One of the first things that they tell you once you have applied for a Green Card is that you should not leave the United States until the card is in your hands. The reason for this is that since your residence status is pending and the border authorities might determine that because you left the country you have decided to abandon your residency application midway. Its understandable however, why one might need to leave the country during the wait; it can take up to a year for the Green Card application to get processed, in which for a number of reasons the applicant might have to leave the country. What if you need to make an emergency that cannot be postponed? Are you willing to risk your Green Card for this? Is there a way around this problem?
Thankfully there is. The Advanced Parole Document allows you to re-enter the United States without worrying about losing your application for residency. The only catch for this, as with all documents is that it takes a while to be processed (3 months). To apply for this document, you need to fill out the Form I-131 (Application for Travel Document). If you know that you will need to travel outside the country while you are waiting for your green card, you can fill out and send both applications together. You can also fill it out after the Green Card application sent. Keep in mind, however, that there is a processing fee of $360 and an additional fee may be charged if you expedite it.
This document is valid for one year, which is approximately the amount of time it takes for the residency application to be processed, so you are safe to travel within that time frame if you have this document. And if for some reason, your application for the Green Card takes a little longer, you can file for it to be renewed.