A lot of controversy surrounds the safety of artificial sweeteners. Most health professionals consider the majority of such sweeteners relatively safe during pregnancy. Some professionals however recommend avoiding them altogether during that period and associate their use to birth defects. Scientific research regarding their general safety is still unfortunately neither comprehensive nor absolute. The ideal approach a pregnant woman can take during her 9 months of gestation is to be prudent and limit her use of such chemical additives. Pregnancy is not the right time to cut on calories anyways so it might be just fine to replace the artificial sweeteners with sugar or honey when you inevitably need that sweet taste (except if you are dealing with carbohydrate intolerance such as gestational diabetes, in which case you need to consult your doctor or your dietician to discuss regulating your sugar consumption). Moreover, a lot of processed foods that contain sugar substitutes are not necessarily calorie-free nor (like in the case of diet soft drinks) do they provide you and your fetus with essential nutrients you both need.
Whatever position you decide to take in relation to artificial sweeteners, you should absolutely avoid Saccharin during pregnancy because it’s proven to be capable of crossing the placenta and getting to your fetus’ developing tissues. Additionally, you should note that the US banned the use of Cyclamate based on some studies that have shown a link to bladder cancer.
Other sugar substitutes like sugar alcohols (such as Maltitol and Xylitol found in sugar-free chewing gums and some other processed foods) are not a big concern when it comes to safety but they do cause digestive problems if you go overboard or in case of a regular intake. Since you are already vulnerable to such discomforts by virtue of being pregnant, you probably don’t want to add to them!