Asafoetida is a herb which finds many medicinal uses. However, on one hand, while it will effectively cure your colicky pain and flatulence, on the other hand can cause a miscarriage too if taken in high amounts. This is due to its abortifascient (the developing embryo is unable to attach to the uterine lining and hence dies) qualities. That is, its hot and stimulating qualities warm the uterus. This will stimulate menstruation and might lead to an abortion.
There is another reason why you should not consume asafoetida when you are pregnant. According to Ayurveda, when you consume this herb, it will reduce the vata or the air elements in your body. This interferes with the development of nerve tissue of the foetus.
Therefore, it is best to completely avoid it in your first trimester. In the later stages of pregnancy, you can, however, consume it – but in very small amounts.