Its not the issue that having the actual test is a problem when you are pregnant. As some people here rightly said, the cervix undergoes a lot of changes during pregnancy, and some normal cells could be misconstrued as being 'abnormal' due to these hormonal changes. Furthermore, if you really did have abnormal cells, it would not be possible to do the treatment during pregnancy, as that DOES interfere with the cervix, so is not done. Therefore, if you are due a smear (and are pregnant), it is always advisable to wait until after the birth, when a) it will be a more accurate interpretation of the cervical cells and b) if there are genuine abnormal cells, you can then get the treatment as soon as possible.
For the record, research has shown that actually having smear tests and infection screening tests performed (like Chlamydia etc) are not risky during pregnancy, so are nothing to worry about - just for those of you who have had it done and may be worrying.
Congrats miaow if you are pg BTW!