The c13, or carbon 13 test, is generally not harmful to the fetus if you are pregnant. Carbon 13 is a respiratory test that can determine the presence of Helicobacter pylori infection. Carbon 13 is a stable isotope of carbon and is virtually non-radioactive. The amount of 13C ingested during the Carbon 13 breath test is also low, resulting in fewer adverse effects on the human body, and can be used to test pregnant women and children. The Carbon 13 Breath Test is safe, non-invasive, painless and relatively quick. It is best to complete the test before pregnancy, and if you must have a carbon 13 test during pregnancy, it must be done in a specialized hospital.