Placenta and Fetal Membranes, Physiological Changes During Pregnancy, and Diagnosis of Pregnancy: A Comprehensive Review
1. Singh Madhav
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
The placenta and fetal membranes form the essential interface between mother and fetus, enabling gas exchange, nutrient transfer, waste elimination, endocrine regulation, and immunological protection throughout gestation. Their orderly development from trophoblast and extraembryonic tissues is critical for successful pregnancy, while abnormalities in formation or function contribute to major obstetric syndromes. Concurrently, profound physiological adaptations occur in virtually every maternal organ system to support the growing fetus and prepare for parturition and lactation. These changes, driven largely by placental hormones and hemodynamic demands, affect the cardiovascular, respiratory, renal, hematologic, and endocrine systems and must be distinguished from pathological states. Accurate diagnosis of pregnancy relies on a combination of clinical symptoms and signs, laboratory detection of human chorionic gonadotropin, and ultrasonographic visualization of the gestational sac, yolk sac, and embryo with cardiac activity. This review integrates current understanding of placental and membrane anatomy and physiology, the spectrum of maternal physiological adaptations, and contemporary approaches to pregnancy diagnosis, drawing on recent literature and clinical guidelines. The aim is to provide medical students and clinicians with a coherent, clinically relevant foundation that links structure, function, and diagnostic practice.
Introduction
Pregnancy represents one of the most remarkable physiological transformations in human biology. From the moment of implantation, a temporary organ—the placenta—begins to form, establishing a unique vascular and endocrine relationship between two genetically distinct individuals. Alongside this, the fetal membranes create a protective fluid-filled environment that cushions the developing fetus, permits movement, and participates in the initiation of labor. At the same time, the mother’s body undergoes extensive adaptations that increase cardiac output, expand blood volume, enhance renal filtration, and alter respiratory mechanics, all coordinated largely by hormones produced by the trophoblast.
Understanding these processes is fundamental to obstetric practice. Disorders of placental development underlie preeclampsia, fetal growth restriction, and stillbirth. Failure of membrane integrity leads to preterm premature rupture of membranes. Misinterpretation of normal physiological changes can result in unnecessary investigation or missed pathology. Timely and accurate diagnosis of pregnancy itself remains the gateway to antenatal care, yet challenges persist in early gestation, particularly in distinguishing viable intrauterine pregnancy from ectopic pregnancy or early pregnancy loss.
This review examines three interconnected domains. First, it describes the development, structure, and functions of the placenta and fetal membranes, incorporating recent insights into membrane formation and the role of oxidative stress and trophoblast invasion. Second, it surveys the major physiological changes that occur across maternal organ systems during pregnancy, emphasizing quantitative changes and their clinical implications. Third, it outlines the clinical, laboratory, and imaging methods used to diagnose pregnancy, with attention to current guidelines on human chorionic gonadotropin interpretation and first-trimester ultrasound criteria. Throughout, the narrative seeks to convey both scientific precision and the clinical relevance that arises when these systems function—or fail—in the care of individual women.
Methods
This narrative review synthesizes peer-reviewed literature, textbook chapters, and clinical guidelines published primarily between 2015 and 2026, with inclusion of classic foundational descriptions that remain authoritative. Searches were performed in PubMed, UpToDate, and major obstetric society resources using terms such as “placental development,” “fetal membranes,” “maternal physiological changes pregnancy,” “diagnosis of pregnancy,” “human chorionic gonadotropin,” and “first-trimester ultrasound.” Priority was given to recent reviews in the American Journal of Obstetrics & Gynecology, StatPearls summaries, ACOG practice bulletins, and mechanistic studies of trophoblast biology and membrane formation. Quantitative values for physiological changes were drawn from consistently reported ranges across sources. Diagnostic criteria reflect current consensus on discriminatory zones and ultrasound landmarks. The synthesis prioritizes clinical applicability and educational clarity while acknowledging areas of ongoing research.
Results
Development and Structure of the Placenta
Placental development begins shortly after fertilization. The blastocyst differentiates into the inner cell mass, which will form the embryo, and the outer trophoblast layer. Around day 6 to 7 after fertilization, the trophoblast attaches to the endometrial epithelium and invades the decidua. The trophoblast rapidly differentiates into an inner cytotrophoblast layer of individual cells and an outer multinucleated syncytiotrophoblast. The syncytiotrophoblast produces human chorionic gonadotropin, which maintains the corpus luteum and thereby progesterone production in early pregnancy, and invades maternal tissues to establish the intervillous space.
