According to Fannie Mae guidelines, lenders are required to include alimony or child support in a borrower's monthly debt figure when calculating a debt-to-income ratio as long as the payments are scheduled to continue for more than ten months. If the borrower is obligated to pay child support for less than ten months then the child support payments are not included in the borrower's debt-to-income ratio. Typically the only exception to the ten month guidelines for child support or alimony payments is if the payments are voluntary. I have included the specific language for the guideline on this topic below.
"When the borrower is required to pay alimony, child support, or maintenance payments under a divorce decree, separation agreement, or any other written legal agreement—and those payments must continue to be made for more than ten months — the payments must be considered as part of the borrower’s recurring monthly debt obligations."