28.3 Fetal Development
4 min read•june 18, 2024
Embryonic and fetal development marks the incredible journey from a single cell to a fully formed baby. This process involves rapid cell division, organ formation, and growth, with distinct stages and milestones along the way.
The fetal circulatory system is uniquely adapted for life in the womb, with special structures that bypass the lungs and prioritize blood flow to vital organs. As pregnancy progresses, the fetus undergoes remarkable changes, developing from a tiny embryo to a fully formed infant ready for birth.
Embryonic and Fetal Development
Embryonic vs fetal periods
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- Embryonic period spans from weeks 3 to 8 of gestation involves rapid and development of major organ systems such as the formation of the (precursor to the brain and spinal cord), heart, and limb buds (arms and legs) with high susceptibility to (harmful substances)
- Fetal period extends from week 9 of gestation until birth characterized by rapid growth and maturation of organ systems, accumulation of body fat and muscle mass, and increased survival rate compared to the embryonic period
- occurs during the embryonic period, involving the formation of tissues and organs from the three germ layers
Sexual differentiation in fetuses
- Genetic sex determined at fertilization by the presence of X or Y chromosome from the sperm
- Gonadal sex
- Presence of Y chromosome and leads to development of testes
- Absence of Y chromosome results in development of ovaries
- Phenotypic sex differentiation driven by testosterone secreted by fetal testes leads to development of male external genitalia (penis and scrotum) while absence of testosterone results in development of female external genitalia (clitoris and labia)
- Internal duct system differentiation
- Testosterone causes to develop into male reproductive structures (epididymis, vas deferens, seminal vesicles)
- Absence of testosterone leads to degeneration of Wolffian ducts and development of into female reproductive structures (fallopian tubes, uterus, upper vagina)
Fetal Circulation and Development
Fetal circulatory system features
- carries oxygenated blood from the to the fetus
- Ductus venosus shunts a portion of oxygenated blood from the directly to the inferior vena cava, bypassing the liver to prioritize oxygenated blood delivery to the fetal brain and heart
- shunts oxygenated blood from the right atrium to the left atrium, bypassing the lungs as they are not functional in the fetal environment filled with
- shunts blood from the pulmonary artery to the aorta, bypassing the non-functional lungs to ensure adequate blood flow to the fetal body
- Umbilical arteries carry deoxygenated blood from the fetus back to the placenta for oxygenation and waste removal (carbon dioxide and urea)
Developmental milestones to birth
- Week 9-12: Fetal period begins, external genitalia begin to differentiate (penis or clitoris), fetal movements can be detected by
- Week 13-16: Fetal skin is transparent and covered with hair (fine, soft hair), fetal liver begins to produce red blood cells, (first feces) starts to accumulate in the intestines
- Week 17-20: (protective waxy coating) covers the skin to protect it from amniotic fluid, fetal heartbeat can be heard with a stethoscope, (first maternal perception of fetal movement) occurs
- Week 21-24: Fetal skin becomes less transparent as fat accumulates beneath it, fetal lungs begin to produce (substance that prevents alveoli collapse), fetal brain undergoes rapid development
- Week 25-28: Fetal eyelids reopen after being fused since week 10, fetal respiratory movements occur but do not exchange gases, fetus has a chance of survival if born prematurely ()
- Week 29-32: Fetal bones are fully developed but still soft, fetal skin is wrinkled due to lack of subcutaneous fat, fetal testes descend into the scrotum in males ()
- Week 33-36: Fetal skin becomes smooth as subcutaneous fat accumulates, fetal fingernails reach the tips of the fingers, hair begins to disappear
- Week 37-40: Fetus is considered full-term, fetal skin is pink and covered with , fetal head engages in the pelvic inlet in preparation for birth ()
Pregnancy Monitoring and Health
- Trimesters divide pregnancy into three approximately 13-week periods, each with distinct developmental milestones
- Ultrasound imaging is used to monitor fetal growth, detect abnormalities, and determine
- supplementation is crucial during early pregnancy to prevent defects
- Amniotic fluid surrounds and protects the fetus, allowing for movement and growth
Key Terms to Review (71)
Amniocentesis: Amniocentesis is a medical procedure used to obtain amniotic fluid from the amniotic sac surrounding a developing fetus. This procedure allows for the analysis of fetal chromosomes, providing critical information about genetic disorders and developmental issues during fetal development.
