Meta-analysis found highly significant heterogeneity for all three measures using the Q –test; pooled RRs were over 1, but given the extensive heterogeneity between countries, they are of limited value (pooled measures: 0.7 (0.7–1.8), 0.2 (0.1–0.3), and 1.3 (1.2–1.4) for all, singleton, and multiple births, respectively).
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