Multivariate analysis stratified on body mass index (BMI) in pregnant women with BMI <30: 23 498 women after in vitro fertilisation (IVF) and 116 960 women with natural pregnancies matched on age and calendar period of delivery
| Variables | Hazard ratio (95 % CI) | ||
|---|---|---|---|
| BMI <25 | BMI 25-29.9 | Adjusted by conditioning on BMI | |
| Natural pregnancies | 1=reference | 1=reference | 1=reference |
| IVF pregnancies: | |||
| First trimester (weeks 1-12) | 5.21 (2.68 to 10.14) | 2.40 (0.85 to 6.80) | 4.13 (2.37 to 7.17) |
| Second trimester (weeks 13-25) | 0.91 (0.34 to 2.46) | 1.78 (0.53 to 5.93) | 1.18 (0.55 to 2.51) |
| Third trimester (week 26 to <3 days before birth) | 1.17 (0.53 to 2.58) | 1.38 (0.57 to 3.33) | 1.25 (0.69 to 2.25) |
| ≤3 days before to 42 days after birth | 1.10 (0.56 to 2.17) | 1.27 (0.59 to 2.75) | 1.16 (0.69 to 1.93) |
| P value for test of difference* | <0.001 | 0.77 | <0.001 |
| Test of equal effect in all time periods† | 0.001 | 0.42 | 0.0017 |
| No of older siblings: | |||
| None | 1=reference | 1=reference | — |
| ≥1 | 0.92 (0.65 to 1.30) | 1.36 (0.58 to 3.19) | 0.83 (0.63 to 1.1) |
| Single birth | 1=reference | 1=reference | |
| Multiple births | 2.52 (1.49 to 4.25) | 1.33 (0.57 to 3.13) | 2.06 (1.32 to 3.21) |
| Smoking at start of pregnancy: | |||
| No | 1=reference | 1=reference | — |
| Yes | 0.91 (0.49 to 1.68) | 0.89 (0.45 to 1.77) | 0.89 (0.56 to 1.41) |
| Education (years): | |||
| ≤9 | 1=reference | 1=reference | — |
| 10-12 | 1.16 (0.57 to 2.39) | 0.55 (0.27 to 1.12) | 0.88 (0.53 to 1.44) |
| >12 | 1.26 (0.61 to 2.59) | 0.80 (0.40 to 1.62) | 1.06 (0.64 to 1.75) |
| Maternal age at delivery (years): | |||
| <35 | 1=reference | 1=reference | — |
| ≥35 | |||
| First trimester (weeks 1-12) | 0.84 (0.43 to 1.63) | 2.34 (0.85 to 6.45) | 1.14 (0.67 to 1.96) |
| Second trimester (weeks 13-25) | 0.60(0.25 to 1.43) | 2.25 (0.73 to 6.88) | 0.92 (0.48 to 1.75) |
| Third trimester (week 26 to <3 before birth) | 0.60(0.30 to 1.23) | 1.20 (0.58 to 2.49) | 0.84 (0.51 to 1.38) |
| ≤3 before to 42 days after birth | 1.53 (0.91 to 2.57) | 2.64 (1.41 to 4.96) | 1.88 (1.26 to 2.78) |
| Country of birth: | |||
| Sweden | 1=reference | — | — |
| Other country | 1.14 (0.72 to 1.83) | 3.06 (1.45 to 6.44) | 1.62 (1.09 to 2.41) |
| Calendar period: | |||
| 1990-2001 | 1=reference | 1=reference | 1=reference |
| 2002-08 | 1.55 (1.12 to 2.14) | 2.64 (1.64 to 4.25) | 1.86 (1.43 to 2.43) |
| Marital status: | |||
| Married to or cohabiting with father of child | 1=reference | 1=reference | 1=reference |
| Other | 0.81 (0.29 to 2.21) | 1.34 (0.53 to 3.36) | 1.03 (0.52 to 2.03) |
*Wald χ2 for difference between IVF and no IVF.
†Wald χ2.