Congratulations!
Non-issue. The rules allow for you to use your pay prior to maternity leave. Tick that off the list of things to worry about!
https://www.gov.uk/guidance/immigration-rules/immigration-rules-appendix-fm-se-family-members-specified-evidence5. In respect of statutory or contractual maternity, paternity or adoption pay all of the following, and in respect of parental leave in the UK only the evidence at paragraph 5(c), must be provided:
(a) Personal bank statements corresponding to the same period(s) as the payslips at paragraph 5(b), showing that the salary has been paid into an account in the name of the person or in the name of the person and their partner jointly.
(b) Payslips covering:
(i) a period of 6 months prior to the date of application or to the commencement of the maternity, paternity or adoption leave, if the applicant has been employed by their current employer for at least 6 months (and where paragraph 13(b) does not apply); or
(ii) any period of salaried employment in the period of 12 months prior to the date of application or to the commencement of the maternity, paternity or adoption leave, if the applicant has been employed by their current employer for less than 6 months (or at least 6 months but the person does not rely on paragraph 13(a)).
(c) A letter from the employer confirming:
(i) the length of the person’s employment;
(ii) the gross annual salary and the period over which it has been paid at this level;
(iii) the entitlement to maternity, paternity, parental or adoption leave; and
(iv) the date of commencement and the end-date of the maternity, paternity, parental or adoption leave.