diff --git a/telethon/extensions/markdown.py b/telethon/extensions/markdown.py
index 2284a283..2beb3d91 100644
--- a/telethon/extensions/markdown.py
+++ b/telethon/extensions/markdown.py
@@ -90,7 +90,11 @@ def parse(message, delimiters=None, url_re=None):
                 for ent in result:
                     # If the end is after our start, it is affected
                     if ent.offset + ent.length > i:
-                        ent.length -= len(delim)
+                        # If the old start is also before ours, it is fully enclosed
+                        if ent.offset <= i:
+                            ent.length -= len(delim) * 2
+                        else:
+                            ent.length -= len(delim)
 
                 # Append the found entity
                 ent = delimiters[delim]