SAKURA IS SARADA'S MOTHER.
1. Karin said "My desk is a part of me", is that object biologically related to her?
According to the japanese and chinese translation, it is more accurate to say "That thing on my desk is LIKE a part of me", which means it is just a metaphor to show the importance of that object. Hence, the object is very obviously not containing Karin's DNA.
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2. What is that object?
Probably the umbilical cord of Sarada. Please take a close look of the object shown in the manga. In addition, suigetsu mentioned " I thought this was from when karin was born " It's probably something that comes out during delivery.
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3. Why Sarada's?
The machine state that the object MATCHES Sarada's DNA. If you read the manga carefully or you understand some basic concept relating to DNA, the only case when the two DNA matches is when the object BELONGS TO YOU. In general and to be short, you have half of your DNA from your father and the rest is from your mother. Even if the object is from Karin's body, it is impossible to say that they fully MATCH.
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4. Why is Karin keeping Sarada's umbilical cord?
Well, this is just an assumption from the fandom. As we all know, Sarada has no birth record in Konoha, We may assume that Sakura gave birth to Sarada with Karin's help outside the village for whatever reasons. What i mean is, Karin being the midwife during delivery. Given that Sakura is Sasuke's wife and a lady once saved Karin's life, is it not possible for Karin to offer a help? Karin loves Sasuke, is it not possible for her to keep something important, something from Sasuke's baby, something she treasure a lot that she see it as "part of me"?
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5. What about suigetsu saying "Your mother is Karin"?
Suigetsu is a troller, as always.
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6. Why are you so sure that Sakura is Sarada's mother?
Didn't you hear that Sasuke said "MY WIFE"?
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7. Sakura has been taking care of Sarada more than ten years! How could Sarada give up on her that easily?
Please understand it is not that easy for a 12 year-old child to accept the "truth" that her mother is not her "real" mother in just a short moment. In the last page of the chapter it seems that Sarada is not going to save Sakura. However the official preview of the next chapter states "What is Sarada's decision? Can Sarada save Sakura, who is now trapped?" We all know the result will be Sarada saying Yes to save Sakura.
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I believe everything will be solved in the end of this series. To be honest, I never thought that Sarada being Sakura's daughter can be such controversial even in the SS fandom. This again proved fans' lack of confidence to this couple. Please, if you love them, please put faith in them. Not to mention that Naruto is a shonen manga, not a Korean drama. I hope this little article helps.