Amniotic Fluid: Amniotic fluid is the protective liquid that surrounds and cushions the developing fetus within the mother's uterus. It is a key component in fetal development and the changes that occur during pregnancy, labor, and birth.
Apical ectodermal ridge: The apical ectodermal ridge is a critical structure in embryonic development, consisting of a thickened area of ectoderm at the distal end of the limbs that plays a key role in directing limb growth. It signals underlying mesodermal tissue to grow and elongate, thereby shaping the eventual form of arms and legs.
Blastocyst: A blastocyst is an early stage embryo, approximately five to six days after fertilization, consisting of a sphere made up of an outer layer of cells (trophectoderm), a fluid-filled cavity (blastocoel), and a cluster of cells (inner cell mass) from which the embryo develops. It is the structure that implants into the uterine wall during the process of embryonic development.
Blastocyst: The blastocyst is a critical stage in early embryonic development, formed after the fertilized egg undergoes several cell divisions. It is a hollow, fluid-filled structure that contains an inner cell mass, which will eventually form the embryo, and an outer layer of cells called the trophoblast, which will form the placenta.
Cordocentesis: Cordocentesis, also known as percutaneous umbilical blood sampling (PUBS), is a prenatal diagnostic procedure in which a sample of fetal blood is obtained directly from the umbilical cord. This procedure is primarily used to assess fetal health, detect genetic disorders, and monitor fetal development during pregnancy.
Differentiation: Differentiation in the context of anatomy and physiology is the process by which unspecialized cells become specialized to perform specific functions. This transformation allows cells to assume roles necessary for the body's growth, repair, and maintenance.
Differentiation: Differentiation is the process by which unspecialized cells become specialized, acquiring distinct structures and functions to perform specific roles within an organism. This fundamental biological process is crucial for the development and growth of multicellular organisms, including the formation of various tissues and organs during embryonic and fetal development.
Ductus arteriosus: The ductus arteriosus is a temporary blood vessel in fetal circulation that connects the pulmonary artery to the descending aorta, allowing most of the blood to bypass the lungs which are not yet in use. After birth, it normally closes and becomes the ligamentum arteriosum within a few days.
Ductus Arteriosus: The ductus arteriosus is a temporary blood vessel present in the fetus that connects the pulmonary artery to the aorta, allowing blood to bypass the lungs before birth. It is a crucial structure in the development of the fetal cardiovascular system and the transition to postnatal circulation.
Ectoderm: The ectoderm is one of the three primary germ layers that form during embryonic development. It is the outermost layer of cells that gives rise to the skin, nervous system, and various other structures in the body.
Embryogenesis: Embryogenesis is the process by which a fertilized egg develops into a fully formed embryo, involving a series of complex cellular and molecular events. This critical stage lays the foundation for all subsequent stages of development, including organ formation and growth. The intricacies of embryogenesis encompass cell division, differentiation, and patterning, which are essential for the proper establishment of body plans in various organisms.
Endoderm: The endoderm is one of the three primary germ layers in the early embryo that gives rise to the lining of the digestive and respiratory systems, among other internal structures. It forms during the process of gastrulation and differentiates to become various organ linings and glandular tissues.
Endoderm: The endoderm is one of the three primary germ layers that form during embryonic development. It is the innermost of the three germ layers and gives rise to the lining of the digestive tract, as well as other important internal organs and structures.
Fibroblast Growth Factor: Fibroblast growth factor (FGF) is a family of proteins that play a crucial role in various cellular processes, including cell growth, differentiation, and tissue repair. FGFs are involved in a wide range of developmental and physiological functions, particularly during fetal development.
Folic Acid: Folic acid, also known as vitamin B9, is a water-soluble vitamin that is essential for proper fetal development. It plays a crucial role in the prevention of neural tube defects and supports various physiological processes during pregnancy.
Foramen Ovale: The foramen ovale is an opening in the fetal heart that allows blood to bypass the lungs and flow directly from the right atrium to the left atrium, bypassing the pulmonary circulation. It is a critical feature of fetal cardiovascular development and the transition to postnatal circulation.
