I honestly think it started out as black marker drawings as part of a "defacing" and "violation" fetish that sparked a trend in some breeding, impregnation and insemination fetishes. Someone probably thought "hey, that would make a great tattoo!" and must have really enjoyed hardcore sex.
Imaging guy wanting to pin her down and knock her up just tunnel visioning on that tattoo like he's using x-ray vision to see himself fertilizing her ovaries at the peak of climaxing. After 9 months, that tattoo stretched out over the baby bump as a less than subtle reminder, "breed me! I'm ready for more babies!"
At least, that's how I think it started. I like my version of the story better, clinical input on the topic is just less exciting.