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Table 4 Small-for-gestational age (SGA) rates, rate ratios and rate differences by gestational age among singleton live births to smokers 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 0.3 0.3 +0.06 1.20 (1.15–1.26)
34–36 weeks 1.4 1.6 +0.22 1.16 (1.14–1.18)
37–38 weeks 4.2 5.5 +1.28 1.30 (1.29–1.32)
39–40 weeks 14.3 13.2 −1.14 0.92 (0.91–0.93)
41 weeks 9.0 9.0 −0.01 1.00 (0.97–1.02)
≥42 weeks 9.9 9.5 −0.38 0.96 (0.90–1.03)
Total 17.6 15.7 −1.95 0.89 (0.88–0.89)
  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 0.3 0.4 +0.01 1.04 (0.99–1.09)
34–36 weeks 1.6 1.6 −0.01 0.99 (0.97–1.02)
37–38 weeks 5.5 6.0 +0.45 1.08 (1.07–1.10)
39–40 weeks 13.2 13.4 +0.20 1.02 (1.01–1.03)
41 weeks 9.0 9.2 +0.20 1.02 (0.99–1.06)
≥42 weeks 9.5 10.0 +0.55 1.06 (0.94–1.18)
Total 15.7 15.9 +0.21 1.01 (1.01–1.02)