Placenta Previa

Contributor: Shelley Chapman, MD
Last Update: 4/18/17

Definition:

• Placental previa- implantation of the placenta over the internal cervical os or just reaching the cervical os by transvaginal scan in the third trimester.
• Low lying placenta- placenta edge less than 2 cm from the internal cervical os.

Incidence: 0.3-0.5% of livebirths. The risk of previa following a prior cesarean delivery is 1-5% with a linear increase as the number of prior cesareans increases. With four or more cesareans, the risk for previa approaches 10%. AMA >35 years old increases the risk to 2% and >40 years old to 5%. Multiparity, prior suction curettage and smoking are all associated with an increased risk for previa.

Classical Clinical Presentation: painless bleeding typically in the third trimester as the lower uterine segment develops.

Diagnosis: Any suspicion of placenta previa due to either clinical history or transabdominal ultrasound examination should be confirmed by transvaginal ultrasound.

Management:

• Antenatal (Define location of edge of placenta)

1. Recommend pelvic rest at the time of diagnosis if in the third trimester or vaginal bleeding is present.
2. Recommend measures to decrease constipation including stool softeners and high-fiber diet.
3. Counsel the patient to seek immediate medical attention for any vaginal bleeding.
4. Schedule ultrasound to assess placentation and fetal growth at 32 weeks. The incidence of placenta accreta is increased particularly in the setting of a prior uterine surgery.
   1. If previa at 32 weeks, plan to deliver as below at 36-37 weeks.
   2. If <2 cm from internal cervical os, ie low lying, repeat transvaginal ultrasound at 36 weeks, if patient remains asymptomatic.
5. If vaginal bleeding occurs, hospitalization may be required with consideration of antenatal corticosteroid therapy and hematocrit assessment with iron therapy or blood transfusion as needed to keep the maternal Hct above 30. These patients will have delivery timing individualized.
6. If resolution of previa or low-lying placenta, perform transvaginal ultrasound to assess for vasa previa at 34-36 weeks gestation.

• Timing of delivery

Placenta previa can result in severe obstetric hemorrhage with subsequent maternal shock, need for transfusion, DIC, hysterectomy, damage to surrounding organs, ICU admission and even death. Placenta previa is likely to result in hemorrhage before delivery of the fetus. Suboptimal timing of delivery can result in decreased resources, fetal/neonatal hypoxemia or acidemia resulting from maternal shock. In a study of 230 cases, the risk of emergent bleed was 4.7% at 35 weeks, 15% at 36 weeks, 30% at 37 weeks, and 59% at 38 weeks. A decision analysis and expert opinion recommend delivery at 36-37 weeks of gestation in patients with uncomplicated placenta previa. Based on this information, we recommend a scheduled cesarean at 36-37 weeks without amniocentesis for FLM if good dating criteria exist. Of note, the incidence of RDS at 36 weeks is 7% and at 37 weeks 3.5%.

• Operative planning

1. Have preoperative HCT >30% if possible.
2. Type and cross 2-4 units of blood.
3. Consult anesthesia to plan for possible intraoperative hemodynamic changes, need for blood products and longer operative time.
4. Have appropriate surgeons available for possible caesarean hysterectomy.

Ninety percent of “low lying placentas” in early pregnancy resolve by the third trimester. Placentas located <2 cm from the internal cervical os as documented by a late third trimester transvaginal scan should be offered cesarean section. Special consideration can be made after informed consent with the non-bleeding patient as to the possibility of vaginal delivery in the patient with a placental edge 1-2 cm from the internal cervical os.

​References

1. Hull AD, Resnik R. Placenta Previa, Placenta Accreta, Abruptio Placentae and Vasa Previa in Creasy and Resnik’s Maternal-Fetal Medicine: Principles and Practice 6th ed. By Saunders, an imprint of Elsevier Inc; 2009.
2. Mercer B, Verghella V, Foley M et al. Placenta Accreta. Am J Obstet Gynecol 2010 Nov;203(5):430-9.
3. Robertson PA, Sniderman SH, Laros et al. neonatal morbidity according to gestational age and birth weight from five tertiary care centers in the US 1983-1986. Am J Obstet Gynecol 1992 Jun;166(6 Pt 1): 1629-41; PMID 1615970.
4. Reddy, U, Abuhamad, A, Levine, D, Saade, G. Fetal Imaging. J Ultrasound Med 2014 May; 33:745-57.