We have explained the ruling on deliberate abortion in the answer to question no. 42321, please refer to it. We have also explained the rulings on loss of the foetus at various stages of pregnancy in the answer to question no. 12475.
If a woman miscarries a foetus and it has no discernible human features, such as a head or limbs, then the bleeding that accompanies that is irregular bleeding, which does not stop her from praying and fasting. If there are discernible human features, then the bleeding is nifaas. The minimum period in which human features appear is eighty-one days, as is stated in the answer to question no. 37784.
If you broke your fast because you thought that the bleeding was nifaas, then you found out that it was irregular bleeding and you made up the day you missed and the prayers, then you do not have to do anything lese. If you have not made up the prayers of that day, then hasten to do so.
May Allaah help us and you to obey Him and seek His pleasure.
And Allaah knows best.