Hi,
How is your wife? Are there other symptoms present? Is there a particular time or activity that causes the symptom? This could indeed be related to her pregnancy and changing hormones as mentioned above. However, not all medications are safe during this time, so it is best that she checks with her doctor for proper management. Rest and avoidance of triggers may help provide relief. Take care and best regards.
Since it's not likely you've ever been pregnant,:) I can see why you don't understand how it can be head pain, but not a headache. During the end of the second trimester, the weight of the baby is starting to press on the nerves of the spine, this can cause all kinds of strange sensations and sometimes pain. With a headache, normally it feels like the pain is inside your skull. If it is nerve pain, it feels closer to the surface of the skin. Have her check with her OB to see if it would be ok to visit a chiropractor. Normally it's ok during pregnancy, but her doctor would know if there is any reason against it. Eventually the baby will shift to a new position, but if she is very uncomfortable, the chiropractor can help. I saw my chiropractor during two of my three pregnancies and it relieved a lot of nerve pain for me.