Section 4G. Any subscription certificate under an individual or group medical service agreement which shall be delivered or issued or renewed in this commonwealth shall provide as benefits to all individual subscribers and members within the commonwealth and to all group members having a principal place of employment within the commonwealth, for expense for services of a certified nurse midwife; provided, however, that such services are reimbursed when rendered by any other duly licensed practitioner; and provided, further, that such services are within the lawful scope of practice for a certified nurse midwife.