Table 5

 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

VariablesHazard ratio (95 % CI)
BMI <25BMI 25-29.9Adjusted by conditioning on BMI
Natural pregnancies1=reference1=reference1=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 birth1.10 (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*<0.0010.77<0.001
Test of equal effect in all time periods†0.0010.420.0017
No of older siblings:
 None1=reference1=reference
 ≥10.92 (0.65 to 1.30)1.36 (0.58 to 3.19)0.83 (0.63 to 1.1)
Single birth1=reference1=reference
Multiple births2.52 (1.49 to 4.25)1.33 (0.57 to 3.13)2.06 (1.32 to 3.21)
Smoking at start of pregnancy:
 No1=reference1=reference
 Yes0.91 (0.49 to 1.68)0.89 (0.45 to 1.77)0.89 (0.56 to 1.41)
Education (years):
 ≤91=reference1=reference
 10-121.16 (0.57 to 2.39)0.55 (0.27 to 1.12)0.88 (0.53 to 1.44)
 >121.26 (0.61 to 2.59)0.80 (0.40 to 1.62)1.06 (0.64 to 1.75)
Maternal age at delivery (years):
 <351=reference1=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 birth1.53 (0.91 to 2.57)2.64 (1.41 to 4.96)1.88 (1.26 to 2.78)
Country of birth:
 Sweden1=reference
 Other country1.14 (0.72 to 1.83)3.06 (1.45 to 6.44)1.62 (1.09 to 2.41)
Calendar period:
 1990-20011=reference1=reference1=reference
 2002-081.55 (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 child1=reference1=reference1=reference
 Other0.81 (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.