A woman can carry an embryo created from another woman’s egg, a standardized practice in modern reproductive medicine. This process separates the biological roles of genetic contribution and physical gestation using advanced assisted reproductive technologies. This approach allows individuals and couples who cannot use their own uterus to have a child, often one that is genetically related to one or both parents.
Defining the Roles: Genetic vs. Gestational Mother
The process relies on separating the two primary biological roles involved in reproduction. The woman whose egg is used is the genetic mother, or egg donor, because her oocyte provides half of the baby’s genetic blueprint. This egg contains the donor’s mitochondrial and nuclear DNA, which determines the child’s inherited traits.
The woman who carries the pregnancy is the gestational carrier, or gestational mother, providing the necessary uterine environment. The gestational carrier has no genetic link to the fetus, as the embryo was formed from a different person’s egg. Her role is to nourish and protect the developing fetus for the full term of the pregnancy.
The gestational carrier’s uterus serves as a specialized incubator, providing oxygen, nutrients, and waste disposal through the placenta. The child’s genetic makeup is established at fertilization, long before the embryo is placed into the carrier’s body. The carrier’s physical contribution is providing the optimal environment for fetal growth and development until birth.
This distinction contrasts with traditional surrogacy, where the woman carrying the pregnancy also provides her own egg and is the genetic mother. Modern gestational carrier arrangements offer a path to parenthood by separating the genetic material and the uterine environment.
The Medical Process: In Vitro Fertilization and Embryo Transfer
This separation of roles requires a precise medical procedure: In Vitro Fertilization (IVF) followed by an embryo transfer. The process begins by preparing the two women involved—the egg donor and the gestational carrier—through hormonal synchronization. This ensures the carrier’s uterus is receptive at the exact moment the donor’s eggs are ready for fertilization.
The egg donor undergoes ovarian stimulation, typically involving daily injections of gonadotropins, such as Follicle-Stimulating Hormone (FSH), for about 8 to 14 days. These hormones stimulate the ovaries to mature multiple follicles simultaneously. The donor is closely monitored with blood tests and ultrasounds until the eggs are mature enough for retrieval.
Once the follicles reach the appropriate size, a final injection of human Chorionic Gonadotropin (hCG), the “trigger shot,” is administered to induce the final maturation phase. About 36 hours later, the eggs are retrieved in a minor surgical procedure. The collected eggs are then fertilized in the laboratory with sperm from the intended father or a donor, creating an embryo.
Simultaneously, the gestational carrier is prepared to receive the embryo through a controlled hormone regimen. She is given estrogen supplements to thicken the endometrial lining of her uterus. This thickening creates a rich, blood-flow-heavy environment necessary to support implantation, ideally reaching 8 to 10 millimeters.
Progesterone supplementation is started after the lining is prepared, usually three to five days before the transfer. Progesterone makes the uterine lining more secretory, preparing it to welcome and nourish the embryo. The embryo must be transferred at the precise developmental stage that matches the carrier’s uterine receptivity, known as the “window of implantation.”
The embryo transfer is a simple, non-surgical procedure where a physician uses a thin catheter to place one or two viable embryos directly into the carrier’s uterus. The carrier continues taking estrogen and progesterone until the pregnancy is established, often through the first trimester, to ensure the uterine environment remains stable and supportive.
Situations Requiring Egg Donation and Gestational Carriers
The combination of egg donation and a gestational carrier is pursued when a woman cannot contribute a viable egg or a viable uterus.
Uterine Issues
One common indication is a medical issue resulting in the absence of a uterus, such as a prior hysterectomy or being born without a uterus (Mayer-Rokitansky-Küster-Hauser, or MRKH syndrome). These women require a gestational carrier even if they have functional ovaries.
Health Risks
Serious medical conditions that pose a severe health risk during pregnancy also necessitate a gestational carrier. These include severe heart disease, specific autoimmune disorders, or uncontrolled high blood pressure that could be life-threatening. In these cases, the woman may use her own eggs but requires the carrier to assume the physical burden of pregnancy.
IVF Failure and Age
Recurrent In Vitro Fertilization (IVF) failure or unexplained recurrent pregnancy loss, even with high-quality embryos, can indicate an issue with the uterine environment. If multiple implantation attempts have failed, a gestational carrier offers a path forward. Advanced maternal age often leads to diminished egg quality, requiring an egg donor, sometimes alongside a gestational carrier.