Prob. Code 21620 and 21622 deal differently with pretermitted heirs born after and those born before the testator’s death.  Section 21620 establishes a presumption that the heirs born after the testator’s death were omitted unintentionally and thus are entitled to inherit, unless an opposing party proves otherwise.  Under 21622, however, the burden is on the heirs born before the testator’s death to prove he unintentionally excluded them from the will because he didn’t know of their existence and would not have excluded them had he known they were alinve.  A general disinheritance clause–disinheriting all those not expressly provided for in the will–can be effective to disprove intent to benefit unknown but existing heirs.