Unbirth is usually separate from genital vore, in that no actual 'consumption' takes place, only envelopment.  Unbirthers can be any character equipped with a womb.  The unbirther draws their unbirthee (some use the terms mother and child, or mother and cub for furries) through their vagina and up through the cervix into the womb.  Often, an umbilical cord will autonomously attach itself to the unbirthee, supplying oxygen and nutrients for the duration of his/her stay in the womb.  Amniotic fluid is often, but not always present in the womb. 

Many fetishes tie into this one, including pregnancy, infantalism, endosomatophilia, age regression, dominance/submission, wet-and-messy, lactation, transformation, and maternal instincts.  Age regression is a common element, wherein the unbirthee is regressed from their usual age to a much younger stage, sometimes to the point that they become an egg and join the others in the unbirther's fallopian tubes.  More often they regress to baby/cub age, and are eventually rebirthed.  Another common element is transformation, the unbirther often changing the unbirthee's species to match her own or hybridize the two.  Some focus points following an unbirth scene may be the new 'mother' experiencing difficulty getting around with her swollen belly, showing off her gravid state to others with proper maternal pride, and experiencing many of the side effects (both positive and negative) a natural pregnancy brings.

Some unbirth practitioners tie a gluttony element in with their pregnancy fetish, and simulate litters by unbirthing multiple creatures, even to the point of total immobility.

In macro-play, a very large character (think parade-balloon-character size or larger) may use an average-sized character to masturbate with, pushing them deep into the vagina.  This is a bit different from unbirth, but close enough that a separate term has not yet come into popular use.  'Living dildo' and 'insertion' are two possibilities for the term.  The act is often non-consensual, and the living sex toy frequently barely survives the experience due to a lack of air, massive muscles crushing in around them, and the drowning risk amidst sexual fluids.

[There are as of yet no authoritative publications from academics on the subject of vore as we know it, only on the loosely-related subject of cannibalism, so I rely on my identity as a vore/unbirth fetishist for over a decade, author of many erotic vore and unbirth stories, and frequent vore/unbirth-scene roleplayer to lend authority to my definitions.]