if after a month you srart bleeding then u consider it as haidh but if it continues more than 10 days then you follow rules of istihaza. If the woman is not affected by fasting, and fasting is not too difficult for her, and she does not fear for her child, then she is obliged to fast, and it is not permissible for her not to fast. so u have to decide that your child should not be effected by fasting