# Female Reproductive System: Anatomy and Function

The female reproductive system is the set of organs that produces oocytes (egg cells), transports them, supports fertilization and pregnancy, and delivers young. In mammals it consists of paired ovaries, paired oviducts (uterine tubes), a uterus, a cervix, a vagina, and the external vulva, all coordinated by cyclic hormones.

Understanding this system matters for three practical reasons. It explains why estrous cycles differ so much between species, from roughly 17 days in the ewe to about 21 days in the cow and sow. It explains why pregnancy diagnosis, breeding timing, and reproductive exams are species-specific. And it gives students and owners a shared vocabulary for reading veterinary records, ultrasound reports, and histology slides without guessing what each structure does.

## The Core Organs at a Glance

<figure class="article-figure">
  <img src="https://thumb.wikimedia.org/wikipedia/commons/thumb/6/68/Scheme_female_reproductive_system-en.svg/1280px-Scheme_female_reproductive_system-en.svg.png" alt="Labeled frontal diagram of female reproductive organs" loading="lazy" decoding="async" width="1000" height="589" />
  <figcaption>This labeled frontal view maps the core organs-ovaries, oviducts, uterus, cervix, and vagina-discussed throughout the article. Image: CDC, Mysid, Public domain, via <a href="https://commons.wikimedia.org/wiki/File:Scheme_female_reproductive_system-en.svg" rel="noopener noreferrer">Wikimedia Commons</a>.</figcaption>
</figure>

Every mammalian female reproductive tract follows the same basic plan, but the proportions and shapes vary enormously. The table below summarizes the structures covered in this guide and the function each one performs.

| Structure | Position | Primary function |
|--|--|--|
| Ovary | Paired, near the pelvic inlet | Produces oocytes and steroid hormones |
| Oviduct | Paired, coiled tube beside each ovary | Site of fertilization, transports the embryo |
| Uterus | Midline, between oviducts and cervix | Supports fetal development, contracts at birth |
| Cervix | Caudal uterus | Seals the uterus, dilates at parturition |
| Vagina | Canal between cervix and vulva | Copulatory canal, birth canal |
| Vulva | External opening | Entry point, includes the clitoris |
| Ovarian bursa | Fold around the ovary (species-dependent) | Encloses the ovary, guides the oocyte |
| Mesosalpinx | Mesentery of the oviduct | Supports and suspends the tube |

The two supporting structures named above are frequently confused. The ovarian bursa is a peritoneal fold that partly or completely encloses the ovary in some species, most notably the bitch and queen. The mesosalpinx is the sheet of mesentery that suspends the oviduct and carries its blood supply. They sit next to each other but do different jobs.

## The Ovary

The ovary is the primary gonad. It has two jobs that run in parallel: producing oocytes through folliculogenesis and secreting steroid hormones, chiefly estradiol and progesterone, plus peptide hormones such as inhibin.

### Follicles and Their Stages

A follicle is an oocyte surrounded by supporting somatic cells. Follicles progress through recognizable stages that a histologist or ultrasonographer can identify:

1. Primordial follicle: a resting oocyte with a single flat layer of granulosa cells.
2. Primary follicle: granulosa cells become cuboidal.
3. Secondary follicle: multiple granulosa layers, theca cells appear, and the zona pellucida forms around the oocyte.
4. Antral (tertiary) follicle: a fluid-filled cavity called the antrum develops, and the follicle becomes visible on ultrasound.
5. Preovulatory (Graafian) follicle: the dominant follicle reaches its maximum size and prepares to ovulate.

After ovulation, the collapsed follicle becomes the corpus luteum, a progesterone-secreting gland. If pregnancy does not occur, the corpus luteum regresses into a corpus albicans.

### Ovarian Morphology Differs by Species

Ovarian shape is a practical identification point. The mare has a large, kidney-shaped ovary with the vascular zone (medulla) on the outside and the functional zone (cortex) inside, the reverse of most mammals. The cow and sow have oval ovaries. The bitch and queen have small, oval ovaries enclosed within the ovarian bursa, which makes transabdominal ultrasound harder and transrectal or laparoscopic approaches more useful in some cases.

## The Oviduct

The oviduct, also called the uterine tube or fallopian tube, is a muscular, ciliated tube divided into four regions: the infundibulum with its fimbriae, the ampulla, the isthmus, and the intramural (interstitial) segment that opens into the uterine horn.

