No one really knows why some pregnant women crave pickles and ice cream, or any other seemingly strange food combination. There are a number of theories about pregnancy cravings, but it has been difficult to pin down a solid biological or psychological reason for cravings. Medical experts do generally agree that pregnancy cravings are very normal, and that unless a craving is unhealthy, satisfying it is probably a good idea.
Some studies have been conducted by surveying pregnant women to see what they crave and when. Salt, sour, bitter, sweet, and hot tastes are often craved at some point during pregnancy, though not necessarily together. Survey data seems to suggest, for example, that women shy away from bitter foods in the first trimester of pregnancy, perhaps because bitterness can signal the presence of a toxin that could hurt the fetus. Women tend to experience increased cravings for salty and sour flavors, like pickles, in the advanced stages of pregnancy, and sweet flavors are often desired in the early stages.
One explanation for a craving like pickles and ice cream is simply that the woman's body is searching for specific nutrients. Many midwives have suggested that pregnancy cravings indicate some sort of vitamin or mineral deficiency, and that the body is trying to make up for this by making foods with the needed nutrition seem especially appealing. This is certainly true in the case of pica, a syndrome in which people eat or crave things that are actually inappropriate for eating, like clay and glass.
Other researchers have dismissed this explanation, arguing that pregnancy cravings are linked to hormonal changes in the body. This theory is supported by the fact that many women develop strange food cravings at the points in their menstrual cycle that involve an especially high level of hormones. This doesn't explain why cravings change from pregnancy to pregnancy, however. A woman might want pickles and ice cream all the time in her first pregnancy, and abhor it in the next.
Pregnancy cravings have also been linked to psychological causes. Some people think that women may experience cravings for unusual things because they expect to experience them, in a self-fulfilling prophecy. Women who have experienced the intensity of pregnancy cravings might laugh this off, but the human relationship with food is very wrapped up in psychology. The inner workings of the mind may well be the reason why some pregnant women salivate at the thought of eating spicy crackers, while others crave potato chips.
Although pickles and ice cream are a very stereotypical pregnancy craving, this particular urge actually seems to be comparatively rare. Although some pregnant women crave pickles, they don't generally want them with ice cream, but that doesn't mean that weird food combinations don't happen in pregnancy. Peanut butter and pickle sandwiches, for example, are popular with some women, as are many other blends of traditionally salty and sour foods.