Most pregnancies carried by a gestational surrogate are realized by in vitro fertilization (IVF). IVF is when an embryo is made in a lab utilizing an egg and sperm in the supposed parents or an egg and/or sperm donor. What is considered a human appropriate is in some cases controversial; https://medium.com/@scaliforniasurrogacy/southern-california-surrogacy-c4408a88a1c1