The infundibulum catches the oocyte at ovulation. Cilia and smooth muscle then move the oocyte toward the ampulla, which is the usual site of fertilization. After fertilization, the developing embryo travels to the isthmus and then into the uterus, arriving at the stage appropriate for that species. In the cow, the embryo enters the uterus around day 4 to 5 as a morula or early blastocyst. In the mare, it arrives around day 5 to 6 and remains mobile in the uterus for over a week before fixing in place.

The mesosalpinx is the mesentery that suspends the oviduct and supplies it with blood vessels and nerves. It is continuous with the broad ligament (mesometrium around the uterus), and it forms the wall of the ovarian bursa in species that have a closed bursa.

## The Uterus

The uterus is the muscular organ where the embryo implants or attaches and where the fetus develops. Its wall has three layers: the endometrium (inner mucosa), the myometrium (smooth muscle), and the perimetrium (outer serosa).

### Uterus Types Across Mammals

Uterus classification is one of the most examinable topics in comparative reproduction. There are four main types:

- Simplex: a single uterine body with no horns. Found in primates, including humans.
- Bicornuate: two uterine horns joined over part of their length, with a single cervix. Found in most domestic mammals, including the cow, ewe, sow, bitch, and queen.
- Bipartite: two horns with a partial internal septum and a single cervix. Seen in some carnivores.
- Duplex: two completely separate horns, each with its own cervix. Found in rabbits and some rodents.

The bicornuate uterus of the cow, ewe, sow, bitch, and queen is the key contrast with the simplex uterus of primates. This difference matters clinically. A bicornuate uterus allows multiple fetuses to occupy separate horns, which is normal in the sow (litter-bearing) and possible in the bitch and queen. It also means that a single pregnancy can be missed on palpation if only one horn is examined.

### Endometrial Glands and Placentation

The endometrium contains glands whose secretions nourish the early embryo before placentation. The type of placenta that forms depends on how fetal membranes attach to the maternal endometrium.

| Species | Uterus type | Placenta type | Estrous cycle length |
|--|--|--|--|
| Cow | Bicornuate | Cotyledonary (synepitheliochorial) | ~21 days |
| Mare | Bicornuate | Diffuse (epitheliochorial) | ~21 to 22 days |
| Bitch | Bicornuate | Zonary (endotheliochorial) | ~6 to 8 months (monoestrous) |
| Queen | Bicornuate | Zonary (endotheliochorial) | Seasonal, polyestrous |
| Sow | Bicornuate | Diffuse (epitheliochorial) | ~21 days |

Note that cycle length is not the same as cycle type. The bitch and queen are both zonary-placenta carnivores, but the bitch cycles once or twice a year while the queen cycles repeatedly during the breeding season.

## The Cervix

The cervix is a thick-walled muscular and fibrous valve between the uterus and vagina. Its job is to remain closed during pregnancy, sealing the uterus against ascending infection, and then to dilate and soften at parturition.

Cervical anatomy varies by species. The cow has a prominent, firm cervix with interlocking folds that form a spiral canal. The ewe has a cervix with transverse rings, which is why transcervical insemination in sheep is difficult and why laparoscopic insemination is often used. The sow has a long cervix with interdigitating pads that lock around the boar's spiral penis during mating. The bitch has a relatively short cervix that dilates noticeably at estrus.

Cervical mucus changes with the cycle. Under estrogen dominance, mucus becomes copious and thin, which helps sperm transit. Under progesterone dominance, it becomes thick and sticky, forming a plug that protects the pregnancy.

## The Vagina and Vestibule

The vagina is the copulatory canal and the lower part of the birth canal. It is lined by stratified squamous epithelium that keratinizes under estrogen influence. The cranial vagina is separated from the vestibule by the hymen or its remnants in some species.

The vestibule is the short caudal section that opens to the outside. It contains the urethral opening on its floor. In the cow, the vestibule is a useful landmark for artificial insemination because the cervix can be grasped through the rectal wall and guided onto the insemination rod.

## The Vulva and Clitoris

The vulva is the external opening of the reproductive tract. It includes the labia majora and minora, the vestibule, and the clitoris. The clitoris is homologous to the penis and contains erectile tissue and sensory nerve endings.

Vulvar appearance changes with the cycle. In the cow, the vulva swells and reddens near estrus, and a clear mucus discharge is common. In the mare, the vulva relaxes and the clitoris may evert during estrus. In the bitch, vulvar swelling is one of the earliest signs of proestrus and is often the first thing an owner notices.

The vulva is also the site of several clinical problems, including lacerations after breeding or whelping, and neoplasms such as transmissible venereal tumor in dogs.

## The Ovarian Cycle: Follicular and Luteal Phases

The ovarian cycle is divided into two functional phases separated by ovulation.

