Filing jointly may be best for 2017 depending on facts and circumstances and whether you can agree to do that. If not, then filing separately may be the only option.
However, there are special rules to see if you can be "considered unmarried." If the custodial parent meets these rules then she can take the exemption and use Head of Household (HOH) filing status instead of Married Filing Separate (MFS) which creates penalty situations.
Considered unmarried. (by clicking the links you will access the IRS site for more detail).
You are considered unmarried on the last day of the tax year if you meet all the following tests.
You file a separate return. A separate return includes a return claiming married filing separately, single, or head of household filing status.
You paid more than half the cost of keeping up your home for the tax year.
Your spouse didn’t live in your home during the last 6 months of the tax year. Your spouse is considered to live in your home even if he or she is temporarily absent due to special circumstances. See Temporary absences , later.
Your home was the main home of your child, stepchild, or foster child for more than half the year. (SeeQualifying person , later, for rules applying to a child's birth, death, or temporary absence during the year.)
You must be able to claim an exemption for the child. However, you meet this test if you can’t claim the exemption only because the noncustodial parent can claim the child using the rule described later inSpecial rule for divorced or separated parents (or parents who live apart) under Exemptions for Dependents. The general rules for claiming an exemption for a dependent are shown in Table 3.
