Anatomy of the Female Reproductive Organs, the Fetus, Fetal Skull, and Maternal Pelvis: A Comprehensive Review
1. Verma Pradeep Kumar
2. Osmonova Gulnaz Zhenishbaevna
(1. Student, International Medical Faculty, Osh State University, Osh, Kyrgyz Republic
2. Teacher, International Medical Faculty, Osh State University, Osh, Kyrgyz Republic.)
Abstract
A thorough understanding of the anatomy of the female reproductive organs, the developing fetus, the fetal skull, and the maternal pelvis remains fundamental to the practice of obstetrics and gynecology. These structures interact dynamically during pregnancy, labor, and delivery, determining the feasibility of vaginal birth and influencing maternal and neonatal outcomes. This review synthesizes current anatomical knowledge drawn from classical descriptions, modern imaging studies, and biomechanical research. The female reproductive tract is examined from external genitalia through the internal organs responsible for gametogenesis, fertilization, implantation, and gestation. Fetal development is traced from fertilization through the embryonic and fetal periods, with particular attention to the adaptive features of the fetal skull. The maternal bony pelvis is described in terms of its planes, diameters, and morphological variants according to the Caldwell-Moloy classification. Emphasis is placed on the functional relationships among these elements that enable or impede parturition. Clinical implications, including molding of the fetal head, cephalopelvic relationships, and the assessment of pelvic capacity, are integrated throughout. The review aims to provide medical students, midwives, and clinicians with a coherent, up-to-date anatomical foundation that supports safe obstetric practice.
Introduction
The successful progression of human pregnancy and childbirth depends upon the precise anatomical and physiological interplay between the maternal reproductive system and the developing fetus. Few areas of clinical anatomy carry such immediate practical consequence as the female genital tract, the fetal skull, and the maternal pelvis. From the moment of fertilization within the uterine tube to the final expulsion of the neonate through the birth canal, each structure must fulfill its role with remarkable efficiency. A deficiency in any one component—whether a contracted pelvic diameter, an incompletely flexed fetal head, or an anomalous uterine configuration—can transform an otherwise normal labor into a high-risk event requiring intervention.
Historically, detailed descriptions of these structures emerged from dissection and clinical observation in the nineteenth and early twentieth centuries. The work of Caldwell and Moloy in the 1930s formalized the classification of pelvic types that continues to inform teaching today, while the classic measurements of the fetal skull diameters remain central to understanding presentation and position. More recent advances in ultrasonography, magnetic resonance imaging, and three-dimensional modeling have refined our appreciation of dynamic changes, particularly the molding of the fetal skull during labor and the biomechanical stresses placed on both maternal soft tissues and the fetal brain. Contemporary genetic and evolutionary studies further illuminate the constraints that shaped the human pelvis and the relatively large fetal head, sometimes referred to as the obstetrical dilemma.
Despite the abundance of primary literature, students and practitioners often encounter fragmented accounts that treat the reproductive organs, the fetus, and the pelvis in isolation. The purpose of this review is therefore to present an integrated description that follows the natural sequence of events in pregnancy and birth. The article examines the anatomy of the female reproductive organs as the environment of early development, surveys the morphology and growth of the fetus with special reference to the skull, and then analyzes the maternal pelvis as the bony framework through which the fetal head must pass. Clinical correlations are woven into the anatomical narrative so that the reader can appreciate not only structure but also function and potential points of failure.
The knowledge assembled here is intended to support undergraduate medical and midwifery education as well as the continuing professional development of clinicians. By consolidating classical measurements with insights from recent imaging and biomechanical research, the review seeks to offer a reliable reference that remains clinically relevant in the second decade of the twenty-first century.
