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Table 2 Small-for-gestational age (SGA) rates, rate ratios and rate differences by gestational age among singleton live births in the United States (1990–1992 vs. 2002–2004; 2002–2004 vs. 2008–2010)

From: The association between temporal changes in the use of obstetrical intervention and small- for-gestational age live births

Gestational Age (weeks) Fetuses-at-Risk (FAR) Approach
1990–1992 2002–2004 2002–2004 vs. 1990–1992
SGA per 100 FAR SGA per 100 FAR Rate Difference Rate Ratio (95 % CI)
24–33 weeks 0.21 0.25 +0.05 1.22 (1.20–1.25)
34–36 weeks 0.82 0.92 +0.10 1.12 (1.11–1.13)
37–38 weeks 2.30 2.84 +0.54 1.24 (1.23–1.24)
39–40 weeks 7.99 7.27 −0.72 0.91 (0.91–0.91)
41 weeks 4.72 4.88 +0.16 1.03 (1.02–1.05)
≥42 weeks 5.21 4.88 −0.33 0.94 (0.91–0.97)
Total 10.1 8.89 −1.23 0.88 (0.88–0.88)
  2002–2004 2008–2010 2008–2010 vs. 2002–2004
  SGA per 100 FAR SGA per 100 FAR Rate Difference Rate Ratio (95 % CI)
24–33 weeks 0.25 0.25 0.0 0.99 (0.97–1.00)
34–36 weeks 0.92 0.91 −0.01 1.00 (0.99–1.01)
37–38 weeks 2.84 3.10 +0.26 1.09 (1.09–1.10)
39–40 weeks 7.27 7.44 +0.17 1.02 (1.02–1.03)
41 weeks 4.88 5.00 +0.12 1.02 (1.01–1.04)
≥42 weeks 4.88 5.20 +0.32 1.07 (1.01–1.12)
Total 8.89 9.11 +0.21 1.02 (1.02–1.03)