TMZ has learned Mitt Romney's son Tagg -- who had twins this year through a surrogate -- signed an agreement that gave the surrogate, as well as Tagg and his wife, the right to abort the fetuses in non-life threatening situations ... and Mitt Romney covered some of the expenses connected with the arrangement

"If in the opinion of the treating physician or her independent obstetrician there is potential physical harm to the surrogate, the decision to abort or not abort is to be made by the surrogate."

Translation: Tagg and Jen gave the surrogate the right to abort the fetuses even if her life wasn't in danger. All the surrogate has to show is "potential physical harm," which could be something like preeclampsia -- a type of high blood pressure that could damage the mother's liver, kidney or brain, but is not necessarily life-threatening.