Methods
This article is a narrative review of the anatomical and obstetric literature concerning the female reproductive organs, fetal development with emphasis on the skull, and the maternal bony pelvis. Primary sources included standard textbooks of anatomy and obstetrics (Gray’s Anatomy, Williams Obstetrics, and equivalent contemporary editions), peer-reviewed review articles and original research published in journals such as the American Journal of Obstetrics and Gynecology, Ultrasound in Obstetrics & Gynecology, and Proceedings of the National Academy of Sciences, as well as open-access anatomical resources from NCBI StatPearls and established educational platforms. Searches were performed using combinations of terms including “female reproductive anatomy,” “fetal development stages,” “fetal skull diameters sutures fontanelles,” “maternal pelvis Caldwell-Moloy,” “fetal head molding,” and “cephalopelvic disproportion” across PubMed, Google Scholar, and specialized obstetric repositories. Preference was given to sources published or updated between 2015 and 2026 to incorporate advances in imaging and biomechanical modeling, while classical measurements that have remained stable across editions were retained. Where quantitative values for diameters and incidence rates of pelvic types showed minor variation across sources, the most consistently reported figures were selected and noted as approximate averages. The synthesis prioritizes structural description, functional relationships, and clinical significance, presented in continuous prose to facilitate reading as a coherent anatomical account rather than a series of disconnected lists.
Results
Anatomy of the Female Reproductive Organs
The female reproductive system comprises external and internal genitalia that together enable sexual function, gamete production, fertilization, implantation, and support of the developing fetus until birth. The external genitalia, collectively termed the vulva, occupy the perineum and serve primarily protective and sensory roles. Anteriorly, the mons pubis forms a rounded pad of adipose tissue overlying the pubic symphysis and is covered by hair after puberty. The labia majora are two longitudinal folds of skin and subcutaneous fat that extend posteriorly from the mons and meet in the midline behind the vaginal orifice; they contain sebaceous and sweat glands and, after puberty, hair on their outer surfaces. Medial to the labia majora lie the thinner, hairless labia minora, which enclose the vestibule. At the anterior junction of the labia minora the clitoris is formed; this erectile structure is homologous to the male penis and consists of a glans, body, and paired crura attached to the ischiopubic rami. The vestibular bulbs, also erectile, lie deep to the labia minora on either side of the vaginal orifice. Within the vestibule open the urethra anteriorly and the vagina posteriorly. The greater vestibular (Bartholin) glands, located posterolaterally, secrete mucus that lubricates the vestibule during sexual arousal, while the lesser vestibular (Skene) glands open near the urethral meatus.
The vagina is a fibromuscular canal approximately 7.5 to 10 cm in length that extends from the vestibule to the cervix. Its anterior wall is shorter than the posterior wall, and the cervix projects into its upper end, creating the vaginal fornices—anterior, posterior, and two lateral. The posterior fornix is the deepest and is related to the rectouterine pouch (of Douglas). The vaginal wall consists of an outer adventitia continuous with surrounding pelvic connective tissue, a muscular layer of inner circular and outer longitudinal smooth muscle, a lamina propria rich in elastic fibers, and a non-keratinized stratified squamous epithelium that is thrown into transverse rugae. These rugae, together with the inherent elasticity of the wall, allow marked distension during intercourse and parturition. The vagina receives blood from the vaginal branches of the uterine and internal iliac arteries and is innervated by both autonomic fibers (via the uterovaginal plexus) and somatic fibers (pudendal nerve) in its lower portion.
The uterus is a thick-walled, pear-shaped muscular organ that lies in the true pelvis between the bladder anteriorly and the rectum posteriorly. In the nulliparous adult it measures approximately 7.5 cm in length, 5 cm in width, and 2.5 cm in anteroposterior thickness, yet it is capable of enormous enlargement during pregnancy. It is divided into the fundus (the portion above the entrance of the uterine tubes), the body (corpus), the isthmus (a narrow transitional zone), and the cervix. The cervix is cylindrical, approximately 2.5 cm long, and projects into the vagina; its canal communicates with the uterine cavity via the internal os and with the vagina via the external os. The uterine wall comprises three layers: the outer serosa or perimetrium (peritoneum covering the fundus and body), the thick myometrium of interwoven smooth-muscle bundles, and the inner endometrium. The endometrium itself consists of a functional layer that undergoes cyclic changes under the influence of ovarian hormones and a basal layer that regenerates after menstruation. During pregnancy the endometrium is transformed into the decidua, which participates in the formation of the placenta. The uterus is supported by the broad ligaments, the round ligaments, the cardinal (transverse cervical) ligaments, and the uterosacral ligaments, which maintain its anteverted and anteflexed position in most women.
