Egger’s regression test was highly significant for comparative risk studies ( p < 0.001), and the trim-and-fill method imputed 15 and 11 hypothetically missing studies for comparative risk and standardized ratio analyses, respectively.
lowest duration category was 1.48 (95% CI 1.26–1.74; I 2 = 50.4%), with a significant trend across categories (P for trend = 0.03).
The comparative risk estimate (RR 2.06; 95% CI 0.78–5.45) did not reach statistical significance, reflecting both limited study numbers ( n = 5) and substantial heterogeneity (I 2 = 57%).