When Marty was conceived, there was a chance he could have belonged to either Luis or Antonio, as Sheridan had been married to Antonio while having an affair with Luis. Sheridan was kidnapped when she was only a few weeks from her due date, and during that time it was decided that the baby's father was Luis, although there has never been any sort of testing done an dboth men were within the right time frame. Marty was born in Beth Wallace's basement, and she immediately took over the boy, claiming her as her own child. After Sheridan was left to die in the ocean, she was made to believe her baby boy had died.

Since the moment he was born Marty has been raised as Beth's son with Luis. His father hasn't been around much, so he has been primarily raised by Beth, with the help of her mother, Edna, and their orangutan helper, Precious. As an infant, he was breastfed by Sheridan as Beth was unable to do so (Beth had never actually been pregnant and had only pretended that she was), prompting Sheridan to believe for quite some time that Marty was her son. Eventually Sheridan accepted that her child was dead, but has maintained a close relationship with Marty. People eventually started noticing that Marty looked a lot like Sheridan, and it wasn't long before Sheridan herself admitted she had never stopped thinking of Marty as her son.

The miracle that Sheridan had been waiting for finally occurred and she was able to prove, through DNA, that Marty was her son (and Beth her half-sister). But before she was able to take custody of him, Beth fled the country with him, with Alistair's help.