The following is based on Van Buren and has not been confirmed by canon sources.
Born ghouls are ghouls who are not mutated humans, but ghouls who were actually born as ghouls. Through cruel experimentation on humans by Dr. Sebastian at the Reservation, these ghouls gestated and were birthed. Reveling in their infrequent success (the ghoul procreation experiments have a 2% success rate), Dr. Sebastian and Dr. Willem Clark keep their born ghoul progeny close at hand and comfortable. To them, the born ghouls represent the future of the ghoul "species", and they will do everything in their power to make sure their "children" are well taken care of. There are only three born ghouls, including Measles and Belle.
At first glance they look like any other ghoul, but a closer inspection reveals that their lesions, exposed bone, and hair patches are very uniform and look like they serve some kind of purpose. On top of that, these ghouls are a little tougher, a little stronger, and a little faster than normal ghouls.
If successful, this could also mean that over time the pregnancies could phase out the feral ghouls, as these are produced when radiation destroys the mind of the ghouls, and if simply born there would be no particular afflictions to the ghouls to fear. This would mean that if the feral type were killed, there would be none to replace them, therefore ghouls may one day be rid of the primary source of humanity's stigma towards them.