In the Ring novel Jotaro Nagao claims that when he raped Sadako (shortly before murdering her), he discovered she had Testicular Feminization Syndrome; despite having the external appearance of a beautiful woman, chromosonally she is XY, a male. The only external sign of her condition is the fact that she has external testes. Internally, she lacks a uterus or ovaries, meaning she could never conceive a child. This aspect of Sadako's character is not included in the film versions of Ring (or its American remakes), though that aspect was included in the television serials and the Korean remake.
When Sadako is reborn in Spiral she has changed, physically; The "new" Sadako has a womb and ovaries, and also states in a letter to Ando that "the man in me can ejaculate." However, it would appear that she does not have a penis as such; rather, her ejaculations are internal - into herself.
Although Sadako inseminates herself twice in Spiral, she never carries an egg fertilised only by herself to term - and as such it cannot be said how such a pregnancy would operate. The following details are based on her "resurrections".
By removing the DNA in one of Sadako's fertilized eggs and replacing it with that of another (then returning the egg to her womb) Sadako can "resurrect" the dead (or potentially clone the living). Her pregnancies, in these cases, last about a week, and the offspring grows back to their age/level of physical development from when the sample was taken in another week. The offspring retain all their memories from when the sample was taken, which Suzuki explains by claiming that memories are stored in the intron of DNA.