For what it's worth, it is mentioned on Dallas's Wikipedia page and also on the character's (Kristin Shepard) Wiki page:
At the end of the third season, Kristin returned yet again and soon thereafter was found drowned in the Southfork Ranch swimming pool. She had overdosed on drugs and, in her stupor, fell and hit her head and went over the balcony railing, landing in the pool. After Kristin's death, it was learned that her husband, Jeff Faraday, may have been the father of her child, who had become Bobby and Pam's adopted son, Christopher Ewing. Jeff claimed Kristin told him she had miscarried the child she had conceived with J.R and that the child she gave birth to was his.