The "Fifty Shades Trilogy" is one of the best-selling book series in recent years with 20 million copies sold only in the last four months. And it may create a baby boom as well as a publishing boom, ABC News reports.

The three erotic novels by British author E.L. James -- "Fifty Shades of Grey," "Fifty Shades Darker" and "Fifty Shades Freed" -- are the crest of a wave called "mommy porn."

Linda Murray, global editor in chief of BabyCenter.com, told "Good Morning America" the popularity of these books could affect birth rates.

"Reading 'Fifty Shades of Grey' is acting like an aphrodisiac for women," she said. "They're having more sex and they're ultimately getting pregnant faster."

In the story, Christian Grey insists that 21-year-old college student Anastasia Steele signs a contract to submit to him as a sexual slave. The virginal Ana agrees and engages in sadomasochistic sex with the domineering Mr. Grey.

The mainstream success of the book is unprecedented since the subject matter heavily involves BDSM, or bondage, discipline, dominance and submission. ABC's Susan Donaldson James writes that the series' popularity "raises the question how sexual submission, especially when pain is involved, could be such a turn-on for many young, well-educated suburban women who are empowered economically and enlightened sexually."

Couples like Carly and Leroy Gibson of Spokane, Wash., have said the BDSM books have helped their sex life, and made it easier for them to conceive a child. After sex had become monotonous, Carly began to read the novels, and Leroy said "She just put down the book and she was all over me."

ABC News featured the Gibsons, along with two other couples who also had trouble getting pregnant. After the women in these relationships read James' novels, their sex lives improved and conception followed soon after, with all three birth dates estimated for this February.

All of the birth dates falling in the same month is no coincidence. ABC News reports that the dates are approximately nine months after the "Fifty Shades" book series saw its biggest sales increase.

The popularity of the books will inevitably lead to a feature film. The movie "Fifty Shades of Grey" is currently in the early stages of production, but rumors already are whirling.

Producers of the film have denied all of the rumors, such as the casting of Emma Watson to play Anastasia. Gather reported Monday that "no one has been approached for any role in the movie because it doesn't have a script for actors and actresses to read."

Questions have also been raised regarding the story's inspiration by the "Twilight" franchise. The series is based on "Twilight" fan fiction, which could cause copyright issues when the film is made.

Sites like Celebuzz have debunked this idea by saying, "If there was an infringement on the copyright, or anything like that, out of the Twilight [books... they would have stopped it at that point." The article continues, "If anybody would have had an issue, it would have already happened."