The Female Reproductive System: A Physician’s Guide to Fertility and Surrogacy

The female reproductive system is one of the most precisely coordinated biological systems in the human body. Every organ has a specific job. When one component is compromised — by disease, anatomy, or prior treatment — the path to pregnancy changes. For many intended parents, that changed path leads to gestational surrogacy.

At Physician’s Surrogacy, our in-house OB/GYNs work with this science every day. They design surrogate screening protocols, review clinical notes after every prenatal appointment, and consult peer-to-peer with surrogates’ managing physicians. This guide explains how the female reproductive system works — and exactly how it functions (and is prepared) in the context of gestational surrogacy.

The Female Reproductive System — Organ by Organ

Five primary internal structures make up the female reproductive system. In natural conception, they work in a tightly sequenced chain. In gestational surrogacy, that chain is deliberately altered — some organs become irrelevant, others become the entire focus of clinical preparation.

The Ovaries

The ovaries are paired almond-shaped glands, one on each side of the uterus. They serve two distinct functions: producing eggs (oocytes) and secreting hormones — primarily estrogen and progesterone — that drive every phase of the reproductive cycle.

A woman is born with roughly 1–2 million immature follicles. By puberty, about 300,000 remain. Over a lifetime, only 400–500 will ovulate. Each cycle, follicle-stimulating hormone (FSH) recruits a group of follicles. One dominant follicle matures and releases its egg.

After ovulation, the emptied follicle transforms into the corpus luteum — the temporary gland that produces progesterone to sustain early pregnancy.

In gestational surrogacy, the surrogate’s ovaries play no role. The embryo comes from the intended parents’ egg and sperm (or from donors). During the transfer cycle, medications suppress the surrogate’s natural hormonal cycle entirely.

Her ovaries don’t ovulate, and her corpus luteum doesn’t form. Exogenous estrogen and progesterone replace those functions through the first 8–12 weeks of pregnancy.

The Fallopian Tubes

The fallopian tubes are 10–12 cm muscular channels connecting each ovary to the uterus. In natural conception, this is where fertilization occurs. Sperm swim up the tube, meet the released egg in the ampulla (the widest section), and form a zygote. Ciliated cells lining the tube then carry the developing embryo toward the uterus over 3–4 days.

In gestational surrogacy, the fallopian tubes are bypassed completely. A laboratory-grown blastocyst — already 5–6 days old — is placed directly into the uterine cavity through a thin catheter passed through the cervix.

Blocked tubes, a history of ectopic pregnancy, or even surgical removal of the tubes (salpingectomy) does not disqualify someone from being a gestational carrier. The tubes simply aren’t part of the process.

The Uterus and Endometrium

The uterus is the organ that matters most in surrogacy. This pear-shaped muscular organ measures roughly 7.5 cm long when not pregnant. It has three distinct layers: the perimetrium (outer covering), the myometrium (thick muscular wall capable of labor contractions), and the endometrium — the inner lining where everything critical for surrogacy happens.

The endometrium has two sublayers. The basalis is the permanent base that regenerates each cycle. The functionalis is the active layer — it thickens under estrogen influence, transforms under progesterone into a secretory state, and becomes the receptive surface for an embryo.

If no implantation occurs, it sheds as a period. In surrogacy, this layer is prepared artificially through hormone medications, timed precisely to match the arrival of a frozen embryo. You can read about how our physician-designed protocol evaluates every candidate at our Physician’s Advantage page.

The Cervix

The cervix is the narrow lower portion of the uterus, approximately 3–6 cm deep. It functions as a gatekeeper: opening slightly during ovulation to allow sperm through, forming a mucus plug during pregnancy that seals the uterine cavity from bacteria, and dilating to 10 cm during labor.

In an embryo transfer, the fertility clinic passes a thin catheter through the cervix to deposit the embryo. The procedure takes roughly 15–20 minutes and causes minimal discomfort. Most surrogates describe it as similar to a routine Pap smear. To learn whether embryo transfer is painful, our dedicated post covers the full experience.

Cervical length is also monitored throughout pregnancy. A cervix shorter than 25 mm is associated with a significantly elevated preterm delivery risk — one reason cervical evaluation forms part of every surrogate’s initial medical screening.

The Vagina

The vagina is the muscular canal connecting the cervix to the body’s exterior. In surrogacy it serves three practical functions.

It accommodates the transvaginal ultrasound probe used during monitoring appointments — the primary tool for measuring endometrial thickness. It receives progesterone suppositories that are part of the transfer medication protocol. And it becomes the birth canal during delivery.

