diff --git a/mercurial/commands.py b/mercurial/commands.py
--- a/mercurial/commands.py
+++ b/mercurial/commands.py
@@ -2233,14 +2233,14 @@ def merge(ui, repo, node=None, **opts):
         node = parent == bheads[0] and bheads[-1] or bheads[0]
 
     if opts.get('preview'):
-        p1 = repo['.']
-        p2 = repo[node]
-        common = p1.ancestor(p2)
-        roots, heads = [common.node()], [p2.node()]
+        # find nodes that are ancestors of p2 but not of p1
+        p1 = repo.lookup('.')
+        p2 = repo.lookup(node)
+        nodes = repo.changelog.findmissing(common=[p1], heads=[p2])
+
         displayer = cmdutil.show_changeset(ui, repo, opts)
-        for node in repo.changelog.nodesbetween(roots=roots, heads=heads)[0]:
-            if node not in roots:
-                displayer.show(repo[node])
+        for node in nodes:
+            displayer.show(repo[node])
         displayer.close()
         return 0
 
diff --git a/tests/test-merge-default b/tests/test-merge-default
--- a/tests/test-merge-default
+++ b/tests/test-merge-default
@@ -50,4 +50,11 @@ hg commit -mf
 echo % should fail because merge with other branch
 hg merge
 
+# Test for issue2043: ensure that 'merge -P' shows ancestors of 6 that
+# are not ancestors of 7, regardless of where their least common
+# ancestor is.
+echo % merge preview not affected by common ancestor
+hg up -q 7
+hg merge -q -P 6         # expect: 2, 4, 5, 6
+
 true
diff --git a/tests/test-merge-default.out b/tests/test-merge-default.out
--- a/tests/test-merge-default.out
+++ b/tests/test-merge-default.out
@@ -34,3 +34,8 @@ created new head
 % should fail because merge with other branch
 abort: branch 'foobranch' has one head - please merge with an explicit rev
 (run 'hg heads' to see all heads)
+% merge preview not affected by common ancestor
+2:2d95304fed5d
+4:f25cbe84d8b3
+5:a431fabd6039
+6:e88e33f3bf62