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If she is regularly and exclusively breastfeeding the baby, she will not have her periods due to the lactation amenorrhea. Exclusively breastfeeding the baby gives signals to ovary to stop releasing the eggs thus not letting the periods to occur. However, if you have been sexually active without protection (and if she has had occasional ovulation/egg release from ovary at the time), then chances of pregnancy are also possible. She should consider visiting her doctor/gynecologist for further evaluation and assistance. If pregnancy gets ruled out, and if you intend to be sexually active, she should also consider getting started on safer birth control measures to prevent repeat pregnancy.
Take care.