All perfect praise be to Allah The Lord of the Worlds and may His peace and blessings be upon our Prophet Mohammad and all his family and companions.
When postpartum bleeding (Nifaas) stops and a woman sees the sign of purity-vagina is dry or there is a white transparent discharge-then her Nifaas has ended. Therefore, she should make Ghusl (Ritual bath), fast Ramadan and perform prayer. This is because scholars are agreedthat the minimum of Nifaas is a moment; thus, whenever a woman sees the sign of purity, she should perform Ghusl, pray, and fast.
The eminent scholar Ibn Hajar Al-Haitami was once asked about: "a woman who became pure after three days from her second delivery, although, in her first delivery, she became pure after forty days. Should she perform Ghusl and pray in case she saw no blood? And in case it ceased for three days, then resumed; what is the ruling on the prayers that she had offered in those three days? He (May Allah have mercy on his soul) replied: "Once the woman in menstruation or Nifaas sees no blood-in the sense that if she inserted a piece of cotton into her vagina it would come out white and pure-she should make Ghusl and pray. It is also permissible that her husband makes love to her. This is regardless of whether the blood stopped before the end ofthe forty days of Nifaas or not. If the Nifaas blood resumed after less than fifteen days from its stoppage, then those are days of menstruation or Nifaas. However, if the Nifaas blood resumed after fifteen days from its stoppage, then it is menstrual blood and those fifteen days are days of purity (Tuhr), and so she should make up for the prayers that she had missed in them.
In case of menstruation, if the blood resumed after fifteen days from stoppage, then it indicates a new period and the stoppage is a time of purity (Tuhr), so prayers missed during these fifteen days should be made up for. If the blood resumed after less than fifteen days from its stoppage, then it is menstrual blood." {Al-Fatawa Al-Fiqhia Al-Kobra, vol.1/pp.79}. And Allah Knows Best.