The paired uterine (fallopian) tubes extend laterally from the uterine cornua toward the ovaries. Each tube is approximately 10–12 cm long and is divided into four regions: the intramural (interstitial) portion within the uterine wall, the narrow isthmus, the wider ampulla (the usual site of fertilization), and the funnel-shaped infundibulum with its finger-like fimbriae that sweep over the ovarian surface. The mucosa is lined by ciliated and secretory cells that facilitate transport of the ovum and the early embryo toward the uterus. The ovaries are almond-shaped organs, roughly 3–5 cm long, suspended from the posterior leaf of the broad ligament by the mesovarium and attached to the uterus by the ovarian ligament and to the pelvic side wall by the suspensory ligament containing the ovarian vessels. Each ovary consists of a cortex containing follicles at various stages of maturation and a medulla of connective tissue and blood vessels. At birth the ovaries contain several hundred thousand primordial follicles; only a few hundred will reach ovulation during reproductive life. The ovaries produce both gametes and the steroid hormones estrogen and progesterone that regulate the menstrual cycle and maintain early pregnancy.
Blood supply to the internal genitalia is derived principally from the uterine and ovarian arteries, with extensive anastomoses ensuring collateral circulation. Venous drainage parallels the arteries, forming the pampiniform plexus around the ovarian vessels and the uterine venous plexus that ultimately empties into the internal iliac veins. Lymphatic drainage follows the vessels to the external iliac, internal iliac, and para-aortic nodes. Innervation is dual: sympathetic fibers from the hypogastric plexus and parasympathetic fibers from the pelvic splanchnic nerves (S2–S4) reach the uterus and upper vagina, while the lower vagina and external genitalia receive somatic innervation via the pudendal nerve.
The Fetus
Human prenatal development is conventionally divided into the germinal (pre-embryonic), embryonic, and fetal periods. Fertilization normally occurs in the ampulla of the uterine tube, producing a zygote that undergoes cleavage as it travels toward the uterus. By approximately day 5–6 a blastocyst has formed, consisting of an outer trophoblast that will contribute to the placenta and an inner cell mass that will form the embryo proper. Implantation into the endometrium begins around day 6–7 and is completed by the end of the second week, at which time the bilaminar embryonic disc and the amniotic cavity are established.
The embryonic period, lasting from the third to the eighth week after fertilization (approximately weeks 5–10 of gestational age counted from the last menstrual period), is the time of organogenesis. Gastrulation produces the three primary germ layers—ectoderm, mesoderm, and endoderm—from which all tissues and organs derive. The neural tube closes, the heart begins to beat, limb buds appear, and the basic body plan is established. By the end of the eighth week the embryo is approximately 3 cm in crown-rump length and possesses recognizable human features, including eyes, ears, fingers, and toes. This period is also the time of greatest vulnerability to teratogens.
From the beginning of the ninth week after fertilization until birth the developing organism is termed a fetus. The fetal period is characterized by rapid growth, maturation of organ systems, and refinement of form rather than the appearance of new organs. Crown-rump length increases from roughly 3–4 cm at nine weeks to approximately 36 cm at term, while weight rises from a few grams to an average of 3 200–3 500 g. The head, which occupies a disproportionately large fraction of body length in the early fetus, gradually assumes a more balanced relationship with the trunk. External genitalia become distinguishable by ultrasound around 12–14 weeks. Fetal movements (“quickening”) are usually perceived by the mother between 16 and 20 weeks. By 24 weeks the lungs have entered the canalicular stage and begin to produce surfactant, although viability outside the uterus remains limited. The third trimester is marked by further accumulation of subcutaneous fat, maturation of the central nervous system, and preparation of the lungs for air breathing. Throughout this period the fetus is nourished and oxygenated via the placenta and is protected by the amniotic fluid, which also permits free movement and prevents adhesion of fetal parts.
