Marginal Placenta Previa

Marginal placenta previa is a placenta whose lower edge reaches the margin of the internal cervical os without covering it. It is often found at the mid pregnancy scan, and many move away from the os by term. It matters because bleeding can occur in labour, and the position of the placental edge decides whether vaginal birth is safe.

• Risk factors: previous caesarean section, previous previa, multiple pregnancy, age above 35 years, smoking, assisted conception.
• An edge to os distance of 0 to 10 mm carries a higher chance of bleeding in labour.
• Repeat scanning at 32 and 36 weeks guides birth.

Women often have no symptoms. Some have painless light bleeding after 28 weeks or after intercourse, with a soft uterus and a normal fetal heart.

• Painless bleeding that settles on its own.
• Presenting part often high.
• Differential diagnosis: low lying placenta, partial placenta previa, placental abruption, vasa previa, cervical polyp or ectropion, show in labour.
• Complications: antepartum haemorrhage, haemorrhage in labour, anaemia, preterm birth, placenta accreta spectrum, postpartum haemorrhage, emergency caesarean section.

The approach is transvaginal ultrasound with measurement of the edge to os distance.

• Transvaginal ultrasound: placental edge reaching the margin of the os (0 mm), with no overlap; rescan at 32 and 36 weeks.
• Colour Doppler for accreta spectrum when there is a previous caesarean section.
• Haemoglobin, blood group and cross-match (2 units) for bleeding; Kleihauer test in RhD negative women.
• Fetal growth and liquor at 28, 32 and 36 weeks; no vaginal examination.

The principle is surveillance with repeat scans, planned caesarean section at 37 to 38 weeks and 6 days if the edge stays within 10 mm of the os, and vaginal birth only if the edge moves 11 mm or more away.

• Supportive: avoid intercourse; avoid strenuous work; teach to attend at once for any bleeding or contractions; admit for bleeding; two cannulae in heavy bleeding; blood in the bank for caesarean section.
• Pharmacological: ferrous sulphate 200 mg oral 8-hourly for anaemia; betamethasone 12 mg IM twice, 24 hours apart, between 24 and 34 weeks and 6 days when delivery within 7 days is likely; tranexamic acid 1 g IV over 10 minutes for bleeding; 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) for atony, avoided in asthma; misoprostol 800 micrograms sublingual for atony.
• Procedures: caesarean section at 37 to 38 weeks and 6 days when the edge remains 0 to 10 mm from the os at 36 weeks; vaginal birth with senior review and caesarean theatre ready when the edge is 11 mm or more from the os; urgent caesarean section for heavy bleeding or fetal compromise; balloon tamponade or compression sutures for atony.
• Special groups: RhD negative women give anti-D immunoglobulin 500 IU IM within 72 hours of each bleed; previous caesarean section with suspected placenta accreta spectrum needs birth in a tertiary unit.
• Red flags: bleeding soaking more than 2 pads in an hour, contractions with bleeding, shock, abnormal fetal heart rate.
• Follow-up: ultrasound at 32 and 36 weeks; haemoglobin every 4 weeks; antenatal review every 2 weeks; postnatal haemoglobin and iron.