Aspartame and saccharin are the two most widely used artificial sweeteners. Recently there has been controversy over the safety of aspartame. I advise you to substitute foods that do not contain the sweeteners for products you usually use because currently we just don't know about its safety for pregnant women and their developing babies.
Also, aspartame is a source of phenylalanine in the diet. Pregnant women who suffer from phenylketonuria must follow a low-phenylalanine diet or their babies may be born mentally retarded and suffer from delayed development.
Saccharin is not used as much today as in the past, but it is still found in many foods, beverages and other substances. The Center for Science in the Public Interest reports saccharin is not safe for use during pregnancy. Without further evidence, it is probably better to avoid using this product while you're pregnant.