> A woman's maternity is never in doubt but a man's paternity can be.
That's not always true. See the Lydia Fairchild case [1], and the earlier Karen Keegan case [2]. These are both cases where women were told, wrongly, based on genetic testing, that they were not the mothers of their children.
Fairchild was pregnant when the controversy over her maternity arose, and the court overseeing that controversy ordered a witness to attend the birth and ensure that blood samples were taken for testing from both Fairchild and the newly born child. Those samples were tested, and the result was that she was not the mother of that child.
How could this be? The answer turned out to be that both Fairchild and Keegan were chimeras. A chimera is a single organism composed of cells from different zygotes. This can happen when multiple fertilized eggs merge. In other words, both of these women were conceived as part of a set of fraternal twins, but sometime during development they merged with their twin.
Some organs and systems developed from one of the twins, some from the other, and so when you do a DNA test you get the DNA of one or the other, depending on where you sample. In Fairchild's case, for instance, DNA taken from her skin and hair did not match her children's DNA, but DNA from a cervical smear did.
Human chimeras are thought to be very very rare.
[1] https://en.wikipedia.org/wiki/Lydia_Fairchild
[2] http://www.nejm.org/doi/full/10.1056/NEJMoa013452