Well yes you are right except there are some exceptions. Historically the sentence would have been "ac yr ydw i'n disgwyl babi", this then became "ac rydw i'n disgwyl babi" (this is still used in more formal situations) and then became "ac dw i'n disgwyl babi".
Ah, yes - thank you - subtle, but makes perfect sense. And as spoken by native South Walians, "ac wi'n disgwyl..." BTW, you gotta feel sorry for the poor girl - pregnant and expecting a baby - burdens never come singly, it seems.