The Fetal Skull
The fetal skull is of paramount obstetric importance because it is the largest and least compressible part of the fetus and must negotiate the maternal birth canal. At term the skull consists of the vault (calvaria) and the base. The vault is formed by the two frontal bones, the two parietal bones, and the squamous portions of the occipital and temporal bones. These bones are separated by membranous sutures that remain unossified at birth, allowing a degree of mobility. The principal sutures are the frontal (metopic) suture between the two frontal bones, the coronal sutures between frontal and parietal bones, the sagittal suture between the two parietal bones, and the lambdoid sutures between the parietal and occipital bones.
Where three or more bones meet, wider membranous gaps called fontanelles are present. The anterior fontanelle (bregma) is diamond-shaped, measuring approximately 2–3 cm in each direction, and is formed by the junction of the frontal, coronal, and sagittal sutures; it normally closes between 12 and 24 months after birth. The posterior fontanelle (lambda) is triangular and smaller, formed by the sagittal and lambdoid sutures, and closes by 6–8 weeks of postnatal life. Two smaller sphenoidal and mastoid fontanelles lie laterally but are of lesser obstetric significance.
The regions of the fetal skull used as landmarks during labor are the vertex (the area bounded by the anterior and posterior fontanelles and the parietal eminences), the occiput (the area behind the posterior fontanelle), the sinciput or brow (the area in front of the anterior fontanelle), and the face. The attitude of the head—degree of flexion or extension—determines which diameter presents. In complete flexion the suboccipitobregmatic diameter (from the nape of the neck to the center of the bregma) measures approximately 9.5 cm and is the most favorable for vaginal delivery. Slight deflexion produces the suboccipitofrontal diameter of about 10 cm. Further deflexion yields the occipitofrontal diameter of 11.5 cm. Extreme extension produces the mentovertical diameter of 13–14 cm (brow presentation) or, with further extension, the submentobregmatic diameter of 9.5 cm (face presentation). The biparietal diameter, the greatest transverse diameter between the parietal eminences, measures approximately 9.5 cm and is the critical transverse dimension that must pass through the pelvic planes.
During labor the sutures and fontanelles permit molding: the bones of the vault can override one another, temporarily reducing the presenting diameters by up to 1 cm or more. Modern imaging studies using three-dimensional ultrasound and MRI have confirmed that molding occurs in the majority of vaginal deliveries, with the parietal bones typically shifting beneath the frontal or occipital bones. This plasticity is essential for successful passage through the relatively rigid maternal pelvis, yet excessive or prolonged molding may be associated with increased intracranial pressure and, in rare cases, adverse neonatal outcomes.
The Maternal Pelvis
The bony pelvis is formed by the two hip bones (each consisting of ilium, ischium, and pubis), the sacrum, and the coccyx. It is divided into the false (greater) pelvis above the pelvic brim, which has little obstetric relevance, and the true (lesser) pelvis below the brim, which constitutes the birth canal. The true pelvis is further subdivided into the inlet (brim), the cavity, and the outlet.
The pelvic inlet is bounded by the sacral promontory, the alae of the sacrum, the sacroiliac joints, the iliopectineal lines, the superior pubic rami, and the upper border of the symphysis pubis. Its principal diameters are the anatomical (true) conjugate of approximately 11 cm (from sacral promontory to the upper border of the symphysis), the obstetric conjugate of 10–10.5 cm (the shortest anteroposterior diameter, measured to the most prominent point on the posterior surface of the symphysis), the diagonal conjugate of about 12.5 cm (from promontory to the lower border of the symphysis, measurable clinically), the transverse diameter of 13 cm, and the oblique diameters of 12 cm. The inlet is normally slightly transverse oval.