Primary chorionic villi form as cytotrophoblast columns covered by syncytiotrophoblast. Secondary villi develop when extraembryonic mesoderm invades the core, and tertiary villi appear once fetal blood vessels form within the mesoderm. By the end of the first trimester, the placenta has assumed its definitive discoid shape. The fetal surface is covered by the chorionic plate, from which the umbilical cord arises, while the maternal surface consists of the basal plate and cotyledons separated by septa derived from the decidua. The functional unit of exchange is the terminal villus, where fetal capillaries lie in close proximity to the syncytiotrophoblast bathed by maternal blood in the intervillous space.
At term the placenta typically measures 15 to 20 cm in diameter, 2 to 3 cm in thickness, and weighs approximately 500 g. Its dual circulation—maternal blood in the intervillous space and fetal blood within villous capillaries—allows transfer of oxygen, carbon dioxide, nutrients, and waste products while maintaining immunological separation. The syncytiotrophoblast lacks classical MHC class I expression, contributing to immune tolerance of the fetal allograft. In addition to transport, the placenta is a major endocrine organ, producing progesterone, estrogens, human placental lactogen, placental growth hormone, and corticotropin-releasing hormone that regulate maternal metabolism and prepare the uterus and mammary glands for parturition and lactation.
Fetal Membranes
The fetal membranes consist of the amnion and chorion. The amnion, derived from the epiblast, forms the innermost layer lining the amniotic cavity. It comprises a single layer of epithelial cells resting on a basement membrane, followed by a compact layer, fibroblast layer, and spongy layer of extracellular matrix. The amnion provides most of the tensile strength of the membranes and is relatively avascular. The chorion, derived from trophoblast and extraembryonic mesoderm, lies external to the amnion and is thicker. In the early gestational sac the chorion is covered by villi (chorion frondosum). As the sac expands, villi over the greater part of the surface regress, forming the smooth chorion laeve.
Recent work has clarified that membrane formation involves regression of approximately two-thirds of the early villi, associated with localized oxidative stress as maternal arterial circulation begins preferentially in the periphery around 6 to 8 weeks. The decidua capsularis overlying the expanding sac undergoes necrosis and does not contribute to the mature membranes. Around 16 weeks the chorionic sac fuses with the decidua parietalis, and cytotrophoblast cells of the chorionic epithelium proliferate to form a stratified layer. The amnion and chorion remain loosely adherent until later gestation, when they become more closely applied. Amniotic fluid, initially a transudate and later largely fetal urine, is regulated by fetal swallowing, intramembranous absorption, and membrane transport. The membranes serve as a protective barrier, participate in immune modulation, and contribute to the biochemical cascade that initiates labor through prostaglandin production and matrix remodeling.
Physiological Changes During Pregnancy
Maternal physiology adapts profoundly to support fetal growth and prepare for delivery. Cardiovascular changes begin early. Cardiac output rises by 30 to 50 percent, initially through increased stroke volume and later through a rise in heart rate of 10 to 20 beats per minute. Systemic vascular resistance falls due to progesterone-mediated vasodilation and the low-resistance placental circuit, producing a mid-pregnancy decline in diastolic blood pressure that returns toward baseline near term. Blood volume expands by 40 to 50 percent, with plasma volume increasing more than red-cell mass, resulting in the physiological anemia of pregnancy. The gravid uterus can compress the inferior vena cava in the supine position, reducing venous return and causing supine hypotensive syndrome; left lateral tilt mitigates this effect.
Respiratory adaptations include an increase in tidal volume of approximately 40 percent driven by progesterone stimulation of the respiratory center, raising minute ventilation and producing a compensated respiratory alkalosis with arterial PCO₂ falling to 28–32 mmHg. Functional residual capacity decreases as the uterus elevates the diaphragm. Oxygen consumption rises to meet the metabolic demands of the fetus, placenta, and maternal organs.
Renal plasma flow and glomerular filtration rate increase by up to 50 percent, lowering serum creatinine and urea. The collecting system dilates, more prominently on the right, creating physiological hydronephrosis. Sodium and water retention occur under the influence of the renin–angiotensin–aldosterone system, yet plasma osmolality falls slightly.
Hematologic changes encompass the dilutional anemia already noted, a mild leukocytosis, and a progressive hypercoagulable state with elevated clotting factors and fibrinogen that protects against postpartum hemorrhage but increases the risk of venous thromboembolism. Platelet counts may decline modestly.
Endocrine changes are dominated by placental hormones. Human chorionic gonadotropin peaks in the first trimester. Progesterone and estrogens rise steadily. Human placental lactogen and placental growth hormone induce insulin resistance, ensuring glucose availability for the fetus. The thyroid axis is stimulated by hCG, and cortisol-binding globulin increases, elevating total cortisol levels. These adaptations are largely reversible after delivery, although some residual changes may persist.
