at 32 weeks and 36 weeks.
• Colour Doppler and, where available, MRI for placenta accreta spectrum in women with previous caesarean section (loss of the retroplacental clear zone, placental lacunae, bladder wall interruption).
• Haemoglobin, group and cross-match (2 to 4 units), coagulation screen if bleeding is heavy; Kleihauer test in RhD negative women.
• Where ultrasound is unavailable, any painless bleeding after 24 weeks is previa until shown otherwise, and the woman is referred without vaginal examination.

The principle is to resuscitate if bleeding, give steroids, correct anaemia and plan caesarean birth at 36 to 37 weeks and 6 days, or at once for heavy bleeding.

• Supportive: admit with bleeding; two large-bore cannulae; Ringer's lactate 1 litre rapidly if shocked; left lateral position; transfuse if haemoglobin is below 7 g/dL or there is shock; avoid vaginal examination and intercourse; plan birth in a facility with blood, theatre and neonatal care.
• Pharmacological: ferrous sulphate 200 mg oral 8-hourly; ferric carboxymaltose 15 mg/kg (maximum 1000 mg) IV once after 13 weeks if haemoglobin stays below 9 g/dL; betamethasone 12 mg IM twice, 24 hours apart, between 24 and 34 weeks and 6 days when bleeding or early delivery is likely; magnesium sulphate 4 g IV over 20 minutes then 1 g per hour until delivery for 24 hours at most before 32 weeks; tranexamic acid 1 g IV over 10 minutes for bleeding, repeated once after 30 minutes if needed; nifedipine 20 mg oral once then 10 to 20 mg every 6 to 8 hours for up to 48 hours for contractions with light bleeding in a stable woman; cefazolin 2 g IV before caesarean section; oxytocin 10 IU IM at birth then infusion of 20 IU in 1 litre sodium chloride 0.9 percent at 125 mL per hour for 4 hours; carboprost 250 micrograms IM every 15 minutes (maximum 8 doses) if there is atony (not in asthma); misoprostol 800 micrograms sublingual for atony.
• Procedures: caesarean section at 36 to 37 weeks and 6 days when stable, at once for heavy bleeding at any gestation; high skin and uterine incision for anterior placenta; uterine balloon tamponade (Foley or condom catheter filled with 300 to 500 mL sodium chloride 0.9 percent), compression sutures, uterine artery ligation or hysterectomy for uncontrolled bleeding.
• Special groups: RhD negative women give anti-D immunoglobulin 500 IU IM within 72 hours of each bleed; placenta accreta spectrum needs birth at 34 to 36 weeks in a tertiary unit with 4 to 6 units of blood, anaesthetic and surgical teams ready; women who decline transfusion need iron optimised and cell salvage.
• Red flags: bleeding soaking more than 2 pads in an hour, systolic blood pressure below 90 mmHg, pulse above 110 per minute, contractions with bleeding, abnormal fetal heart rate.
• Follow-up: antenatal review every 2 weeks; haemoglobin every 4 weeks; ultrasound at 32 and 36 weeks; postnatal haemoglobin at 24 hours, with ferrous sulphate 200 mg oral 8-hourly for 3 months.