The mid-pelvis, or plane of least pelvic dimensions, lies at the level of the ischial spines. Here the transverse (interspinous) diameter is approximately 10–10.5 cm and is often the narrowest dimension of the pelvis. The anteroposterior diameter at this level is about 12 cm. The pelvic cavity between inlet and mid-pelvis is relatively roomy, with diameters approaching 12–12.5 cm in all directions.
The pelvic outlet is diamond-shaped. Its anteroposterior diameter measures approximately 13 cm when the coccyx is displaced backward during the second stage of labor; the bituberous (transverse) diameter between the ischial tuberosities is about 11 cm. The subpubic angle is normally 85–100 degrees in the gynecoid pelvis.
Four classical morphological types were described by Caldwell and Moloy. The gynecoid pelvis (approximately 40–50 percent of women) has a rounded or slightly transverse oval inlet, straight side walls, blunt ischial spines, a wide subpubic arch, and a well-curved sacrum; it is the most favorable for vaginal delivery. The android pelvis (20–30 percent) resembles the male form, with a heart-shaped or triangular inlet, convergent side walls, prominent ischial spines, and a narrow subpubic arch; it predisposes to deep transverse arrest. The anthropoid pelvis (20–25 percent) has a long oval inlet with elongated anteroposterior diameters and shortened transverse diameters; persistent occiput-posterior positions are more common. The platypelloid pelvis (less than 5 percent) is flat, with shortened anteroposterior and elongated transverse diameters, often leading to asynclitism or obstructed labor. Most pelves encountered clinically are mixed rather than pure types.
The axis of the birth canal is curved: the fetal head descends first downward and backward along the axis of the inlet, then changes direction at the level of the ischial spines to pass downward and forward through the outlet. This J-shaped pathway, together with the changing orientation of the longest pelvic diameters (transverse at the inlet, anteroposterior at the outlet), necessitates the cardinal movements of labor—engagement, descent, flexion, internal rotation, extension, restitution, and external rotation.
Discussion
The anatomical relationships described above form the structural basis of human parturition. The female reproductive organs create and maintain the intrauterine environment that supports the fetus from implantation to term. The uterus, in particular, undergoes extraordinary hypertrophy and hyperplasia, increasing its capacity many-fold while retaining the contractile power necessary for labor. The cervix must soften, efface, and dilate under the influence of hormonal and mechanical factors, transforming from a firm barrier into a continuum with the lower uterine segment and vagina.
The fetal skull’s capacity for molding is a remarkable evolutionary adaptation that partially compensates for the constraints of the bipedal pelvis. Recent MRI and ultrasound studies have demonstrated that overlapping of the cranial bones occurs routinely during the second stage of labor and that the resulting reduction in presenting diameters facilitates passage through the mid-pelvis and outlet. At the same time, excessive molding or prolonged compression can transmit force to the underlying brain, a finding that has prompted renewed interest in the biomechanics of labor and the possible contribution of head compliance to outcomes previously attributed solely to cephalopelvic disproportion.
Assessment of the maternal pelvis remains both clinical and, when indicated, imaging-based. Clinical pelvimetry, including measurement of the diagonal conjugate and evaluation of the subpubic angle and sacral curve, continues to provide useful information, although its predictive value for successful vaginal birth is limited. Modern imaging—ultrasound, magnetic resonance, and computed tomography—can furnish precise measurements and three-dimensional reconstructions, yet no single modality has replaced the trial of labor as the ultimate test of pelvic capacity. The recognition that pelvic morphology is highly heritable and that fetal head size and pelvic dimensions show genetic correlation further underscores the co-evolution of these structures.
From a clinical standpoint, knowledge of these anatomical details guides the diagnosis of presentation and position, the interpretation of progress in labor, and the recognition of potential obstruction. Vertex presentation with well-flexed head and occipito-anterior position presents the most favorable diameters to the successive pelvic planes. Deflexed attitudes, malpositions, and pelvic variants increase the likelihood of dystocia and may necessitate operative intervention. Understanding the anatomy also informs the safe performance of instrumental delivery, the placement of pudendal nerve block, and the management of perineal trauma.
