It is rare that another gandhabba can take over a living human body (possession).
– The person “being possessed” must have a weak mind and must willingly “hand over” the body to another gandhabba. Usually, that other gandhabba is a deceased person who had authority while living.
– For example, someone with a weak mind could be possessed by the gandhabba of a deceased relative. In Sri Lanka, there have been a few credible accounts.
On the other hand, it is of the more common occurrence of the case Daniel mentioned at the end. It is possible for a gandhabba who had taken possession of a zygote to leave and for another gandhabba to take possession of that zygote.
– That may happen within several days or weeks of pregnancy.
To learn about the zygote and gandhabba, see “Buddhist Explanations of Conception, Abortion, and Contraception“