I have in front of me the 16th edition (1995) and 18th edition (2006). They are two entirely different books.
The 16th edition is comprehensive and explains things in detail to assist understanding. The 18th edition misses out all of this detail and provides only one short paragraph on many important common pathologies e.g. Bartholin's cyst. It COMPLETELY OMITS certain other common pathologies e.g. mid-cycle ovulatory pains (which are included in the 16th edition).
And whilst managing all of these feats, the 18th edition is somehow also physically thicker than the 16th edition :S. Perhaps they used a larger font size? Because in terms of useful content, its much less.
I'm just lucky I didn't buy the 18th edition. I borrowed it from a library.