The association shall be comprised of all insurers possessing a certificate of authority to write and engaged in writing medical malpractice insurance within this state and general casualty companies. Every insurer shall be a member of the association and shall remain a member as a condition of its authority to continue to transact business in this state. Only licensed midwives under chapter 18.50 RCW, certified nurse midwives licensed under chapter 18.79 RCW, or birth centers licensed under chapter 18.46 RCW may participate in the joint underwriting authority.