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If you have been breast feeding the child regularly then temporaray cessation of periods can occur and that is usual.By now I am assuming the child must not be breast feeding and if the cycles have not started yet then you need to get yourself clinically examined and get a Hormonal Analysis done.
Usually after child birth hormonal derangements can be expected.Kindly visit a Gynaecologist for evaluation and if required she might prescribe medications for starting the periods.
Take care!