There is no distinction between a human life born in marriage or a human life born from a dish. Afterall Mary the mother of Jesus was a surrogate supplied with sperm. She was not a wife but a birthing Channel to give birth to Jesus. She gave birth by means of In Vitro Fertilization an assisted reproductive technolog commonly referred to as IVF. IVF is the process of fertilization by extracting eggs, retrieving a sperm sample, and then manually combining an egg and sperm in a laboratory dish, but in Mary's case the sperm went directly to her uterus.

There are no sins involved with being a surrogate i.e., adultery or fornication. Because the vagina remains off limits to any copulation. If In Vitro Fertilization is off limits it would be because of laws that were against organ transplants which there are none. A surrogate is similar in kind to Organ transplantation a medical procedure in which an organ is removed from one body and placed in the body of a recipient, to give life. IVF is manually combining an egg and sperm in a laboratory dish to facilitate life in the natural mother or a surrogate.

At what point should a Christian couple use a surrogate? After praying for several months or years or does it matter that a surrogate could be the answer to those prayers. Jesus went against nature and brought back sick Lazarus and a 12 year old girl named Jairus from the dead so why is science any different than miracles when used to facilitate life.

All of it is by divine miracles and science through the will of God. So, yes, a Christian can use a surrogate with prayer.