Before birth, a baby develops through a carefully coordinated sequence of changes that begins with a single fertilized egg and ends with a newborn whose organs and body systems are prepared to function outside the uterus. The process takes about 40 weeks when pregnancy is measured from the first day of the last menstrual period, although fertilization usually occurs about two weeks into that count.
Development is not simply a matter of getting bigger. Cells divide, specialize, move into precise locations, and communicate with one another to form the brain, heart, lungs, limbs, organs, and other tissues. At the same time, the placenta and umbilical cord provide the developing baby with oxygen and nutrients and help remove waste.
The three stages of prenatal development
Prenatal development is often described in three overlapping periods: the germinal period, the embryonic period, and the fetal period.
The germinal period begins at fertilization and lasts roughly two weeks. The fertilized egg, called a zygote, repeatedly divides as it travels toward the uterus. It develops into a structure called a blastocyst, which implants in the lining of the uterus. Implantation establishes the early connection between the pregnancy and the mother’s blood supply.
The embryonic period extends from about the third through the eighth week after fertilization. This is a particularly important period for forming the basic structures of the body. The developing organism is called an embryo.
The fetal period begins around the ninth week after fertilization and continues until birth. The developing baby is then called a fetus. During this period, organs and body systems that began forming during the embryonic period grow, mature, and become increasingly coordinated.
Pregnancy dating can make these terms confusing because doctors usually count pregnancy from the last menstrual period rather than from fertilization. As a result, the commonly used “40 weeks of pregnancy” corresponds to about 38 weeks after fertilization in a typical pregnancy.
What happens during the first few weeks?
Fertilization occurs when a sperm cell and an egg cell combine, producing a cell with genetic material from both parents. That cell begins dividing rapidly.
As the early cell population develops, different groups of cells take on different roles. After implantation, cells form structures that will eventually become the embryo as well as tissues that support the pregnancy.
One of the most important early events is the formation of the three primary germ layers: the ectoderm, mesoderm, and endoderm. These layers give rise to different parts of the body. The ectoderm contributes to the nervous system and skin, the mesoderm contributes to structures such as muscle, bone, blood, and connective tissue, and the endoderm contributes to the lining of the digestive and respiratory systems and several internal organs.
This early organization provides the foundation for later development.
How do the brain and nervous system develop?
The nervous system begins taking shape very early. A structure called the neural tube forms from specialized tissue and eventually develops into the brain and spinal cord.
The brain does not appear fully formed at this stage. Instead, its regions develop progressively, and nerve cells are produced, migrate to appropriate locations, and establish connections. Throughout pregnancy, the brain undergoes extensive growth and organization.
Later in pregnancy, the nervous system becomes increasingly capable of coordinating movement and responding to sensory information. Movements that may initially be small or difficult to detect become more organized as the brain, spinal cord, muscles, and sensory systems mature.
Brain development continues after birth. Many neural connections are refined during infancy, childhood, and adolescence, so birth marks an important transition rather than the completion of brain development.
When does the baby’s heart develop?
The heart is one of the earliest organs to begin functioning. Its initial form develops during the first several weeks after fertilization, and the developing heart begins beating early in the embryonic period.
The early heart is much simpler than the heart of a newborn. As development proceeds, it bends and changes shape, and internal partitions and valves develop. These changes create the basic four-chambered organization of the mature heart.
The fetal circulation is also different from circulation after birth. Before birth, the lungs are not being used for oxygen exchange, so blood follows pathways that direct much of the circulation away from the lungs. The placenta performs the oxygen and nutrient exchange that the lungs will handle after birth.
At delivery, the newborn takes its first breaths. Blood flow through the lungs increases, and the fetal circulation pathways gradually close or change as the circulatory system adapts to life outside the uterus.
How do the arms, legs, and other body parts form?
The limbs begin as small buds during the embryonic period. They lengthen and develop recognizable regions, while cells become organized into bones, muscles, connective tissues, blood vessels, and nerves.
Hands and feet initially have a paddle-like appearance. Individual digits then become distinct as tissue between them is removed through a normal developmental process called programmed cell death, or apoptosis.
Facial development is similarly gradual. Structures that will form the forehead, nose, mouth, jaw, eyes, and ears develop and change position and shape as the embryo grows. Many of the features recognizable on an ultrasound or at birth are the result of several tissues growing and joining in carefully coordinated ways.
What happens to the organs during the fetal period?
By the beginning of the fetal period, the major organ systems have been established in their basic forms. Development then shifts increasingly toward growth, refinement, and functional maturation.
The digestive system develops its organs and begins coordinated activity. The fetus swallows amniotic fluid, and the digestive tract continues maturing throughout pregnancy.
The kidneys begin producing urine during fetal development. Fetal urine contributes substantially to the amniotic fluid later in pregnancy. This is not the same as an infant’s postnatal urinary function because the placenta is still handling many waste-removal tasks.
