SPOILERS: So I am on my second read through the DT series and this time being very careful to understand every complicated part I missed the first time around.

In Song of Susannah, Mia originally lies to Susannah and says that she was a demon elemental able to change sexes, and was both the original oracle in The Gunslinger AND the demon that Detta/Susannah tangled with in The Wastelands.

After a while, still in Song of Susannah, Mia comes clean and tells/shows Susanna the truth (which King never hints at as being a lie, at least not the GIST of it). This truth is that Mia was originally a low-ranking creature of the prim, a kind of succubus whose real form is a small cloud of darkness. Men would see her and fall in lust, she would screw them to death, getting nothing out of it. One day she saw a baby boy, a rare occurrence in her where and when, and fell in love. The family was protected by some aura, though. Oh, Discordia.

Flagg came to Mia and saw her true form. Heard her tale. Offered her a deal: 7 years of motherhood (known even then by Flagg to be a lie), but she would become mortal. She accepts. Flagg takes her to a kind of Dogan and uses a machine to make her mortal. For a week or two or three, she only remembers a red haziness.

It is then explaines that Mia never got any sperm. In Susannah's words, "I caught the fuck." Susannah was impregnated in The Wastelands while pulling Jake through (how Roland's sperm got there is obvious. The Crimson King's ... I disremember, but I believe it had previously been given to that demon by Flagg. The original Oracle in The Gunslinger had nothing to do with it). Then, some machinery from the old people "faxed" the baby from Susannah to Mia one cell at a time throughout the period of Susannah's pregnancy and the two's shared birth.

But then, in The Dark Tower, all this is retconned! King(not a character, but just narration - so King himself, the author) says that after Mia was made mortal, the Crimson King either screwed her during the "red haziness (paraphrase)" or had his sperm transported to her and that Roland's sperm was already inside her! No further explanation is offered. This frustrates me more than it should, because so far, everything else I've been confused on during my first read has been made clear now, and so was this, in Song of Susannah. Until The Dark Tower retconned it all. I can't even explain it as Susannah misremembering, since it's not her speaking this, but just King's narration/recap of it. I also can't explain it as Mia having still been lying, because I honestly think she was telling the truth, except about less important stuff like why she accepted the deal.