If you have a pending Application to Register Permanent Residence or Adjust Status, you should carefully examine your situation and ideally contact your immigration lawyer prior to making any international travel plans.
The U.S. Citizenship and Immigration Service filing fee for Applications to Register Permanent Residence or Adjust Status include an Application for Advance Parole. Most aliens need Advance Parole to reenter the United States after traveling abroad if they have pending applications for immigration benefits. However, if you are an H, L, V or K3/K4 nonimmigrant, you must present a valid visa upon reentry to the United States.