Limitations of the present review include the reliance on averaged measurements that inevitably mask individual variation, the incomplete integration of soft-tissue dynamics (pelvic floor, ligaments, and uterine lower segment), and the still-emerging nature of biomechanical modeling. Future research combining advanced imaging, finite-element analysis, and large-scale genetic data is likely to refine our understanding of the interplay between fetal skull plasticity and maternal pelvic architecture.
In summary, the anatomy of the female reproductive organs, the fetus, the fetal skull, and the maternal pelvis constitutes a coherent functional system whose successful operation underpins safe childbirth. Mastery of this anatomy remains indispensable for every clinician charged with the care of the pregnant woman and her infant. Continuous correlation of classical descriptions with contemporary imaging and biomechanical insights will ensure that this foundational knowledge continues to support evidence-based obstetric practice.
References
Caldwell WE, Moloy HC. Anatomical variations in the female pelvis and their effect in labor with a suggested classification. Am J Obstet Gynecol. 1933;26(4):479-505.
Caldwell WE, Moloy HC. Anatomical variations in the female pelvis: their classification and obstetrical significance. Proc R Soc Med. 1938;32(1):1-30.
Standring S, editor. Gray’s Anatomy: The Anatomical Basis of Clinical Practice. 42nd ed. Elsevier; 2020.
Cunningham FG, Leveno KJ, Bloom SL, et al. Williams Obstetrics. 26th ed. McGraw-Hill Education; 2022. (Note: 27th edition forthcoming/available in some markets.)
Dutta DC. DC Dutta’s Textbook of Obstetrics: Including Perinatology and Contraception. 9th ed. Jaypee Brothers Medical Publishers; 2018 (or later reprints).
Moore KL, Persaud TVN, Torchia MG. The Developing Human: Clinically Oriented Embryology. 11th ed. Elsevier; 2019.
Ami O, Maran JC, Gabor P, et al. Three-dimensional magnetic resonance imaging of fetal head molding and brain shape changes during the second stage of labor. PLoS One. 2019;14(5):e0215721. doi:10.1371/journal.pone.0215721
Kahrs BH, Usman S, Ghi T, et al. Fetal molding examined with transperineal ultrasound and associations with position and delivery mode. Am J Obstet Gynecol. 2020;223(6):909.e1-909.e8.
Rankin J, editor. Physiology in Childbearing: With Anatomy and Related Biosciences. 4th ed. Elsevier; 2017. (Chapter on the nature of bone: the female pelvis and fetal skull.)
National Center for Biotechnology Information. Anatomy, Abdomen and Pelvis, Female Internal Genitals. StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025.
National Center for Biotechnology Information. Anatomy, Abdomen and Pelvis, Female External Genitalia. StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025.
National Center for Biotechnology Information. Anatomy, Abdomen and Pelvis, Pelvic Inlet. StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023.
El-Mowafi DM. Anatomy of the Female Pelvis. Obstetrics Simplified. Geneva Foundation for Medical Education and Research (GFMER). Available from: https://gfmer.ch/Obstetrics_simplified/anatomy_of_the_female_pelvis.htm (accessed 2026).
Bamberg C, Deprest J, Sindhwani N, et al. Evaluating fetal head dimension changes during labor using open magnetic resonance imaging. J Perinat Med. 2016. doi:10.1515/jpm-2016-0005.
VanSickle C, Liese KL, Rutherford JN. Textbook typologies: Challenging the myth of the perfect obstetric pelvis. Anat Rec (Hoboken). 2022;305(4):952-967. doi:10.1002/ar.24880.
Xu L, et al. The genetic architecture of and evolutionary constraints on the human pelvic form. Science. 2025;388(6743). doi:10.1126/science.adq1521.