To determine whether adenomyosis diagnosed by ultrasound using explicitly reported criteria is associated with postpartum hemorrhage (PPH) and placenta accreta spectrum (PAS). MEDLINE, Embase, Scopus, Web of Science, and CENTRAL were searched to September 9, 2026. Observational studies were eligible if adenomyosis was diagnosed sonographically using explicitly reported criteria in at least 80% of exposed cases and if an unexposed comparator was available. Studies relying on magnetic resonance imaging alone, histology, self-report, or administrative coding, and studies in which ultrasound- and MRI-based diagnoses could not be disaggregated were excluded. Co-primary outcomes were PPH and PAS. Conformity with the Morphological Uterus Sonographic Assessment (MUSA) criteria was examined in a prespecified sensitivity analysis. Risk of bias was assessed with ROBINS-I and certainty of evidence with GRADE. Odds ratios (OR) were pooled using a DerSimonian-Laird random-effects model. Of 12 038 records, five studies were included (1356 exposed and 11 270 unexposed pregnancies). Three studies using the same PPH threshold (> 500 mL after vaginal and ≥ 1000 mL after cesarean birth) contributed to the primary analysis: the pooled OR was 1.56 (95% CI 0.69-3.49; p = 0.284) with substantial heterogeneity (I2 = 62.3%; τ2 = 0.298; 95% prediction interval 0.41-5.94). The pooled estimate was unstable on leave-one-out analysis, ranging from 1.10 (0.41-2.96) when the study recruiting from a symptomatic tertiary population was removed to 2.33 (1.36-4.00) when the registry-based study was removed. One study reporting severe PPH (≥ 1000 mL vaginal/≥ 1500 mL cesarean) reported an OR of 6.52 (2.24-19.03). Only two studies reported PAS, with 0/90 and 3/311 events among exposed women; these were not pooled. All studies were at serious risk of bias and certainty of evidence was very low for every outcome. When eligibility is restricted to adenomyosis diagnosed by ultrasound with explicitly reported criteria, an association with PPH is not demonstrable, and the pooled estimate depends entirely on whether the source population is symptomatic or unselected. Evidence on PAS is insufficient for synthesis. Published associations may in part reflect how cases were ascertained rather than the disease itself. Trail Registration: PROSPERO CRD420261495931.