http://www.dwp.gov.uk/publications/spec ... owance-ma/
Maternity Allowance is not normally regarded as Public Funds, so should not affect your future application for PR.
However, you have to consider if you are intending to return to employment after the birth of your child.
If your job has been kept open for you, you remain employed by your employer, then you will be considered to be exercising treaty rights as a worker.
If you resign from your job to be a full-time mother, you will be considered voluntarily unemployed and not exercising treaty rights as a worker / jobseeker.
Unless you continued to exercise treaty rights as self-sufficient with sufficient funds and CSI for all family members when not a worker ( if you did resign), then you would break the five continuous years of exercising treaty rights that are required to acquire PR.