Does your wife want to eat apple during pregnancy? If she does, it maybe a sign that the body is lacking of certain nutrients. The urge to eat certain food during pregnancy is a sign of certain nutrient deficiency.
For example, because my wife has been taking at least 3 scoops of protein powder per day, she doesn't have the urge to eat soya food or egg to get protein.
Now, let's study the apple. It contains at least 4 gram of dietary fiber. Also, rich in phyto-nutrients. High in potassium and it has about 16g of sugar. Check this link. Isn't that interesting? Thus, does your wife have any low blood sugar or low blood pressure problem? Or she doesn't have the habit of eating fruits before she was pregnant? Or does she have any constipation problem before she was pregnant? If your answer is yes, then, this maybe why the body is always asking for apple.
Try taking Double X and see how. Who knows, she may not want to eat apple anymore. However, there is one thing for sure, eating apple is good for the baby. Check this link too.
However, do remember to scrap away the wax coating on the apple before eating it. They are toxic. The apple itself already has a layer of natural wax. However, producers have put another layer to ensure its freshness. That is why organic apples always look dull but non-organic ones always look shiny.