Foramen ovale of the middle cranial fossa: The foramen ovale is an oval-shaped opening in the sphenoid bone of the middle cranial fossa through which the mandibular branch of the trigeminal nerve (cranial nerve V3) passes. It serves as a crucial pathway for both nerves and blood vessels connecting the cranium to other parts of the head.
Gastrulation: Gastrulation is a crucial phase in embryonic development where a single-layered blastula reorganizes into a multilayered structure called the gastrula, consisting of three primary layers: ectoderm, mesoderm, and endoderm. This process establishes the foundational layers that will differentiate into various tissues and organs of the body.
Gastrulation: Gastrulation is a crucial stage in embryonic development where the single-layered blastula transforms into a three-layered embryo known as the gastrula. This process establishes the basic body plan and lays the foundation for the formation of the major organ systems.
Gestational Age: Gestational age refers to the length of pregnancy, calculated from the first day of the last menstrual period to the current date. It is a critical measure in fetal development, as it provides insight into the stage of growth and maturation of the fetus.
Gonadal and Placental Hormones: Gonadal hormones are steroid hormones produced by the gonads (ovaries in females and testes in males) that regulate reproductive functions and secondary sexual characteristics. Placental hormones are produced by the placenta during pregnancy, supporting fetal development and modifying maternal physiology to sustain the pregnancy.
Human Chorionic Gonadotropin: Human chorionic gonadotropin (hCG) is a hormone produced during pregnancy by cells formed in the placenta, which is the organ that develops in the uterus during pregnancy to feed the egg after it has been fertilized and becomes attached to the uterine wall. This hormone is essential for the development and function of the corpus luteum, a temporary endocrine structure in the ovary that produces progesterone to maintain the early stages of pregnancy.
Implantation: Implantation is the process by which a fertilized egg (embryo) attaches itself to the wall of the uterus to begin pregnancy. This critical step occurs about one week after fertilization and marks the start of the embryo's growth within the uterine environment.
Implantation: Implantation is the process by which a fertilized egg (zygote) attaches to the wall of the uterus, marking the beginning of pregnancy. This critical event occurs about 6-12 days after fertilization and is a key step in the embryonic development process.
Inner Cell Mass: The inner cell mass is a group of cells within the blastocyst, the early-stage embryo formed after fertilization. It is the group of cells that will eventually give rise to the embryo proper and the fetus during fetal development.
Lanugo: Lanugo is the fine, soft hair that covers the body and limbs of a fetus or newborn baby. It is typically shed before birth, around the seventh or eighth month of pregnancy, but can also be seen on babies born prematurely.
Lanugo: Lanugo is a fine, downy hair that covers the body of a fetus during development. It typically appears around 20 weeks of gestation and helps regulate the fetus's body temperature and protect the skin.
Lightening: Lightening, also known as fetal lightening, refers to the descent of the fetus into the pelvic cavity that typically occurs a few weeks before the onset of labor. This process marks an important milestone in fetal development and the progression towards childbirth.
Meconium: Meconium is the earliest stool of a mammalian infant, composed of materials ingested during the time the infant spends in the uterus. It is typically thick, sticky, and greenish-black in color, and is usually passed within the first few days after birth.
Mesoderm: The mesoderm is one of the three primary germ layers in the early embryo that gives rise to various tissues and organs, including muscles, bones, and the circulatory system. It forms during the process of gastrulation and lies between the ectoderm and endoderm.
Mesoderm: The mesoderm is one of the three primary germ layers that form during embryonic development. It is the middle layer that gives rise to a variety of tissues and organs, including the skeletal, muscular, circulatory, and urogenital systems.
Morula: A morula is an early stage in embryonic development that consists of a solid ball of cells resulting from the division of a fertilized ovum. It forms before the blastocyst stage and after the zygote stage.
Morula: The morula is a solid ball of cells formed during the early stages of embryonic development, shortly after fertilization. It is a crucial step in the process of embryogenesis, marking the transition from the zygote to the blastocyst stage.
Müllerian Ducts: The Müllerian ducts are paired embryonic structures that develop into the female reproductive tract, including the uterus, fallopian tubes, and the upper portion of the vagina. They are a crucial component in the development of the male and female reproductive systems.