How Hormones Drive Reproduction — and How Surrogacy Replicates Them

The menstrual cycle runs on a four-hormone feedback loop between the hypothalamus, the pituitary gland, and the ovaries. In a natural cycle, the body runs this loop automatically. In a frozen embryo transfer (FET) cycle for a gestational carrier, medications replicate it deliberately — with precise timing that the body’s own cycle cannot guarantee.

Surrogates take specific hormone medications before embryo transfer to build and prepare the uterine lining. Estrogen and progesterone supplementation continue for 8–12 weeks post-transfer until the placenta matures enough to take over hormone production — a process called the luteal-placental shift.

Stopping progesterone prematurely before this transition can cause pregnancy loss, which is why the fertility clinic tracks serum levels closely throughout the first trimester. For what this preparation looks like from the inside, our post on what cycling means in surrogacy walks through the full medical calendar.

The Endometrial Lining: Why It Matters for Embryo Transfer

The endometrium is the single most closely watched measurement in a frozen embryo transfer cycle. Before the fertility clinic proceeds with transfer, two criteria must be confirmed via transvaginal ultrasound.

The first is thickness. The lining must measure at least 7 mm — ideally between 8 and 12 mm. Below 7 mm, the endometrium is considered thin and unlikely to support implantation. Above 14 mm is less common and may prompt clinical review.

The second is pattern. The ideal endometrium shows a “trilaminar” or triple-line appearance on ultrasound — three distinct layers visible in cross-section. This reflects a fully proliferative lining that has responded properly to estrogen and is ready for progesterone-driven transformation.

A homogeneous, bright white pattern on ultrasound typically signals the lining has either already entered secretory phase or is not developing as needed.

When estrogen supplementation builds the lining successfully, progesterone is then added — typically as intramuscular injections, vaginal suppositories, or both. Progesterone in oil injections are the most reliable delivery method.

Progesterone converts the proliferative lining to a secretory state, triggering the opening of the implantation window — a narrow 4-day period during which the uterus can accept an embryo.

For a day-5 blastocyst transfer, progesterone is started exactly 5 days before transfer day. Timing this window correctly — not too early, not too late — is what separates a successful cycle from a failed one. This level of precision is why surrogacy requires fertility clinic involvement at every step.

How the Uterus Accepts a Genetically Unrelated Embryo

This is the question most intended parents — and most prospective surrogates — eventually ask. How does a surrogate’s body accept an embryo that is genetically someone else’s entirely?

The answer lies in the immunology of normal pregnancy. Even between a mother and her own child, pregnancy is a remarkable act of immune tolerance.

In every pregnancy, the fetus is genetically 50% different from the mother — it carries the father’s DNA. The maternal immune system must tolerate this “foreign” tissue rather than attacking it.

Decidual NK (natural killer) cells, regulatory T cells, and macrophages at the maternal-fetal interface create a carefully calibrated immune environment that suppresses rejection while still protecting against infection.

In gestational surrogacy, the embryo is 100% genetically unrelated to the carrier. The immune challenge is greater — but the same mechanisms apply.

Research on immunology at the maternal-fetal interface confirms that the placenta actively manages maternal immune tolerance, and that healthy surrogates with proven prior pregnancies demonstrate the immune competence needed to sustain that tolerance throughout gestation.

This is also one reason that prior successful pregnancy is a non-negotiable requirement for gestational carriers. It shows that a woman’s immune system has already navigated this tolerance process at least once — and that her uterus supported a full-term pregnancy without complications.

Does a Surrogate Share DNA With the Baby?

A phenomenon called microchimerism does occur during all pregnancies. A small number of fetal cells cross the placental barrier and enter the mother’s bloodstream — and maternal cells transfer to the fetus in return. These microchimeric cells can persist in both the surrogate and the child for decades.

But they don’t change the baby’s genetic identity. They don’t alter inherited traits, and they don’t create a legal or biological parental relationship. The child’s chromosomal makeup remains entirely that of the egg and sperm providers.

The surrogate’s uterine environment can influence gene expression through epigenetics — how genes are expressed during development, shaped by the carrier’s nutrition, stress hormones, and general health. This is another reason why selecting the right surrogate matters beyond just medical eligibility.

Gestational Surrogacy vs. Traditional Surrogacy: The Biological Difference

Traditional surrogacy uses the surrogate’s own egg, fertilized by the intended father’s sperm or donor sperm. This makes the surrogate both the genetic and gestational mother. The legal and emotional risks are significant — and the practice is now rare.

