Optimal delivery method in the presence of low-lying placenta

Placenta Previa is a pregnancy complication where the placenta partially or completely covers the cervix. It typically is characterized by painless vaginal bleeding that starts in the second or third trimester and is a major risk factor for postpartum hemorrhage. Mothers are at a higher risk if they are older than 35 years old, are multiparity, a smoker or cocaine user, or have a prior history of curettage, cesarean sections, or placenta previa. The definitive diagnosis is made through transvaginal ultrasound. Depending on the symptom severity, patients may need to be hospitalized to managed for bleeding; however, approximately 90% of placenta previa cases diagnosed early in pregnancy resolve by the third trimester. When the placenta is completely covering the cervix, it prevents the mother from having a safe vaginal delivery, resulting in majority of individuals being scheduled for elective cesarean sections at 36-37 weeks. However, a vaginal delivery is not an absolute contraindication in cases of low-lying placenta. 

In a 2019 systematic review, researchers sought out to determine the probability of a successful vaginal delivery in women with low-lying placentas as the preferred method of delivery remains unclear. The researchers used OVID EMBASE, OVID MEDLINE, and clinicaltrials.gov to find studies published from inception to February 2017 with a primary outcome of successful vaginal deliveries in women with low-lying placenta and a secondary outcome of antepartum, intrapartum, and postpartum hemorrhage. Of the 10 articles identified, results showed that vaginal delivery was most successful if the placenta was lying at 11-20mm or more from the internal os with a significant lower emergency cesarean rate due to hemorrhage. No statistically difference was seen when the distance from the internal os was 0-10mm. The results are summarized below. 

Internal os distance (IOD) (mm)	Successful vaginal deliveries	Emergent Cesarean rate due to hemorrhage
0-10	43% (95% CI 28-59)	45% (95% CI 22-29)
11-20	85% (95% CI 70-96)	14% (95% CI 4.2-29)
>20 mm	82% (95% CI 58-97)	10% (95% CI 2.2-22.3)

Results of the secondary outcome showed a minimal difference between IOD and antepartum and postpartum hemorrhage. Interestingly the researchers found that women with larger IOD and women who delivered cesarean delivery required more blood transfusions due to increased intrapartum hemorrhage among caesarean deliveries compared to vaginal delivery. 

This study has demonstrated that a trial of labor is possible in women with a low-lying placenta within 20mm of the internal os in the third semester, with a significantly higher chance of vaginal success rate if the placenta lays more than 11 mm away and a smaller emergent cesarean rate due to hemorrhage if the placenta is more than 20mm away. Ultimately extensive education and counseling should be given to the patient and a shared decision should be made before labor. 

Limitations of the study include majority of the studies being retrospective studies, increasing the risk of selection bias with a possibility of overestimation and/or underestimation of the association between low-lying placenta and delivery outcome. Further results should focus on prospective studies that focus on reducing the time between the last ultrasound and delivery to determine the optimal delivery method. Other factors such as in the presence of posterior or anterior placenta or marginal sinus placenta previa or speed to which the placenta migrates per week should be assessed in whether there is an association between higher rates of successful vaginal deliveries or a need for emergent cesarean deliveries.