Applying for a visitor's visa needs a valid reason for the visit. So, I disagree with Picklehead about his point about not mentioning the baby situation, especially the point that grandparents taking care of the child is a skilled work. This simply is not true. being there for the birth of your grandchild is a greater occasion than witnessing a graduation (for example).  So, I think it's better you mention it and highlight that this is the reason your parents want to visit. Of course, also provide documents to convince the consular that they would be back In Nepal after the baby is born.

However, I know of a case where a set of parents applied for vistor's visa specifically for this purpose, but didn't get the visa because they didn't provide proof that there really was a baby coming up. So, what I would suggest is that you also provide a doctor's or a hospital's certification, which confirms 1) you/spouse is actually pregnant  and 2) the expected delivery date, just so you are covered.