Gestational surrogacy uses in vitro fertilization (IVF) to create an embryo from the intended parents’ gametes (or donor eggs and sperm). That embryo transfers to the carrier. The surrogate gestates a pregnancy she has no genetic stake in. Physician’s Surrogacy facilitates gestational surrogacy exclusively — it’s the safer, cleaner, and legally better-supported arrangement.

For a full breakdown, see our article on gestational vs. traditional surrogacy. Once the embryo implants and pregnancy is confirmed, the biology is identical to any other pregnancy — the same fetal development stages, the same placental function, the same prenatal milestones.

How IVF Prepares a Surrogate’s Uterus for Pregnancy

For intended parents who haven’t been through IVF before, preparing a surrogate’s uterus involves two parallel tracks happening simultaneously.

On the intended parents’ side (or the egg donor’s side), the fertility clinic uses injectable FSH to stimulate multiple follicles to mature. Follicle growth is tracked by serial ultrasounds. When leading follicles reach 18–22 mm diameter, a trigger shot releases them for retrieval.

The eggs are fertilized in the lab — either through conventional insemination or intracytoplasmic sperm injection (ICSI). The resulting embryos are cultured for 5–6 days to the blastocyst stage.

High-quality blastocysts are biopsied for preimplantation genetic testing for aneuploidy (PGT-A) to confirm chromosomal normalcy before transfer.

On the surrogate’s side, the FET protocol runs in parallel. Cycle suppression comes first, using a GnRH agonist to quiet her natural hormonal cycle. Estrogen supplementation follows — given over 10–14 days — to build her endometrial lining.

When the lining reaches the required 7–14 mm with a trilaminar pattern, confirmed by ultrasound and serum estradiol blood draw, progesterone is added. Transfer is then scheduled precisely to the progesterone start date.

After transfer, a serum beta-hCG blood test is performed 10–14 days later. Levels above 100 mIU/mL indicate pregnancy. The rate of hCG rise — ideally doubling every 48–72 hours — matters as much as the initial number. Both estrogen and progesterone supplementation continue until the placenta is producing these hormones independently, typically by weeks 10–12 of gestation.

Uterine Health Requirements for Gestational Surrogates

The ASRM 2022 Practice Committee Opinion on gestational carriers — the authoritative U.S. clinical standard — outlines specific uterine health criteria every surrogate must meet before proceeding to embryo transfer. At Physician’s Surrogacy, our physician-designed screening protocol meets and exceeds these guidelines.

Saline Infusion Sonogram (SIS)

A saline infusion sonogram — also called sonohysterography — is a minimally invasive uterine evaluation performed during initial surrogate screening. A thin 2 mm catheter is inserted through the cervix, sterile saline is infused to separate the uterine walls, and transvaginal ultrasound creates detailed images of the uterine cavity.

SIS detects endometrial polyps with approximately 94% sensitivity, submucosal fibroids with ~96% sensitivity, and intrauterine adhesions with ~93% sensitivity. Its negative predictive value — the reliability of a clean result — is 96–99%. The procedure takes 6–30 minutes, requires no general anesthesia, and is best performed on cycle days 5–9 when the lining is thinnest.

When SIS reveals pathology that needs treatment, hysteroscopy follows — allowing the fertility clinic to directly visualize the cavity and remove polyps, resect fibroids, or address adhesions in the same procedure.

What the Full Uterine Screening Evaluates

• Cavity shape: Free from congenital anomalies — septate, bicornuate, or unicornuate configurations — that reduce capacity or disrupt blood flow.
• Endometrial thickness: Confirmed to reach 7–14 mm under exogenous estrogen during a mock cycle.
• Structural pathology: No significant submucosal fibroids, polyps, adhesions, or septa within the cavity.
• Cervical competence: Normal cervical length (30–50 mm); history of preterm delivery prompts additional evaluation.
• Uterine blood flow: Doppler ultrasound confirms adequate uterine artery flow supporting endometrial development and placentation.
• Hormonal responsiveness: The endometrium must respond correctly to estrogen and progesterone — sometimes evaluated during a mock transfer cycle before committing to a real transfer.

Our surrogate requirements page outlines the full eligibility criteria, including the medical, lifestyle, and history factors our OB/GYNs evaluate during screening.

Reproductive Conditions That Lead to Surrogacy

Gestational surrogacy is one of the most medically sophisticated ways a family can be built — and one of the most human. For many intended parents, it’s not the first choice. It’s the answer to a reproductive system that, for specific and diagnosable biological reasons, cannot support a pregnancy. Understanding those reasons helps intended parents feel grounded rather than defeated by their path.

MRKH Syndrome

Mayer-Rokitansky-Küster-Hauser (MRKH) syndrome is a congenital condition in which the uterus and the upper portion of the vagina are absent or severely underdeveloped. Women with MRKH have functioning ovaries — meaning their eggs can be retrieved and used to create embryos — but no uterus to carry a pregnancy.

