What are the risk factors for placenta previa?

Risk Factors for Placenta Previa

The most critical risk factors for placenta previa are prior cesarean delivery (with risk increasing dramatically with each subsequent cesarean), advanced maternal age, multiparity, prior uterine surgery including abortion and curettage, and smoking. 1, 2, 3

Primary Risk Factors

Prior Cesarean Delivery

• Prior cesarean delivery is the single most important modifiable risk factor, with risk escalating in a dose-dependent manner with each additional cesarean 1, 4, 5
• Women with one prior cesarean have a 2.0-fold increased risk (OR 2.0,95% CI 1.17-3.44) 5
• After two cesarean deliveries, the risk increases 7.32-fold (OR 7.32,95% CI 2.1-25) 5
• The rising incidence of placenta previa over recent decades directly parallels the increased cesarean delivery rate 1, 2

Advanced Maternal Age

• Maternal age >34 years confers significantly elevated risk, even after adjusting for parity 5, 3
• Advanced maternal age is consistently identified across multiple studies as an independent risk factor 1, 2, 6

Multiparity and High Gravidity

• Gravidity of 3 or more increases risk 4-fold (OR 4.0,95% CI 2.5-6.6) 5
• Parity of 2 or more confers 2.76-fold increased risk (OR 2.76,95% CI 1.7-4.3) 5
• High parity is an established risk factor across observational studies 1, 2, 3

Prior Uterine Surgery and Instrumentation

• Prior abortion increases risk 2.8-fold overall (OR 2.8,95% CI 2.04-3.83), with risk escalating to 4.8-fold after just one abortion (OR 4.8,95% CI 2.7-8.3) 5
• Prior curettage and dilatation procedures damage the endometrial-myometrial interface 1
• Myomectomy with endometrial cavity entry creates full-thickness uterine defects similar to cesarean delivery 7
• Asherman syndrome (intrauterine adhesions) is an established risk factor 1

Uterine Abnormalities

• Presence of uterine abnormalities (congenital or acquired) increases risk 8.5-fold (OR 8.5,95% CI 1.75-44.5) 5
• Both congenital and acquired uterine anomalies are recognized risk factors 1

Smoking and Substance Use

• Smoking during pregnancy increases placenta previa risk 2, 3
• Cocaine use during pregnancy is associated with increased risk 3
• One Croatian study paradoxically found lower smoking rates in placenta previa cases, but this contradicts the broader literature and likely reflects population-specific confounding 5

Other Risk Factors

• In vitro fertilization (IVF) is an established risk factor 1
• Multifetal gestation increases risk 2
• Male fetal sex confers modestly increased risk 3
• Prior postpartum hemorrhage 1
• Hypertension 1

Critical Clinical Context: The Placenta Previa-Accreta Connection

When placenta previa occurs in the setting of prior cesarean delivery, the risk of placenta accreta spectrum disorder escalates dramatically, creating a life-threatening combination 1:

• Placenta previa with no prior cesarean: 3% accreta risk 1
• Placenta previa with one prior cesarean: 11% accreta risk 1, 8
• Placenta previa with two prior cesareans: 40% accreta risk 1
• Placenta previa with three prior cesareans: 61% accreta risk 1
• Placenta previa with five or more prior cesareans: 67% accreta risk 1, 8

Epidemiology

• Overall prevalence of placenta previa is approximately 4.0 per 1,000 births (0.4%), with rates ranging from 3.5-4.6 per 1,000 depending on study design 5, 3
• More recent U.S. data suggests rates as high as 1 in 272 birth-related hospital discharges 1
• The incidence has increased substantially from historical rates of 0.26% in the 1970s-1980s 1

Common Clinical Pitfalls

• Do not dismiss multiparity alone as high-risk without concurrent uterine scarring—three uncomplicated vaginal deliveries without prior cesarean or uterine surgery carries minimal placenta previa risk 9
• Always assess for placenta accreta spectrum when placenta previa is diagnosed in women with any prior uterine surgery, as 49% of accreta cases have concurrent previa 1
• Routine second-trimester ultrasound identifies most cases, but the majority of low-lying placentas at 18-22 weeks will resolve before term 6
• If placenta previa persists into the third trimester with prior cesarean delivery, refer to a level III or IV maternal care facility with multidisciplinary expertise and blood bank protocols for massive transfusion 1