Table 2. Association of stillbirth with maternal pre-pregnancy body mass index specified as a continuous variable, by gestational age, race-ethnicity and parity, California 2007–2010. 1 .
| Change in BMI | 20–23 weeks | 24–27 weeks | 28–31 weeks | 32–36 weeks | 37–41 weeks | |
|---|---|---|---|---|---|---|
| RR (CI) | RR (CI) | RR (CI) | RR (CI) | RR (CI) | ||
| Nulliparous | ||||||
| Non-Hispanic White | 1-unit | 1.06 (1.04,1.09) | 1.03 (0.99,1.07) | 1.07 (1.03,1.11) | 1.03 (0.99,1.06) | 1.11 (1.05,1.17) 2 |
| 5-unit | 1.36 (1.21,1.54) | 1.15 (0.93,1.40) | 1.39 (1.16,1.65) | 1.13 (0.95,1.35) | 1.57 (1.26,1.95) | |
| 10-unit | 1.86 (1.47,2.36) | 1.31 (0.87,1.97) | 1.92 (1.35,2.72) | 1.29 (0.91,1.82) | 2.11 (1.52,2.93) | |
| 15-unit | 2.54 (1.78,3.63) | 1.50 (0.82,2.77) | 2.66 (1.58,4.48) | 1.46 (0.86,2.46) | 2.45 (1.59,3.76) | |
| 20-unit | 3.47 (2.15,5.59) | 1.72 (0.76,3.89) | 3.68 (1.83,7.39) | 1.65 (0.82,3.32) | 2.44 (1.22,4.86) | |
| Non-Hispanic Black | 1-unit | 1.04 (1.01,1.07) | 1.00 (0.96,1.06) | 1.05 (1.00,1.10) | 1.04 (1.00,1.09) | 1.03 (0.99,1.08) |
| 5-unit | 1.22 (1.06,1.42) | 1.02 (0.80,1.32) | 1.29 (1.01,1.63) | 1.24 (1.01,1.52) | 1.18 (0.93,1.49) | |
| 10-unit | 1.50 (1.12,2.01) | 1.05 (0.63,1.74) | 1.65 (1.03,2.67) | 1.53 (1.02,2.32) | 1.39 (0.87,2.21) | |
| 15-unit | 1.84 (1.18,2.86) | 1.07 (0.50,2.29) | 2.13 (1.04,4.35) | 1.90 (1.02,3.53) | 1.63 (0.81,3.29) | |
| 20-unit | 2.25 (1.25,4.06) | 1.10 (0.40,3.02) | 2.73 (1.05,7.11) | 2.35 (1.03,5.37) | 1.92 (0.75,4.90) | |
| Hispanic | 1-unit | 1.08 (1.07,1.10) | 1.01 (0.98,1.04) | 1.00 (0.97,1.04) | 1.01 (0.98,1.04) | 1.04 (1.02,1.07) |
| 5-unit | 1.49 (1.38,1.61) | 1.05 (0.88,1.24) | 1.01 (0.84,1.22) | 1.07 (0.92,1.24) | 1.22 (1.08,1.39) | |
| 10-unit | 2.22 (1.90,2.59) | 1.10 (0.78,1.54) | 1.03 (0.70,1.50) | 1.15 (0.85,1.54) | 1.50 (1.17,1.92) | |
| 15-unit | 3.31 (2.63,4.18) | 1.15 (0.69,1.92) | 1.04 (0.59,1.83) | 1.23 (0.79,1.90) | 1.84 (1.26,2.67) | |
| 20-unit | 4.94 (3.63,6.72) | 1.20 (0.61,2.39) | 1.05 (0.50,2.25) | 1.31 (0.73,2.36) | 2.25 (1.36,3.71) | |
| Multiparous | ||||||
| Non-Hispanic White | 1-unit | 1.04 (1.01,1.06) | 1.06 (1.03,1.09) | 1.04 (1.01,1.08) | 1.04 (1.01,1.06) | 1.13 (1.06,1.20) 2 |
| 5-unit | 1.20 (1.06,1.36) | 1.36 (1.19,1.56) | 1.24 (1.05,1.46) | 1.19 (1.04,1.36) | 1.63 (1.29,2.06) | |
| 10-unit | 1.44 (1.12,1.84) | 1.85 (1.41,2.43) | 1.54 (1.11,2.14) | 1.42 (1.09,1.86) | 2.03 (1.45,2.84) | |
| 15-unit | 1.73 (1.19,2.50) | 2.51 (1.67,3.79) | 1.91 (1.17,3.13) | 1.69 (1.13,2.53) | 1.92 (1.22,3.01) | |
| 20-unit | 2.07 (1.26,3.40) | 3.41 (1.98,5.90) | 2.38 (1.23,4.58) | 2.02 (1.18,3.44) | 1.38 (0.64,2.96) | |