The lungs develop progressively but are not simply inactive until birth. Airways branch repeatedly, lung tissue becomes more specialized, and cells that will participate in gas exchange mature. Later in pregnancy, the lungs produce increasing amounts of surfactant, a substance that helps keep the tiny air sacs from collapsing after birth. Lung maturation is one reason that a baby born substantially before its due date may need specialized respiratory support.
The liver, bone marrow, and other tissues also take on increasingly important roles in producing blood cells as development progresses. Blood formation changes location during pregnancy as different organs mature.
How does a baby get oxygen and nutrients before birth?
The developing baby does not breathe air in the womb. Instead, oxygen crosses from the mother’s blood into the baby’s blood through the placenta. Carbon dioxide and other substances move in the opposite direction for removal through the mother’s body.
The umbilical cord connects the fetus to the placenta. It normally contains one vein and two arteries. The umbilical vein carries oxygen-rich blood and nutrients toward the fetus, while the umbilical arteries carry blood from the fetus toward the placenta.
The placenta is more than a simple filter. It is a specialized organ that allows exchange between maternal and fetal circulations while keeping those blood supplies largely separate. It also produces hormones that help maintain pregnancy and support fetal development.
Some substances can cross the placenta more readily than others. Oxygen, nutrients, and certain medications can cross, as can some infectious agents and harmful substances. This is one reason conditions during pregnancy can influence fetal development.
What protects the developing baby?
The fetus develops inside the amniotic sac, which contains amniotic fluid. The fluid cushions the fetus, allows movement, and helps provide an environment in which the lungs and musculoskeletal system can develop.
The uterus also provides a protective muscular environment, while the placenta performs important roles in supporting the pregnancy.
Protection is not absolute. The developing fetus is sensitive to certain environmental exposures, infections, medications, nutritional deficiencies, and other conditions, particularly during periods when specific organs are forming. The effects depend on factors such as the substance or condition involved, the amount or severity, and the stage of development.
How does fetal development change from month to month?
During the first trimester, the most fundamental structures develop. The embryo’s nervous system, heart, limbs, face, and major organs begin forming. By the end of this period, the fetus has a distinctly human body plan, although it is still very small and its organs are immature.
During the second trimester, the fetus grows rapidly and develops increasingly coordinated movement. Bones become more developed, facial features become more distinct, and sensory systems continue to mature. Many pregnant people begin to feel fetal movement during this part of pregnancy.
During the third trimester, growth and organ maturation become especially important. The brain continues developing rapidly, the lungs mature, body fat accumulates, and the fetus gains substantial weight. Movement remains important, although its character can change as space becomes more limited.
These trimester boundaries are useful for describing pregnancy, but development does not occur in three sharply separated stages. Growth and maturation continue continuously throughout pregnancy.
How does the fetus know when to move and respond?
Fetal movement is produced by the developing nervous and muscular systems working together. Early movements are relatively simple, but as neural pathways mature, movement becomes more coordinated.
The fetus can also respond to sensory stimulation before birth. Hearing develops progressively, and later in pregnancy the fetus can respond to sounds. Vision also develops, although the womb is a dim environment and visual experience before birth is very different from seeing after delivery.
These abilities reflect an important principle of prenatal development: organs do not merely appear in finished form. They gradually acquire the ability to function as their underlying tissues and control systems mature.
What happens during the final weeks before birth?
Near the end of pregnancy, the fetus continues gaining weight and accumulating body fat while the organs become better prepared for independent function.
The brain and nervous system continue to mature. The lungs undergo further structural and biochemical development. The digestive, immune, circulatory, and temperature-regulation systems also prepare for the transition from the protected uterine environment to life outside it.
The fetus typically changes position late in pregnancy, often settling head-down in preparation for birth. At the same time, the placenta continues supplying oxygen and nutrients until delivery.
Birth requires a major physiological transition. Once the umbilical cord is no longer supplying oxygenated blood from the placenta, the newborn must obtain oxygen through its lungs, circulate blood through those lungs, regulate its own temperature, and begin obtaining and processing nutrition independently.
Why timing matters so much in prenatal development
Development follows a general sequence, but it is not identical in every pregnancy. Different organs have different periods of rapid formation and maturation, and development continues throughout pregnancy.
The earliest weeks are especially important for the formation of major body structures. Later, the emphasis increasingly shifts toward growth and functional maturation. This distinction helps explain why the same exposure can have different effects depending on when it occurs.
It also explains why premature birth can create different medical challenges depending on how early it happens. A baby born before full term may have immature lungs, brain function, feeding ability, temperature regulation, or other systems. The earlier the birth, the more developmental work remains to be completed outside the uterus.
Prenatal development is therefore best understood not as a checklist of milestones but as a continuous process in which cells, organs, and body systems develop together. From the first divisions of the fertilized egg to the newborn’s first breath, each stage builds on changes that came before while preparing the body for the next environment it will encounter.