### Follicular Phase

The follicular phase begins when a cohort of follicles starts to grow under rising follicle-stimulating hormone (FSH). As follicles grow, they secrete estradiol. Estradiol drives estrus behavior, cervical mucus changes, and the preovulatory luteinizing hormone (LH) surge. The follicular phase ends at ovulation.

### Luteal Phase

After ovulation, the remaining follicular cells form the corpus luteum, which secretes progesterone. Progesterone prepares the endometrium for pregnancy and suppresses further follicular growth. If pregnancy does not occur, the corpus luteum regresses (luteolysis), progesterone falls, and a new follicular phase begins.

### Ovulation Type: Spontaneous vs Induced

Most domestic mammals ovulate spontaneously. The bitch and queen are often described in this context, and the terminology needs care. The bitch is a spontaneous ovulator: she ovulates without mating, typically 2 days after the LH peak, and the oocytes are released as primary oocytes that mature in the oviduct over 2 to 3 days. The queen is an induced ovulator: mating triggers the LH surge and ovulation, which occurs roughly 24 to 48 hours after copulation. The cow, ewe, sow, and mare are spontaneous ovulators.

## Estrous Cycle Lengths and Gestation

Cycle length is species-specific and is measured from one estrus to the next. The cow averages about 21 days, the ewe about 17 days, and the sow about 21 days. The mare averages 21 to 22 days but has a long estrus of 5 to 7 days. The bitch is monoestrous with a cycle of roughly 6 to 8 months, and the queen is seasonally polyestrous, cycling repeatedly while day length is long.

Gestation length also varies. The cow carries a calf for about 283 days. The mare averages about 340 days, the ewe about 147 days, the sow about 114 days, and the bitch about 63 days from ovulation. These numbers are standard textbook values and are used to plan breeding, pregnancy checks, and dry-off periods.

## How the System Is Observed in Practice

Reproductive anatomy and function are assessed in several ways.

Transrectal palpation and ultrasonography are standard in cattle and horses. In the cow, the uterus and ovaries can be felt through the rectal wall, and a corpus luteum or follicle can be identified by size and texture. In the mare, ultrasound is used to track follicle growth and time breeding.

Vaginal cytology is used in the bitch to stage the cycle. The proportion of superficial, intermediate, and parabasal cells changes predictably with estrogen and progesterone. This helps time breeding and predict whelping.

Hormone assays measure progesterone and estradiol in serum or milk. In cattle, milk progesterone profiles are used to detect silent heats and confirm ovulation.

Histology is used in research and diagnostic pathology. Ovarian and uterine tissues are examined for follicular development, luteal tissue, and endometrial changes. Experimental models of reproductive disruption often rely on these endpoints. For example, in a rat model of polycystic ovary syndrome, estrous cycles were tracked and ovarian and uterine morphology were evaluated alongside hormone levels and lipid profiles, with all induced groups showing hormonal imbalance, insulin resistance, multiple follicular cysts on ultrasonography, and histological alterations [1]. In a separate rat study, acrylamide exposure produced degenerative changes in ovarian and uterine tissue, including vascular disruption, and altered estrous cycle characteristics [2].

Molecular and cellular tools add another layer. [Single-cell RNA sequencing](/knowledge/bioinformatics/single-cell-rna-sequencing-from-bulk-to-resolution) has been used to show that porcine epidemic diarrhea virus can infect the porcine uterus and preferentially targets uterine stromal cells, with minimal signal in epithelial cells, and that infection disrupts extracellular matrix organization and inflammatory signaling [3]. This kind of work links anatomy to function at the level of individual cell types.

Urinary extracellular vesicles are an emerging noninvasive window on the reproductive tract. These lipid-bound particles can originate from the uterus, vagina, and placenta in females, and their molecular cargo may reflect reproductive and urogenital pathology, though most clinical studies to date have used male-dominant cohorts, leaving a knowledge gap for female-specific biomarkers [4].

## Comparative Notes for Students

The most common exam trap is mixing up uterus type with placenta type. A bicornuate uterus is about shape, not about how the placenta attaches. The cow has a bicornuate uterus and a cotyledonary placenta. The mare has a bicornuate uterus and a diffuse placenta. The bitch and queen have bicornuate uteri and zonary placentas. Primates have a simplex uterus and a discoid hemochorial placenta.

A second trap is confusing spontaneous and induced ovulation. The bitch ovulates spontaneously but is often grouped with the cat in casual conversation because both have unusual cycle patterns. The cat is the induced ovulator. The bitch is not.

