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Objective: To assess the diagnostic accuracy of ultrasound for detecting placenta accreta spectrum (PAS) during the first trimester of pregnancy and compare it with the accuracy of second‐ and third‐trimester ultrasound examination in pregnancies at risk for PAS. Methods: PubMed, EMBASE and Web of Science databases were searched to identify relevant studies published from inception until 10 March 2023. Inclusion criteria were cohort, case–control or cross‐sectional studies that evaluated the accuracy of ultrasound examination performed at < 14 weeks of gestation (first trimester) or ≥ 14 weeks of gestation (second/third trimester) for the diagnosis of PAS in pregnancies with clinical risk factors. The primary outcome was the diagnostic accuracy of sonography in detecting PAS in the first trimester, compared with the accuracy of ultrasound examination in the second and third trimesters. The secondary outcome was the diagnostic accuracy of each sonographic marker individually across the trimesters of pregnancy. The reference standard was PAS confirmed at pathological or surgical examination. The potential of ultrasound and different ultrasound signs to detect PAS was assessed by computing summary estimates of sensitivity, specificity, diagnostic odds ratio and positive and negative likelihood ratios. Results: A total of 37 studies, including 5764 pregnancies at risk of PAS, with 1348 cases of confirmed PAS, were included in our analysis. The meta‐analysis demonstrated that ultrasound had a sensitivity of 86% (95% CI, 78–92%) and specificity of 63% (95% CI, 55–70%) during the first trimester, and a sensitivity of 88% (95% CI, 84–91%) and specificity of 92% (95% CI, 85–96%) during the second/third trimester. Regarding sonographic markers examined in the first trimester, lower uterine hypervascularity exhibited the highest sensitivity (97% (95% CI, 19–100%)), and uterovesical interface irregularity demonstrated the highest specificity (99% (95% CI, 96–100%)). In the second/third trimester, loss of clear zone had the highest sensitivity (80% (95% CI, 72–86%)), and uterovesical interface irregularity exhibited the highest specificity (99% (95% CI, 97–100%)). Conclusions: First‐trimester ultrasound examination has similar accuracy to second‐ and third‐trimester ultrasound examinations for the diagnosis of PAS. Routine first‐trimester ultrasound screening for patients at high risk of PAS may improve detection rates and allow earlier referral to tertiary care centers for pregnancy management. © 2024 International Society of Ultrasound in Obstetrics and Gynecology. [ABSTRACT FROM AUTHOR]