During a Pap smear, the doctor uses a speculum to examine the cervix and then uses a small brush or spatula to swab a sample of cells from the cervix for testing. Some women may experience light spotting after the test, due to the sensitivity of the cervix during pregnancy, but it's not likely that a Pap would be able to inadvertently cause a miscarriage. Usually the pregnancy is implanted higher up in the uterus and not near the cervix, and even in the event that the baby would be implanted lower, the cervix is quite thick in the first trimester and the light scraping from a pap test would not disturb an implanted pregnancy.
Unfortunately, given that 15 to 20% of confirmed pregnancies end in miscarriage, some women do inevitably miscarry after having a Pap smear. Some may even start having symptoms of miscarriage after having a Pap smear earlier the same day. This does not mean that the Pap smear caused the miscarriage in these cases, but more likely that the miscarriage symptoms coincidentally happened to appear right after the test. Nevertheless, if you are still worried, discuss your concerns with your prenatal care provider -- it's possible that your doctor or midwife will agree to postpone the Pap test until your postpartum checkup, especially if you have a history of normal Pap results.
Pap Smear. American Pregnancy Association. Accessed: 12 Jan 2010. http://www.americanpregnancy.org/womenshealth/papsmear.html
Buchmayer, S., Sparén, P., Cnattingius, S. "Signs of infection in Pap smears and risk of adverse pregnancy outcome." Paediatr Perinat Epidemiol. 2003 Oct;17(4):340-6.