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Stammering or stuttering is not very uncommon in this age group. Most children do stammer at this age, and with time over a few months, it gets resolved usually. I believe that the delivery of this child was smooth, and she had no history of admissions for any illness. Also, her milestones are normal and not delayed.
Now here, the family has to play an important role. Your child has started developing speech, and this is quite normal to happen. So, first of all, you stop worrying. And even if you worry because I know it can be alarming for parents, please do not pass on your anxiety to your kid. Just do not let her feel that it is abnormal. Whenever she speaks, to listen to her and never try to correct her or scold her for this. Pay attention to what she is saying and look at her, and eye contact should be there.
When you talk to her, talk slowly so that she can grasp every word. Kids always try to imitate us. She will also start speaking slowly, which will help her say words correctly and with ease. Never criticize or scold, also tell everyone at your home and even school teachers. Encourage her to speak. Do not pass your anxiety to your kid is most important because if a child gets anxious, it will get more pronounced. Slowly and gradually, you will see that your kid has outgrown this.
If still, you feel that there is no improvement after one to two months, speech therapy may be done. Speech therapy should be started before four to five years. I hope this advice would be helpful to you. I would be there for any help.