A gestational carrier is a woman who carries a pregnancy for another woman. She does not provide a genetic contribution (ovum) to the pregnancy but provides strictly a pregnancy carrier service ("a rented womb") and hands the baby over to the genetical mother at the conclusion of the pregnancy. Thus, gestational carrying is a particular mode of assisted reproductive technology.
A gestational carrier service is a form of surrogacy. Egg donation itself is another example of third party reproduction.
Gestational carrier services may be needed in women who have normal ovarian function but have no uterus, be it the result of a congenital defect (i.e. Mullerian agenesis) or a hysterectomy at young age. Also, in some women, the uterus may be damaged from scarring (Asherman's Syndrome) or leiomyoma.
Women who provide such services may be relatives, friends, or individuals specifically recruited through agencies who do this for a payment, but also altruistically. Careful screening is needed to assure their health as the gestational carrier incurs potential obstetrical risks.