Gestational surrogacy is the primary clinical option for women seeking genetically related children without the capacity to carry them.

Severe Asherman Syndrome

Asherman syndrome involves scar tissue (adhesions) forming inside the uterine cavity, causing the walls to adhere. Approximately 90% of cases follow dilation and curettage (D&C) procedures. Mild-to-moderate Asherman can often be treated via hysteroscopy.

Severe cases — particularly those with dense adhesions covering most of the cavity — may leave the uterus unable to build an adequate endometrial lining. Implantation or sustained pregnancy becomes essentially impossible. Recurrent pregnancy loss in this context is one of the clearest indications for surrogacy as a medical solution.

Uterine Fibroids and Surrogacy

Fibroids affect up to 70–80% of women by age 50, though many are asymptomatic. The critical factor in surrogacy is whether the fibroid distorts the endometrial cavity. Submucosal fibroids directly interfere with implantation — they disrupt the lining surface, alter the local cytokine environment, and impair uterine blood flow to the implantation site. They must be addressed before transfer.

For intended parents whose own fibroids make pregnancy high-risk or impossible, surrogacy offers a clear path forward. See our guide on top medical reasons to use a surrogate.

Endometriosis

Endometriosis — where tissue similar to the endometrial lining grows outside the uterus — affects approximately 1 in 10 reproductive-age women. It causes inflammation, scarring, and, critically for IVF and surrogacy, progesterone resistance in the endometrium.

The lining doesn’t transform into the receptive secretory state needed for implantation, even when progesterone is administered at the right time.

Moderate-to-severe endometriosis is one of the more common medical indications for surrogacy. Our post on endometriosis and surrogacy covers the clinical reasoning in detail.

Congenital Uterine Abnormalities

Congenital uterine anomalies — called Müllerian anomalies — form during fetal development when the uterus doesn’t develop normally. A septate uterus (a fibrous or muscular wall dividing the cavity) is the most common, affecting approximately 5% of women, and carries a 31% preterm birth rate — though it is surgically correctable via hysteroscopy.

A bicornuate uterus (heart-shaped, two horns) carries a 39% preterm rate. Unicornuate and didelphys configurations carry even higher preterm risk and reduced carrying capacity. Where surgical correction is not possible, or where prior pregnancies have shown poor outcomes due to uterine shape, surrogacy following failed IVF is often the most appropriate next step.

Prior Hysterectomy

Women who have undergone hysterectomy — whether for fibroids, cancer treatment, hemorrhage, or other indications — no longer have a uterus to carry a pregnancy. If the ovaries are still intact, eggs can be retrieved and fertilized to create genetically related embryos. Those embryos are then transferred to a gestational carrier.

This is one of the clearest and most medically uncomplicated paths to surrogacy for intended mothers.

Recurrent Implantation Failure

Recurrent implantation failure (RIF) — defined as failure to achieve pregnancy after three or more high-quality embryo transfers — presents a more complex clinical picture. In many cases, the embryos have been chromosomally tested and are normal. The uterus has been cleared by SIS or hysteroscopy. The hormone protocol has been adjusted multiple times.

When the endometrial environment is the suspected factor, surrogacy after failed IVF allows the embryo to transfer into a different uterus — one with demonstrated implantation capacity.

Why Physician Oversight Changes the Outcome

Most surrogacy agencies hand off medical coordination to the fertility clinic and step back. Physician’s Surrogacy is built differently.

Our in-house OB/GYN team doesn’t just review applications — they design the screening protocol that determines who becomes a surrogate, they monitor every pregnancy through physician-reviewed clinical notes, and they provide peer-to-peer consultation with the surrogate’s managing OB when complications arise.

The result is measurable: our preterm delivery rate is 50% below the national average. That’s not a marketing claim — it’s the direct outcome of physician-designed surrogate screening that catches uterine, hormonal, and obstetric risk factors before a match is ever made. No other agency in the U.S. is structured this way.

Our path to parenthood for intended parents is built on this clinical foundation. If you’re at the research stage, our how surrogacy works guide walks through each stage. If you’re ready to talk through your specific situation — including any reproductive history — schedule a free consultation with our team.

Frequently Asked Questions

Julianna Nikolic

Chief Strategy Officer Julianna Nikolic leads strategic initiatives, focusing on growth, innovation, and patient-centered solutions in the reproductive sciences sector. With 26+ years of management experience and a strong entrepreneurial background, she brings deep expertise to advancing reproductive healthcare.