Neural tube: The neural tube is an embryonic structure that develops into the central nervous system, including the brain and spinal cord. It forms from a flat sheet of cells that rolls into a tube, which then differentiates into various structures of the nervous system.
Neural Tube: The neural tube is a structure that forms early in embryonic development and gives rise to the central nervous system, including the brain and spinal cord. It is a critical component in the embryologic, embryonic, and fetal development of the human body.
Neurulation: Neurulation is the process during embryonic development in which the neural plate forms the neural tube, which later develops into the central nervous system comprising the brain and spinal cord. This critical phase occurs after gastrulation and involves the bending and closure of the neural plate along the embryo's dorsal surface.
Neurulation: Neurulation is the embryonic process by which the neural plate folds and closes to form the neural tube, which eventually develops into the central nervous system, including the brain and spinal cord. This critical stage of development is a key component of the embryonic and fetal development processes.
Notochord: The notochord is a flexible rod made out of a material similar to cartilage that provides support in the embryonic stages of all chordates, including humans. It plays a crucial role in the development of the spinal column and the central nervous system.
Notochord: The notochord is a flexible, rod-like structure that forms the main skeletal support of the body in the earliest stages of embryonic development. It is a key feature in the development of the axial skeleton and plays a crucial role in fetal development.
Organogenesis: Organogenesis is the phase during embryonic development where various organs and structures within the embryo begin to form and differentiate. It follows gastrulation and involves the interaction of various cells to generate the distinct functional organs of the organism.
Organogenesis: Organogenesis is the process by which the various organs of the body develop and form during embryonic and fetal development. It is a critical stage in the overall process of embryonic development, following the initial formation of the three germ layers (ectoderm, mesoderm, and endoderm) and the subsequent differentiation of those layers into specialized cells and tissues.
Placenta: The placenta is a temporary organ that develops during pregnancy, connecting the developing fetus to the uterine wall of the mother. It serves as a vital interface for the exchange of nutrients, waste, and gases between the maternal and fetal circulatory systems, enabling the growth and development of the fetus.
Primitive streak: The primitive streak is a structure that forms on the surface of the early embryonic disk, marking the beginning of gastrulation, the process where the three primary germ layers develop. It serves as an organizer for cell movement and differentiation during early embryonic development.
Primitive Streak: The primitive streak is a transient structure that appears early in the development of an embryo and marks the beginning of gastrulation, the process by which the three primary germ layers (ectoderm, mesoderm, and endoderm) are formed. It is a critical feature in the establishment of the body plan and the formation of the major organ systems.
Progesterone: Progesterone is a steroid hormone released by the corpus luteum in the ovary, placenta, and adrenal glands that plays a key role in the female menstrual cycle and maintaining the early stages of pregnancy. It prepares the body for conception, regulates the menstrual cycle, and supports fetal development.
Progesterone: Progesterone is a steroid hormone primarily produced by the ovaries, adrenal glands, and, during pregnancy, the placenta. It plays a crucial role in the regulation of the female reproductive system, pregnancy, and other physiological processes.
Pulmonary surfactant: Pulmonary surfactant is a complex mixture of lipids and proteins produced by cells in the lungs, which reduces surface tension within the alveoli to prevent their collapse during exhalation. This substance is crucial for efficient gas exchange and lung stability.
Quickening: Quickening is the first moment during fetal development when a pregnant woman begins to feel or perceive fetal movements in the uterus, typically occurring between 16 and 25 weeks of pregnancy. It is an important milestone in prenatal care, indicating active fetal health.
Quickening: Quickening is the term used to describe the first noticeable movements of a fetus, typically felt by the pregnant person around the 18th to 25th week of pregnancy. This phenomenon marks an important milestone in fetal development, as it signifies the growing strength and activity of the fetus, reflecting its overall health and well-being. Quickening can be an emotional and bonding experience for expectant parents, highlighting the transition from pregnancy as a physical process to one that involves more personal connection with the developing baby.
Shunt: In the context of fetal development, a shunt is a passage or bypass that allows blood to move from one part of the circulation to another, circumventing certain areas. This mechanism is crucial in the fetal circulatory system for directing blood away from undeveloped organs like the lungs and liver.
Somites: Somites are blocks of mesodermal cells located on either side of the neural tube in a developing embryo. These cells differentiate into structures such as vertebrae, ribs, and skeletal muscles.
