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The current population of the world is around 7.6 billion, so it would not be far fetched to say that the chances for you being born is 1 in 7.6 billion, and in other words, you are one of a kind. It is sad that your mother had a miscarriage. Your unborn sister or brother would have been unique, just like you are. If your mother had not had a miscarriage, you would have had an older brother or sister.
In this life, miscarriages and all sorts of sadness exist but in the next life, in heaven, all sorts of suffering will cease. Ever since sin entered the world, death did also. Jesus came to save us from sin and He succeeded. For God so loved the world that He gave His only begotten Son that whosoever believes will not perish but have everlasting life (John 3:16). You can read more about how the plan of salvation came about in the following link: https://bibleask.org/plan-salvation-come/
In His service,