How Does the Surrogate Mother Get Pregnant? The IVF Process Explained

By Dr. Rachel Whitmore, OB/GYN  |  Updated February 5, 2026

A surrogate mother gets pregnant through in vitro fertilization (IVF) embryo transfer — not through sexual intercourse. An embryo created from the intended parents’ egg and sperm (or donor gametes) is transferred directly into the surrogate’s uterus using a thin catheter. The surrogate takes hormone medications for 3 to 5 weeks before the transfer to prepare her uterine lining for implantation. Success rates for gestational surrogacy transfers range from 60% to 75% per cycle with high-quality embryos. This guide explains every step of the medical process, from initial hormone protocol through pregnancy confirmation.

One of the most common questions about surrogacy is how the surrogate mother actually becomes pregnant. The answer is entirely medical — gestational surrogacy uses assisted reproductive technology, specifically IVF, to achieve pregnancy. The surrogate has no genetic connection to the baby she carries. Understanding this process is important for both prospective surrogates and intended parents because it sets accurate expectations about the medical commitment involved.

Gestational vs. Traditional Surrogacy: A Critical Distinction

Before explaining the medical process, it is important to understand the two types of surrogacy and why they involve completely different methods of conception.

Gestational surrogacy (99% of US surrogacy): The surrogate carries an embryo that was created through IVF using the intended parents’ egg and sperm, or donor gametes. The surrogate provides no genetic material. Pregnancy is achieved through embryo transfer, a clinical procedure performed at a fertility clinic. The surrogate is the gestational carrier — she carries and nurtures the pregnancy but is not the biological mother.

Traditional surrogacy (rare, less than 1%): The surrogate uses her own egg, making her the biological mother of the child. Traditional surrogacy can be achieved through intrauterine insemination (IUI) or IVF. This method is rarely practiced in the US today because it creates legal and emotional complexities — the surrogate has a genetic connection to the baby, which complicates parentage determinations. Most agencies and attorneys do not facilitate traditional surrogacy.

This guide focuses exclusively on gestational surrogacy, which is the standard practice in modern reproductive medicine.

Step 1: Synchronization and Baseline Testing

Before any medications begin, the fertility clinic performs baseline testing on the surrogate to confirm she is ready to proceed.

Baseline ultrasound: A transvaginal ultrasound examines the surrogate’s uterus and ovaries at the start of her menstrual cycle (cycle days 2-4). The doctor checks uterine lining thickness (should be thin at baseline), confirms there are no cysts on the ovaries, and verifies that the uterine cavity is clear of polyps or fibroids.

Blood work: Baseline hormone levels are drawn including estradiol, progesterone, FSH, and TSH. These values confirm that the surrogate’s hormonal environment is appropriate for beginning the medication protocol. Abnormal levels may delay the cycle until the next menstrual period.

Infectious disease screening update: If more than 12 months have passed since the surrogate’s initial screening, she will be retested for HIV, hepatitis B and C, syphilis, gonorrhea, and chlamydia. FDA regulations require current infectious disease testing before embryo transfer.

Mock cycle (sometimes): Some fertility clinics perform a mock embryo transfer cycle before the real one. The surrogate takes the same medications she will use in the actual cycle, and the doctor performs a trial transfer using an empty catheter to map the optimal placement path. Mock cycles add 4-6 weeks to the timeline but can improve transfer success rates by identifying potential issues in advance.

Step 2: The Hormone Medication Protocol

The surrogate begins taking hormones to prepare her uterine lining for embryo implantation. This is the most medically intensive part of the surrogate’s experience before pregnancy.

Estrogen supplementation (weeks 1-3): Estrogen thickens the uterine lining (endometrium) to create a receptive environment for the embryo. Estrogen is administered through one or more of the following methods:

• Oral estradiol tablets — taken 2-3 times daily
• Transdermal estrogen patches — changed every 2-3 days, applied to the lower abdomen or buttocks
• Vaginal estradiol tablets — inserted 1-2 times daily

The surrogate takes estrogen for approximately 12 to 18 days before progesterone is added. During this time, she has 1-2 monitoring appointments with ultrasound and blood work to confirm the lining is thickening appropriately. The target lining thickness is 8mm or greater with a trilaminar (triple-line) pattern.

