Abstract

Objective: We hypothesized that a second- or third-trimester diagnosis of low-lying placenta imparts underappreciated risk for postpartum hemorrhage (PPH) and placenta accreta spectrum (PAS). To quantify this risk and to assess whether it varies by the specific distance of the placenta from the cervical os and low-lying placenta resolution status, we conducted a systematic review and meta-analysis.

Data sources: Systematic searches were conducted in PubMed, ClinicalTrials.gov, EMBASE, and Web of Science from database inception to April 30, 2024.

Methods of study selection: A total of 3,700 results were screened for relevance with the PICO framework: population-singleton pregnancies; intervention-low-lying placenta; comparators-normal placentation; and outcomes-PPH and PAS. Studies published before 2000 were excluded to minimize bias from ultrasound sensitivity.

Tabulation, integration, and results: Twenty-one studies (3,704 patients with low-lying placenta, 2,555 with normal placentation) were included. Data extraction and quality assessment with the Newcastle-Ottawa Scale were performed independently by three reviewers. At any gestational age, low-lying placenta imparted a significant PPH risk (risk ratio [RR] 2.10, 95% CI, 1.02-4.35, P=.05, I2=0.0%) compared with non-low-lying placenta. The incidence of PPH was 16.0% (95% CI, 10.3-24.1%, I2=93.3%) in low-lying placenta 1-20 mm compared with 5.8% (95% CI, 3.8-8.8%, I2=79.9%) in non-low-lying placenta. When parsed by clinically meaningfully strata, a high incidence of PPH persisted with resolved low-lying placenta (resolved: 8%, 95% CI, 4.1-16.3%, I2=85.0%; unresolved: 29.2%, 95% CI, 19.0-42.0%, I2=70.5%; non-low-lying placenta: 5.8%, 95% CI, 3.8-8.8%, I2=79.9%) with no difference in PPH risk at less than 2 cm from the os (low-lying placenta 1-10 mm: 16.6%, 95% CI, 9.2-28.3%, I2=78.4%; low-lying placenta 11-20 mm: 17.5%, 95% CI, 8.8-31.7%, I2=92.2%; RR 0.97, 95% CI, 0.67-1.41, P=.84, I2=0.0%). An important finding is that PAS disorders affected 9.0% (95% CI, 4.7-16.8%, I2=89.9%) of all low-lying placenta cases.

Conclusion: Antepartum diagnosis of low-lying placenta is associated with a twofold increased risk of PPH compared with normal placentation. The pooled proportions of PPH were 16.6% in the 1-10 mm group and 17.5% in the 11-20 mm low-lying placenta group, with no significant difference. This meta-analysis is the first to quantify the risk of PPH associated with low-lying placenta, emphasizing the need for rigorous monitoring and delivery management of pregnancies with low-lying placenta to mitigate the burden of PPH on maternal morbidity.

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