Background: Delayed onset of lactogenesis II (DOL II) is a critical barrier to successful breastfeeding, yet the quality and reliability of existing systematic reviews on its prevalence and influencing factors remain unclear. Objectives: To assess the methodological quality, reporting quality, and evidence certainty of systematic reviews on DOL II, and to synthesize the pooled prevalence and influencing factors. Search Strategy: A comprehensive search was conducted in 12 databases (including PubMed, Cochrane Library, Web of Science, Embase, Scopus, and Chinese databases) from inception to 14 December 2025. Selection Criteria: Systematic reviews, meta-analyses, or systematic reviews with meta-analysis that examined the incidence and/or influencing factors of DOL II in parturients were eligible. Narrative reviews, protocols, and conference abstracts were excluded. Main Results: Fourteen systematic reviews were included. The overall pooled prevalence of DOL II was 27% (95% CI: 24%-30%). Twenty-six influencing factors were identified; the most consistent risk factors were pre-pregnancy overweight/obesity (OR=1.80, 95% CI: 1.42-2.28), caesarean section (OR=1.42, 95% CI: 1.30-1.56), gestational diabetes mellitus (OR=1.58, 95% CI: 1.35-1.84), and primiparity (OR=2.09, 95% CI: 1.67-2.61). Methodological quality (AMSTAR 2) was predominantly low or critically low (79%), reporting quality was similarly limited, and the certainty of evidence (GRADE) for all outcomes was low or very low. Conclusions: DOL II affects about one in four parturients and is associated with several modifiable and non-modifiable risk factors. However, the current evidence base is constrained by the suboptimal methodological quality of existing systematic reviews and the very low certainty of the underlying evidence.