If you say you are resigning ahead of your return date they may take it as the date of the letter which may not give your required notice period.
I would say something along the lines of
"As you may be aware, I am due to return from maternity leave on the XXX. However, a change in circumstances has eventuated and I will not be returning to work on this date, accordingly I submit my 4 weeks notice of resignation, which will take effect from XXX."
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