Ghusl is obligatory when one is in a state of janaabah (impurity)

When performing ghusl because of janaabah (impurity, e.g. after intercourse), both men
and women must wash their heads and ensure that the water reaches the roots of the hair;
it is not sufficient merely to wipe the head. A woman who wears her hair in braids is
excused from having to undo her braids when doing ghusl because of janaabah, due to the
difficulty involved in doing them up again. The evidence (daleel) for this is the hadeeth
of Umm Salamah (may Allaah be pleased with her), who said: “I said, ‘O Messenger
of Allaah, I am a woman who wears her hair in braids. Do I have to undo my braids when I
make ghusl because of janaabah?’ He said, ‘No, it is enough if you pour water on
your head three times, and then over the rest of your body. That will make you
clean.’”

(Reported by Muslim, no. 497).

But in the case of a woman who has ended her period or nifaas
(post-natal bleeding), she has to undo her braids when she does ghusl. And Allaah knows
best.