Board Thread:Theories and Speculation/@comment-26622660-20150715231416/@comment-24949460-20150716143023

Basically guys Kishimoto didn't put a flashback showing Karin with a pregnant Sakura. While Sakura is the Nurture mother, Karin is still the biological mother(Nature).

When the chakra fruit was mention Kishimoto showed what it looked like with a flashback panel. When Kaguya was first introduced was using a semi flashback that Hagoromo showed Naruto. I think Karin created Sarada using the Shin method but using Sasuke and Her DNA then gave the child to Sakura since she is closer to Sasuke then her. Makes sense with the whole why did they keep the birth a secret.

Also:

The chakra strengthening isn't a bloodline ability but something that can be LEARNED. Sakura isn't Tsunade's daughter and yet she learned the HSS and HSJ.