Oh man, I've had this one in the back of my head for ages. I recall it being posted on the old, old Pregchan and it got rave reviews from our little community.
If anybody has a gallery link to the full doujin, please share.
(Same goes for another, similar ero-manga. Mother and daughter are impregnated by bulls, get very large, and give birth. One even has her womb come out with the bull.)