Placenta previa is the presence of placental tissue lying over the cervical canal or right next to it. It complicates 1:250 term pregnancies (over 37 weeks)
Placenta previa occurs when the embryo implants in the lower part of the uterus, near the cervix. This is a very serious and potentially life-threatening situation. As the pregnancy advances and the uterus grows, the lower segment of the uterus will stretch and thin out. This may cause some of the placental blood vessels implanted in this area to spontaneously tear or rupture and may result is significant painless vaginal bleeding. With placenta previa, the placenta is located lower than the developing fetus and obstructs the cervical canal. The only safe option for delivery in this situation is a cesarean section.
Alison Martin, 2010
There are various classification schemes for placenta previa. Traditionally, placenta previa's were classified as marginal, partial or complete, in increasing order of severity:
marginal: placental edge near the cervical os
partial: placental edge next to the cervical os
complete/central: placenta completely
covers the cervical os
With the use of ultrasound in obstetrics today, a more contemporary classification scheme is used:
marginal placenta previa: placental edge lies within 2 cm of the cervical os but does not cover it
placental previa: placenta completely covers the cervical os
Mary Solarin is a 34 year-old woman living in rural Kenya, 32 weeks pregnant with her third child. Her pregnancy has progressed uneventfully, and she has seen no one other than the village community health worker. She awakes to find her bedsheets soaked with blood. She is experiencing no contractions. Her husband rushes her to the nearest hospital, 30 km away, by public transit.
Risk factors for placenta previa include:
Placenta previa is initiated by implantation of the embryo in the
lower segment of the uterus. As the placenta develops and grows,
it covers the internal cervical os.
Bleeding is thought to occur secondary to the thinning of the lower uterine segment which is a normal ongoing process as the uterus enlarges and stretches throughout pregnancy. The placental attachments in the lower segment may tear during this process or with cervical dilatation. Bleeding is typically painless, bright red and unpredictable with most episodes occurring spontaneously without any preceding event.
Most women today have a second trimester ultrasound to review fetal anatomy and for placental location. Consequently, most women are diagnosed with a placenta previa before they develop vaginal bleeding and are therefore aware and educated about their diagnosis before problems arise. If a patient is noted to have a placenta previa or low-lying placenta on their routine second-trimester ultrasound, they should be rescheduled for a follow-up scan in the third trimester for repeat placental localization. In many women with a marginal placenta or placenta previa diagnosed in the second trimester, the placenta will have 'migrated' far enough away from the cervix by term to allow for safe vaginal delivery. It should be noted that the placenta itself does not move throughout pregnancy, however the growth and stretching of the lower segment effectively shifts the placenta away from the cervical canal. Vaginal delivery can be attempted in women with a low-lying placent if the edge is more than 2-3 cm away from the internal cervical os.
Painless bright red bleeding in the third trimester. Bleeding may be severe and cause symptoms of shock.
Vaginal/Cervical examination is contraindicated in patients who present with third-trimester painless vaginal bleeding until a placenta previa can be ruled out by ultrasound. A digital cervical exam in a patient with a placenta previa can result in a catastrophic hemorrhage if the examining fingers tear the placenta during the examination.
Therefore, before examining a patient with vaginal bleeding in pregnancy their chart should be reviewed to identify the location of the placenta if they have had a previous ultrasound.
Transabdominal ultrasound will identify 95% of cases of placenta previa. Transvaginal ultrasound has a diagnostic accuracy of almost 100% and is safe in patients with a placenta previa when performed by an experienced sonographer.
The differential diagnosis for antepartum bleeding in the third-trimester includes:
Management when diagnosed at routine second-trimester ultrasound and no bleeding:
If placental edge is greater than 2-3 cm away from the
internal cervical os, then vaginal delivery can be safely attempted.
Management if presents with antepartum hemorrhage:
If bleeding stabilizes:
Indications for immediate delivery by cesarean section:
Counsel patients with placenta previa about risk of recurrence and ask them to see an obstetrician early on during the next pregnancy.
Recurrence Risk in a subsequent pregnancy is between 4 and 8 percent.
Gabbe et al., Antepartum hemorrhage. Obstetrics: Normal and
problem pregnancies, 5th edition. Mosby 2007.