There are two kinds of surrogate mothers.
Artificial insemination first made surrogacy possible. A traditional surrogate is a woman who is artificially inseminated with the father's sperm. She then carries the baby and delivers it for the parents to raise. A traditional surrogate is the baby's biological mother. That's because it was her egg that was fertilized by the father's sperm. Donor sperm can also be used for a traditional surrogacy.
In vitro fertilization (IVF) now makes it possible to harvest eggs from the mother, fertilize them with sperm from the father, and place the embryo into the uterus of a gestational surrogate. The surrogate then carries the baby until birth. A gestational surrogate has no genetic ties to the child. That's because it wasn't her egg that was used. A gestational surrogate is called the "birth mother." The biological mother, though, is still the woman whose egg was fertilized.
In the U.S., gestational surrogacy is less complex legally. That's because both intended parents have genetic ties to the baby. As a result, gestational surrogacy has become more common than a traditional surrogate. About 750 babies are born each year using gestational surrogacy.
A woman might decide to use a surrogate for several reasons:
Other women choose surrogacy after trying unsuccessfully to get pregnant with a variety of assisted-reproduction techniques (ART), such as IVF.
Surrogates have also made parenthood an option for people who might not be able to adopt a child. Reasons could include:
For instance, when gay men use a traditional surrogate, one of them uses his sperm to fertilize the surrogate's egg through artificial insemination. The surrogate then carries the baby and gives birth. A gay couple might also choose an egg donor, fertilize that donated egg, and have the resulting embryo implanted in a gestational surrogate to carry until birth.