It is generally advised that smear tests should not be done during pregnancy. The usual practice is to delay the smear until several months after delivery. The reasons for this are that smears are less easy to interpret in pregnancy, and that there is a theoretical risk that doing the smear might upset the pregnancy in some way (though there is no evidence for this).
The only reason that a woman would be advised to have a smear done during pregnancy would be if she had had an abnormality picked up on her previous smear, and a follow-up had been requested. In this case, it would be best for the smear to be done at some time between the 12th and the 24th week of pregnancy.