Table 3

 Time to first venous thromboembolic event by trimester in pregnant women after in vitro fertilisation (IVF) and women with natural pregnancies matched on age and calendar period of delivery. Effect of different levels of effect modifier body mass index (BMI) is also given. Values are numbers (percentages) of women unless stated otherwise

VariablesIVF pregnancies (n=23 498)Natural pregnancies (n=116 960)Hazard ratio (95% CI)Proportional hazard regression (95% CI)*
TotalBMI <25BMI 25-29.9BMI >30
First trimester (weeks 1-12)36 (0.15)38 (0.03)4.61 (2.95 to 7.21)4.05 (2.54 to 6.46)6.64 (3.6 to 12.23)2.61 (0.97 to 7.07)1.01 (0.22 to 4.6)
Second trimester (weeks 13-25)23 (0.10)63 (0.05)1.00 (0.51 to 1.97)1.11 (0.54 to 2.29)1.04 (0.40 to 2.72)2.13 (0.66 to 6.93)—†
Third trimester (week 26 to <3 days before birth)33 (0.14)94 (0.08)1.04 (0.64 to 1.69)1.30 (0.77 to 2.19)1.30 (0.61 to 2.80)1.80 (0.81 to 4.01)0.26 (0.04 to 1.91)
≤3 days before to 42 days after birth49 (0.21)192 (0.16)1.69 (1.15 to 2.48)1.59 (1.03 to 2.45)1.66 (0.90 to 3.05)1.55 (0.75 to 3.24)1.45 (0.49 to 4.27)
P value for test of difference‡<0.001<0.0010.070.53
P value for test of equal effect in all time periods§0.0020.00060.860.33

*Women with complete data on BMI.

†Weeks 13-25 are merged with week 26 until three days before delivery owing to few events.

‡Wald χ2 test for difference between IVF and no IVF.

§Wald χ2.