Diagnosis of Pregnancy
Diagnosis begins with clinical suspicion based on amenorrhea, breast tenderness, nausea, urinary frequency, and fatigue. Physical signs such as Chadwick’s sign (bluish discoloration of the cervix and vagina), Goodell’s sign (cervical softening), and Hegar’s sign (softening of the uterine isthmus) appear in the first trimester but are nonspecific.
Laboratory confirmation relies on detection of human chorionic gonadotropin. Urine pregnancy tests are highly sensitive and specific when performed after a missed period. Serum quantitative β-hCG assays detect pregnancy earlier and allow serial monitoring. In a viable intrauterine pregnancy, levels typically double every 48 hours in early gestation, although the rate varies. The discriminatory zone—the β-hCG level above which an intrauterine gestational sac should be visible on transvaginal ultrasound—is generally cited between 1500 and 3500 mIU/mL, but thresholds must be interpreted cautiously and in conjunction with clinical findings.
Ultrasonography is the definitive method for confirming location and viability. Transvaginal ultrasound can detect a gestational sac at approximately 4.5 to 5 weeks, a yolk sac shortly thereafter, and an embryo with cardiac activity by 6 weeks. Crown–rump length provides the most accurate gestational age assessment in the first trimester. Absence of a yolk sac or embryo at expected sizes, or failure of appropriate growth on serial scans, raises concern for early pregnancy loss. Visualization of an adnexal mass or empty uterus with rising hCG prompts evaluation for ectopic pregnancy. Current guidelines emphasize conservative ultrasound criteria to avoid intervening in potentially viable pregnancies.
Discussion
The placenta and fetal membranes exemplify the extraordinary coordination required for human reproduction. Their development from trophoblast and extraembryonic tissues creates a dynamic interface that simultaneously nourishes the fetus, modulates maternal physiology, and protects against immune rejection. Recent studies highlighting the role of oxidative stress in villous regression and membrane formation deepen our understanding of how early events influence later pregnancy complications such as preterm premature rupture of membranes.
Maternal physiological changes are equally impressive in scope. The cardiovascular expansion and respiratory adjustments ensure adequate oxygen delivery, while renal hyperfiltration clears fetal wastes. Clinicians must recognize these norms to avoid misdiagnosing physiological anemia or interpreting a creatinine of 0.7 mg/dL as abnormal. At the same time, the hypercoagulable state and altered drug pharmacokinetics demand careful attention in clinical management.
Diagnosis of pregnancy has become highly accurate with modern hCG assays and high-resolution ultrasound, yet challenges remain in the earliest weeks and in distinguishing normal from abnormal gestations. Serial evaluation and adherence to evidence-based discriminatory criteria reduce both delayed diagnosis of ectopic pregnancy and unnecessary intervention in viable pregnancies.
Limitations of current knowledge include incomplete understanding of the molecular drivers of membrane remodeling and the long-term implications of placental function for maternal and offspring health. Future research integrating single-cell and spatial transcriptomics of the maternal–fetal interface promises further insight. In clinical practice, the integration of anatomical knowledge, physiological awareness, and precise diagnostic methods remains essential for safe and effective maternity care.
Every pregnancy begins with the silent establishment of the placenta and the progressive adaptation of the maternal organism. Accurate recognition of these processes allows clinicians to support the profound transformation that sustains new life.
References
Roberts V, Myatt L. Placental development and physiology. UpToDate. Updated June 2025.
Burton GJ, et al. Formation of the placental membranes and pathophysiological origin of associated great obstetrical syndromes. Am J Obstet Gynecol. 2025.
StatPearls. Physiology, Placenta. NCBI Bookshelf. Updated 2023–2025.
StatPearls. Embryology, Placenta. NCBI Bookshelf.
StatPearls. Physiology, Maternal Changes. NCBI Bookshelf.
Merck Manual Professional Edition. Physiology of Pregnancy. Updated recent years.
American College of Obstetricians and Gynecologists. Ultrasound in Pregnancy. Practice Bulletin No. 175 (reaffirmed).
American College of Obstetricians and Gynecologists. Early Pregnancy Loss. Practice Bulletin No. 200.
Frontiers in Cell and Developmental Biology. Unveiling the human fetal-maternal interface during the first trimester. 2024.
Nature. Single-cell spatiotemporal dissection of the human maternal–fetal interface. 2026.
Additional supporting reviews on maternal cardiovascular, respiratory, renal, and hematologic adaptations (2023–2026).
Clinical guidelines and reviews on hCG interpretation and first-trimester ultrasound criteria from ACOG and related societies.
Contemporary sources on amniotic membrane structure and function.
Foundational embryology texts and recent mechanistic studies of trophoblast invasion and membrane formation.
Population and clinical data on physiological reference ranges in pregnancy.
UpToDate and specialty reviews synthesizing placental endocrine and transport functions.