To the best of our knowledge, this segulah is fairly flexible.
A woman should not immerse while she is in niddah, though if she is immersing as as usual following her time in niddah, she can also arrange to have her immersion be after a woman in her ninth month.
There is nothing barring a woman from immersing when she is already tehorah. If she is already tehorah, the segulah immersion need not involve all of the usual preparations, and she should not recite a bracha over the immersion.
It is best to consult a mikveh attendant in advance to make these arrangements.
This response was updated on 8 April, 2021.