Somites: Somites are segmented blocks of mesoderm that form along the embryonic body axis, giving rise to the axial skeleton, muscles, and other structures. They are a crucial component in the embryonic development of the body's main systems.
Sonic Hedgehog: Sonic hedgehog (Shh) is a signaling protein that plays a crucial role in embryonic development, particularly in the patterning and growth of various body structures, including the appendicular skeleton and fetal development.
SRY Gene: The SRY (sex-determining region Y) gene is a critical genetic factor that plays a pivotal role in male sexual development and the determination of biological sex. This gene is located on the Y chromosome and is responsible for triggering the cascade of events that lead to the formation of male reproductive organs and characteristics during embryonic development.
Surfactant: Surfactant is a complex mixture of lipids and proteins produced by type II alveolar cells in the lungs. It plays a critical role in the respiratory system by reducing surface tension within the alveoli, facilitating gas exchange, and preventing alveolar collapse during exhalation.
Syncytiotrophoblast: The syncytiotrophoblast is a layer of multinucleated cell tissue that forms part of the placenta, arising from the outer layer of the blastocyst and playing a crucial role in nutrient exchange between the embryo and the mother. It embeds the developing embryo into the uterine wall and facilitates the formation of embryonic part of the placenta.
Teratogens: Teratogens are environmental factors that can disrupt the normal development of the fetus, leading to congenital abnormalities or birth defects. These substances can interfere with the growth and differentiation of the developing embryo or fetus, causing structural, functional, or metabolic changes.
Testicular Descent: Testicular descent refers to the process by which the testes, the male reproductive glands, migrate from their initial position in the abdomen to their final location in the scrotum during fetal development. This journey is a crucial step in the proper formation and function of the male reproductive system.
Trimester: A trimester refers to one of the three distinct segments of pregnancy, each lasting about three months, which is crucial for monitoring fetal development and maternal health. Each trimester is characterized by specific developmental milestones for the fetus and unique physiological changes for the mother. Understanding trimesters helps to frame the timeline of prenatal care, labor, and birth.
Trophoblast: The trophoblast is the outer layer of cells that forms the placenta in a developing embryo. It is responsible for implanting the embryo into the uterine wall and facilitating the exchange of nutrients, gases, and waste between the mother and the developing fetus.
Ultrasound: Ultrasound is a medical imaging technique that uses high-frequency sound waves to create images of structures within the body. It is particularly valuable for visualizing soft tissues and organs, making it essential for examining conditions in various medical fields and monitoring fetal development during pregnancy.
Umbilical vein: The umbilical vein is a blood vessel present during fetal development that carries oxygen-rich blood from the placenta to the fetus. It plays a crucial role in providing the fetus with oxygen and nutrients necessary for growth and development.
Umbilical Vein: The umbilical vein is a blood vessel that carries oxygen-rich blood from the placenta to the developing fetus during pregnancy. It plays a crucial role in the fetal circulation and development of the blood vessels.
Vernix caseosa: Vernix caseosa is a white, creamy, naturally occurring biofilm covering the skin of the fetus during the last trimester of pregnancy and can be present at birth. It serves as a protective barrier against infections and helps in maintaining the moisture of the fetus's skin.
Vernix Caseosa: Vernix caseosa is a waxy, cheese-like substance that covers and protects the skin of a fetus during its development in the uterus. It is an important feature of fetal development and the adjustments an infant undergoes at birth and during the postnatal stages.
Viability: Viability refers to the ability of a fetus to survive and thrive outside the mother's womb. It is a critical milestone in fetal development that signifies the fetus has reached a stage where it can potentially live independently with or without medical assistance.
Wolffian Ducts: Wolffian ducts, also known as mesonephric ducts, are a pair of embryonic structures that play a crucial role in the development of the male reproductive system. These ducts are essential for the formation of various male reproductive organs and are closely linked to the topics of the development of the male and female reproductive systems, as well as fetal development.
Zygote: A zygote is the initial cell formed when two gamete cells (sperm and egg) are joined through the process of fertilization. It contains all the genetic information necessary to form a new individual.
Zygote: A zygote is the initial cell formed when a sperm cell successfully fertilizes an egg cell. It is the earliest stage of embryonic development and marks the beginning of a new human life.