| Non-Hispanic Black | 1-unit | 1.04 (1.01,1.07) | 1.01 (0.97,1.06) | 1.00 (0.96,1.05) | 1.02 (0.99,1.06) | 1.18 (1.03,1.34) 2 |
| 5-unit | 1.19 (1.03,1.38) | 1.07 (0.86,1.33) | 1.01 (0.80,1.28) | 1.12 (0.94,1.34) | 1.91 (1.15,3.17) | |
| 10-unit | 1.43 (1.06,1.91) | 1.14 (0.74,1.77) | 1.02 (0.63,1.64) | 1.26 (0.88,1.79) | 2.41 (1.17,4.96) | |
| 15-unit | 1.71 (1.10,2.65) | 1.22 (0.64,2.36) | 1.03 (0.51,2.10) | 1.41 (0.83,2.40) | 2.00 (0.85,4.70) | |
| 20-unit | 2.04 (1.13,3.66) | 1.31 (0.55,3.13) | 1.04 (0.40,2.69) | 1.59 (0.78,3.21) | 1.09 (0.29,4.10) | |
| Hispanic | 1-unit | 1.03 (1.01,1.05) | 1.00 (0.97,1.02) | 1.01 (0.99,1.04) | 1.00 (0.98,1.02) | 1.03 (1.01,1.04) |
| 5-unit | 1.15 (1.07,1.25) | 0.98 (0.85,1.12) | 1.05 (0.93,1.20) | 0.98 (0.89,1.09) | 1.13 (1.04,1.23) | |
| 10-unit | 1.33 (1.13,1.56) | 0.95 (0.73,1.25) | 1.11 (0.87,1.43) | 0.96 (0.79,1.18) | 1.28 (1.08,1.52) | |
| 15-unit | 1.54 (1.21,1.96) | 0.93 (0.62,1.39) | 1.17 (0.81,1.71) | 0.95 (0.70,1.28) | 1.45 (1.12,1.88) | |
| 20-unit | 1.77 (1.29,2.44) | 0.91 (0.53,1.55) | 1.24 (0.75,2.04) | 0.93 (0.62,1.39) | 1.65 (1.17,2.33) | |
1 Relative Risks (RR) reflect estimated risk of stillbirth relative to term (37–41 weeks) live birth adjusted for maternal age, education, and height. Each 5-unit change in BMI reflects the approximate difference in risk between the following categories of BMI: a 5-unit change represents the approximate difference in risk between women with normal BMI (18.5–24.9 kg/m2, with 22.5 taken as the approximate mid-point) versus overweight (25.0–29.9 kg/m2, with 27.5 as the mid-point); a 10-unit change, the difference between women with normal BMI and obese class I (BMI 30.0–34.9 kg/m2, with 32.5 as the mid-point); a 15-unit change, the difference between normal BMI and obese class II (BMI 35.0–39.9 kg/m2, with 37.5 as mid-point); and a 20-unit change, the difference between normal BMI and obese class III (BMI ≥40.0 kg/m2, with 42.5 as reference). Analyses exclude women with gestational or pre-gestational diabetes or pregnancy-induced or chronic hypertension with one exception. For estimates among stillbirth at 20–23 weeks, the comparison group of term live births included the following additional births beyond those shown in Table 1 who had gestational diabetes: non-Hispanic White (n = 386,490), non-Hispanic Black (n = 69,633), and Hispanic (n = 734,548).
2 The quadratic term (BMI-squared) was significant (p<0.10) for the noted models; this is an indication that the association between BMI and risk of stillbirth was not linear, as reflected in the RRs for these groups, which are all relative to a BMI of 22.5 kg/m2.