A third trap is assuming that all domestic mammals have the same cycle length. They do not. The ewe at about 17 days is the shortest of the common farm species listed here. The sow and cow are both near 21 days, which makes them easy to confuse on a flashcard but easy to separate by other features such as litter size and placental type.

## Quick Review

- The tract runs ovary, oviduct, uterus, cervix, vagina, vulva, from cranial to caudal.
- The ovarian bursa encloses the ovary in the bitch and queen. The mesosalpinx suspends the oviduct.
- The ovarian cycle has a follicular phase (estradiol-driven) and a luteal phase (progesterone-driven).
- Most domestic mammals ovulate spontaneously. The queen is an induced ovulator.
- Most domestic mammals have a bicornuate uterus. Primates have a simplex uterus.
- Cycle length is about 21 days in the cow and sow, about 17 days in the ewe.
- Gestation is about 283 days in the cow.

## Common Mistakes and Limitations

The first mistake is treating the ovarian bursa and the mesosalpinx as the same structure. They are adjacent but distinct. The bursa is a fold that encloses the ovary. The mesosalpinx is the mesentery of the oviduct.

The second mistake is assuming that a bicornuate uterus means two cervices. It does not. A bicornuate uterus has two horns and one cervix. A duplex uterus has two cervices.

The third mistake is using human reproductive timelines for animals. Human menstrual cycles are about 28 days with menstruation, but most domestic mammals have estrous cycles without menstruation. The endometrial shedding that defines menstruation is not a general mammalian feature.

The fourth mistake is reading a single ultrasound or palpation finding as a definitive diagnosis. Ovarian structures change daily, and a follicle on Monday may be a corpus luteum by Friday. Serial examinations are more informative than a single snapshot.

The fifth limitation is species coverage. This guide focuses on the cow, mare, bitch, queen, and sow because those are the species most often encountered in US veterinary and animal science settings. Other species, including camelids, rabbits, and rodents, have additional variations in ovulation type, uterus shape, and placental structure that are not covered here.

Individual animals can deviate from textbook values. A veterinarian who examines the animal in person is the right source for any specific diagnosis or breeding decision.

## Frequently Asked Questions

### What is the difference between the ovarian bursa and the mesosalpinx?

The ovarian bursa is a peritoneal fold that partly or completely encloses the ovary, most prominently in the bitch and queen. The mesosalpinx is the mesentery that suspends the oviduct and carries its blood supply. They are separate structures that sit close together.

### Is the cat a spontaneous or induced ovulator?

The cat (queen) is an induced ovulator. Mating triggers the LH surge that leads to ovulation, usually 24 to 48 hours after copulation. The bitch, in contrast, ovulates spontaneously.

### How long is the estrous cycle in a cow, ewe, and sow?

The cow averages about 21 days, the ewe about 17 days, and the sow about 21 days. These values are measured from one estrus to the next and are standard textbook figures.

### What type of uterus do most domestic mammals have?

Most domestic mammals, including the cow, mare, bitch, queen, and sow, have a bicornuate uterus with two horns and a single cervix. Primates, including humans, have a simplex uterus with no horns.

### How long is gestation in the cow?

Gestation in the cow averages about 283 days. Other species differ: the mare is about 340 days, the ewe about 147 days, the sow about 114 days, and the bitch about 63 days from ovulation.

### What are the two phases of the ovarian cycle?

The ovarian cycle has a follicular phase and a luteal phase. The follicular phase is dominated by estradiol and ends at ovulation. The luteal phase is dominated by progesterone from the corpus luteum and ends with luteolysis if pregnancy does not occur.

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## Further Reading

- [Sexual dysfunction after female genital mutilation/cutting: A retrospective cross-sectional study of associations with organ-specific genital anatomy, trauma history, and psychiatric comorbidity.](https://pubmed.ncbi.nlm.nih.gov/42752680/)

## Sources

1. [Evaluation of metabolic, reproductive, and gut microbiota alterations in a comparative study of different preclinical models of polycystic ovary syndrome.](https://pubmed.ncbi.nlm.nih.gov/42698331/)
2. [Olive leaf extract attenuates acrylamide-induced reproductive toxicity through reduced oxidative DNA damage and apoptosis associated with 8-OHdG and Caspase-3 in rats.](https://pubmed.ncbi.nlm.nih.gov/42550302/)
3. [Single-cell RNA sequencing identifies uterine stromal cells as a previously unrecognized target of an alphacoronavirus underlying reproductive disorders.](https://pubmed.ncbi.nlm.nih.gov/42484330/)
4. [Sex differences in urinary extracellular vesicles originating from the genitourinary system in health and disease.](https://pubmed.ncbi.nlm.nih.gov/42530522/)