Mass. Gen. Laws ch. 186, § 16

Any provision of a lease or other rental agreement relating to real property whereby a lessee or tenant enters into a covenant, agreement or contract, by the use of any words whatsoever, the effect of which is to terminate, or to provide that the lessor or landlord may terminate, such lease or agreement if the tenant has or shall have a child or children, who shall occupy the premises covered by such lease or agreement, shall be deemed to be against public policy and void.