Introduction:
Finding the risk factors for thromboembolic (TE) disease and preventing its development in pregnant women is important. Asthma is a common chronic disease in pregnant women. The aim of the study is to find the risk factors.

Material and methods:
From 2004 to 2011, 55,057 pregnant women were recruited from a Taiwan database. They were grouped into AS and non-AS groups. AS subjects were divided into three groups. Group 1: with AS before pregnancy or during pregnancy; without AS after childbirth. Group 2: without AS before pregnancy, with AS during pregnancy or after childbirth. Group 3: with AS before, during pregnancy and after childbirth. The rate of TE (during pregnancy and 60 days after childbirth) was compared between different groups.

Results:
In pregnant women, TE rate was 2.47‰ in AS and 0.71‰ in non-AS subjects. The rate was higher in AS subjects (OR 3.47, adjust p 0.005). In group 2, the rate was 3.97‰, higher than that in non-AS subjects (OR 5.44, p 0.013). The TE rate in groups 1 and 3 was similar to non-AS subjects.

Conclusions:
The rate of TE increases if pregnant women have AS. Physicians need to be alert to the occurrence of TE as early as possible in pregnant women with AS. If AS was re-appeared or worsened during pregnancy (group 2), the rate of TE increased. Pregnant women and physicians should prevent the re-appear of AS or control AS properly during pregnancy to prevent the occurrence of TE.