There are no scientific studies that show adversarial effects on pregnant women who enjoy no more than one diet soft drink per day. While one diet soda a day may not seem like much, most dieticians and doctors recommend limiting your soda intake in this way so as to consume more juice, water and milk which each supply more health benefits and nutrients than soda. There are, however, a few aspects to consider.
NutraSweet, the brand name of aspartame, is a non-caloric sweetener used in diet sodas and many other foods. It is made up of two amino acids. The digestive track breaks down aspartame the same way that it would break down any other protein during digestion.
One of the proteins that make up part of aspartame is phenylalanine. Phenylketonuria (also called PKU) is a rare hereditary condition in which this particular protein is not metabolized. If you have a family history of PKU, you might consider getting tested. If you take in too much phenylalanine while you are pregnant, there is a risk because the high levels of phenylalanine can cause damage to the fetus.
In a healthy person fed sixty 12-oz cans of diet soda at one time, blood phenylalanine levels peak well below the sustained concentration level deemed harmful. Unless you suffer from PKU, phenylalanine does not seem to pose any threat.
Saccharin is another sweetener found in some soft drinks. It has been found to have teratogenic (causing abnormal fetal development and birth defects) effects in rats. It has also been shown to cause cancer in rats as well. Human studies have not found these effects. However, it is probably best to err on the side of caution when it comes to Saccharin.