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 Allah be pleased with her), who said: “I said, ‘O Messenger of Allah, 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 Allah knows best.