Progesterone supplementation (starting 5-6 days before transfer): Progesterone transforms the thickened lining into a receptive state for embryo implantation. Timing is critical — progesterone must begin exactly 5 days before a Day 5 (blastocyst) transfer or 3 days before a Day 3 transfer. This is because progesterone exposure must mirror the natural hormonal timeline that would occur after ovulation.

Progesterone is administered through:

• Intramuscular injections — progesterone in oil, injected into the upper outer quadrant of the buttock using a 1.5-inch needle. This is the most common method and typically requires daily injections. Many surrogates have their partners or a friend administer the injection because the injection site is difficult to reach alone.
• Vaginal suppositories or gel — inserted 2-3 times daily. Some clinics use vaginal progesterone exclusively; others use it in combination with intramuscular injections.
• Oral progesterone — less commonly used for surrogacy transfers because bioavailability is lower than injectable or vaginal routes.

Additional medications: Some protocols include additional medications such as low-dose aspirin (to improve uterine blood flow), doxycycline (a short course of antibiotics around transfer time), methylprednisolone (a brief steroid course to modulate the immune response), or Lupron (to suppress the surrogate’s natural hormone cycle if needed).

Step 3: The Embryo Transfer Procedure

The embryo transfer is the pivotal medical event in gestational surrogacy. It is a relatively simple outpatient procedure, but its success depends on precise timing, careful technique, and the quality of the embryo being transferred.

Day of transfer preparation: The surrogate arrives at the fertility clinic with a comfortably full bladder — a full bladder pushes the uterus into a position that provides better ultrasound visualization during the transfer. She changes into a hospital gown and is positioned on the procedure table.

The transfer procedure itself:

1. The reproductive endocrinologist uses abdominal ultrasound to visualize the uterus in real time throughout the procedure.

2. A speculum is inserted vaginally (similar to a Pap smear).

3. The cervix is gently cleaned with culture medium or saline.

4. A soft, flexible outer catheter (guide catheter) is passed through the cervical canal into the lower uterine cavity.

5. The embryologist loads the embryo into an inner transfer catheter — a thin, flexible tube containing the embryo suspended in a small amount of culture medium.

6. The inner catheter is threaded through the guide catheter and advanced to the predetermined optimal placement position in the upper-middle uterine cavity, approximately 1 to 2 centimeters from the uterine fundus.

7. The embryo is deposited by gently pushing the catheter plunger. On the ultrasound screen, a small flash of brightness confirms the fluid (and embryo) has been released.

8. The catheter is slowly withdrawn and handed to the embryologist, who examines it under a microscope to confirm the embryo is no longer inside the catheter.

Duration: The entire transfer takes 10 to 15 minutes. Most surrogates report that it is less uncomfortable than a Pap smear. No anesthesia or sedation is required, though some clinics offer Valium for relaxation.

Immediately after transfer: The surrogate rests in the clinic for 15 to 30 minutes. There is no medical evidence that prolonged bed rest improves outcomes — a 2019 meta-analysis found no difference in implantation rates between surrogates who rested for 10 minutes versus those who rested for 24 hours. Most clinics recommend light activity for 24-48 hours and avoiding strenuous exercise for one week.

Step 4: The Two-Week Wait and Pregnancy Confirmation

After the embryo transfer, the surrogate enters the “two-week wait” — the period between transfer and the first pregnancy test.

What happens inside the uterus: After the embryo is deposited, it must hatch from its outer shell (zona pellucida) and attach to the uterine lining. This process — called implantation — begins approximately 1 to 2 days after a Day 5 blastocyst transfer. The embryo burrows into the endometrial tissue, establishes a blood supply connection with the mother’s uterine vessels, and begins producing human chorionic gonadotropin (hCG).

Beta hCG blood test: The first pregnancy test is a blood draw measuring beta hCG levels, typically performed 9 to 14 days after embryo transfer. An hCG level above 50-100 mIU/mL generally indicates a positive pregnancy. The surrogate returns for a second blood draw 48-72 hours later to confirm that hCG levels are rising appropriately (they should roughly double every 48 hours in early pregnancy).

First ultrasound: If hCG levels rise normally, the first ultrasound is scheduled at approximately 6 to 7 weeks gestation (4-5 weeks after transfer). This ultrasound confirms the presence of a gestational sac in the uterus, the number of embryos that implanted, and a fetal heartbeat.

