Here are two screenshots of 奶奶 and 姥姥 from the dictionary, Dictionary of Modern Chinese, or 现代汉语词典 in Chinese, published by the Commercial Press, or 商务印书馆 in Chinese.
As you can see, 奶奶 could mean the paternal grandmother. It could also refer to elder women whose ages could be those of the grandmothers. In the old days, it could mean the younger daughters-in-law in the houses of the wealthy or higher social status, such as, officials.
As you can see, 姥姥 could mean the maternal grandmother. It could also mean the midwife.