The National Library of Medicine (NLM) is currently involved in MEDLINE year-end processing (YEP) activities. These include changing the Medical Subject Headings (MeSH) and Substance Names on existing MEDLINE citations to conform with the 2013 version of MeSH, and other global changes.
November 14, 2012: NLM expects to temporarily suspend the addition of fully-indexed MEDLINE citations to PubMed. Publisher-supplied and in process citations will continue to be added.
Mid-December 2012: PubMed MEDLINE citations, translation tables, and the MeSH database will have been updated to reflect 2013 MeSH.
For details about the impact on searching from November 14 to mid-December, see: Annual MEDLINE/PubMed Year-End Processing (YEP): Impact on Searching During Fall 2012.
For background information on the general kinds of changes made annually, see: Annual MEDLINE/PubMed Year-End Processing (YEP): Background Information.