The emotional reality of the two-week wait: This period is psychologically difficult for surrogates and intended parents alike. Symptoms are unreliable indicators — early pregnancy symptoms like breast tenderness, cramping, and spotting can also be caused by the progesterone medications. Most reproductive endocrinologists advise against home pregnancy tests during this period because they can produce false negatives if taken too early, causing unnecessary anxiety.

Step 5: Continuing Medications After Positive Test

A positive pregnancy test does not mean the surrogate stops taking medications. Hormone supplementation continues well into the first trimester.

Progesterone continuation: The surrogate continues daily progesterone injections or suppositories through weeks 10 to 12 of pregnancy. By this point, the placenta has developed sufficiently to produce its own progesterone and sustain the pregnancy independently. Stopping progesterone before the placenta takes over can result in miscarriage.

Estrogen continuation: Estrogen supplementation typically continues until weeks 8 to 10, then is tapered gradually over 1-2 weeks. Abrupt discontinuation of estrogen can cause withdrawal bleeding or hormonal instability.

Monitoring: The fertility clinic continues monitoring the surrogate with blood work and ultrasounds through weeks 8 to 10 before transitioning her care to her OB/GYN for routine prenatal management. Once the OB/GYN assumes care, the pregnancy proceeds like any other healthy pregnancy — the surrogate’s body manages the hormonal environment naturally from the second trimester onward.

Embryo Transfer Success Rates

Understanding success rates helps set realistic expectations for how many transfer cycles may be needed.

Frozen embryo transfer (FET) success rates: The majority of surrogacy transfers use frozen embryos. Success rates for FET in gestational surrogacy range from 60% to 75% per transfer when using euploid (chromosomally normal) embryos tested with PGT-A. Without PGT-A testing, success rates are approximately 45% to 60% per transfer.

Factors that influence success:

• Embryo quality — the single most important predictor. Euploid blastocysts have the highest implantation rates.
• Uterine lining thickness — linings above 8mm with a trilaminar pattern are associated with higher success rates.
• Surrogate age — the surrogate’s age has less impact on success than the egg provider’s age, since the surrogate is not providing eggs. However, uterine receptivity may decline slightly after age 40.
• Transfer technique — experienced reproductive endocrinologists who perform high volumes of transfers achieve higher success rates.
• Number of prior successful pregnancies — surrogates with multiple prior pregnancies tend to have higher implantation rates.

If the transfer fails: Approximately 25% to 40% of first transfers do not result in pregnancy. A failed transfer does not mean something is wrong with the surrogate. The gestational carrier agreement specifies how many transfer attempts are included (typically 2-3). If the first transfer fails, the fertility clinic may adjust the medication protocol, add additional progesterone, or modify the transfer technique for the next attempt.

Common Questions About How Surrogates Get Pregnant

Does the surrogate have sex to get pregnant? No. Gestational surrogates become pregnant exclusively through IVF embryo transfer — a medical procedure. There is no sexual contact between the surrogate and the intended father or any other person for the purpose of achieving pregnancy. Surrogates are typically asked to abstain from sexual intercourse for a period before and after embryo transfer to prevent natural conception.

Is the baby genetically related to the surrogate? No. In gestational surrogacy, the embryo is created from the intended parents’ egg and sperm (or donor gametes). The surrogate contributes no genetic material. The baby’s DNA comes entirely from the egg provider and sperm provider.

Do the hormone injections hurt? Progesterone intramuscular injections involve a 1.5-inch needle and can cause soreness, bruising, and temporary discomfort at the injection site. Most surrogates report that the injections become routine after the first week. Warming the oil and massaging the injection site afterward reduces discomfort. Vaginal progesterone is painless but can be messy.

How many embryos are transferred at once? The standard of care in 2026 is single embryo transfer (SET). Transferring one embryo at a time reduces the risk of twin pregnancy, which carries higher medical risks for both the surrogate and the babies. Some intended parents request dual embryo transfer, which is discussed and agreed upon during the matching process and documented in the gestational carrier agreement.

Can the surrogate’s body reject the embryo? The surrogate’s immune system does not typically reject the embryo. The uterine environment is immunologically privileged — the body naturally suppresses local immune responses during implantation. The medications the surrogate takes further support this immune tolerance. Implantation failure is more commonly related to embryo quality or lining receptivity than immune rejection.