Yes it is. Assuming you meet the other requirements of the MA maternity leave law, you are entitled to unpaid maternity leave if you adopt a child under the age of 18 OR if your adoption involves a child under the age of 23 who is mentally or physically disabled.  For your information, I have included below the text of 804 Code of Massachusetts Regulations 3.01(8), which spells out some of the requirements of the MA maternity leave law.  

(8) Special Provisions Relating to Maternity Leave.

(a) Definition. The term "maternity leave" means a period of time, not exceeding eight weeks, that a female employee is absent from employment for the purpose of giving birth or adopting a child.

(b) Eligibility for Maternity Leave. A female employee is eligible for maternity leave if:
1. she has completed the initial probationary period, if any, set by the terms of her employment; or has been employed by the same employer for at least three consecutive months as a full-time employee; and
2. she is absent from such employment for a period not exceeding eight weeks for the purpose of:
a. giving birth; or
b. adopting a child under the age of 18; or
c. adopting a child under the age of 23, if the child is mentally or physically disabled; and
3. she gives her employer at least two weeks' notice of her anticipated date of departure and intention to return.

Hope that helps.  Follow this link to another post regarding the rights and responsibilities of employees and employers upon the completion of the maternity leave.  Good luck and, when the time comes, congratulations!