Prenatal development unfolds in three distinct stages over approximately 40 weeks: the germinal stage (weeks 1–2), the embryonic stage (weeks 3–8), and the fetal stage (weeks 9–40). Each stage has a specific role, from the first cell divisions after fertilization to the final weeks of weight gain before birth. Here’s what happens during each one.
The Germinal Stage (Weeks 1–2)
Prenatal development begins when a sperm and egg unite in one of the fallopian tubes, forming a single-celled zygote. Within hours, the zygote starts dividing as it travels down the fallopian tube toward the uterus. By the time it arrives, it has become a solid ball of cells called a morula.
The morula then hollows out into a structure called a blastocyst, which contains an inner cluster of cells that will become the embryo and an outer layer that will become the placenta. Around the end of the first week, the blastocyst begins burrowing into the uterine lining, a process called implantation. Implantation is complete by roughly the end of week 2. At this point, the developing organism is smaller than the head of a pin, but the groundwork for the entire pregnancy has been laid.
The Embryonic Stage (Weeks 3–8)
The embryonic stage is when the body’s major organs and systems take shape. This process, called organogenesis, packs an extraordinary amount of development into just six weeks. By the end of this stage, the embryo measures roughly 20 millimeters (about the size of a kidney bean) yet already has the basic blueprint for nearly every organ system.
Week-by-Week Highlights
During week 3, a flat sheet of cells called the neural plate forms. This is the earliest precursor to the brain and spinal cord. Paired tubes that will become the heart also appear. By week 4, those tubes fuse and the heart begins to beat. The neural tube closes (forming the early brain and spinal cord), tiny limb buds emerge, and the liver starts developing. Neural tube closure is a critical milestone: defects like spina bifida occur if this process goes wrong before week 5.
Week 5 brings the beginning of the cerebral hemispheres, the earliest lung buds, and the start of the pituitary gland. In week 6, the heart begins dividing into separate chambers, facial features start to take shape, and nerves grow into the developing limbs. By weeks 7 and 8, bone formation begins in the limbs, the pancreas starts producing hormones, the stomach begins making digestive chemicals, and external genitalia start to differentiate.
Why This Stage Is So Vulnerable
The embryonic stage is the period of highest sensitivity to harmful exposures like alcohol, certain medications, infections, and environmental toxins. Because so many organs are forming simultaneously, a disruption during these weeks can affect multiple body systems at once. Exposure to harmful substances can impact development as early as two weeks after conception, but the risk is greatest during the first eight weeks.
The Fetal Stage (Weeks 9–40)
Starting around week 9, the developing organism is officially called a fetus. The major organs are already in place. From this point forward, the job is growth, maturation, and fine-tuning. The majority of the increase in both weight and length happens during this stage.
Sensory Development
The senses come online gradually. Teeth and taste buds start forming around week 9. By week 16, the ears are developed enough for the fetus to hear external voices, and even with closed eyes, the fetus can react to light by turning away from it. The brain region responsible for all five senses begins developing around week 20. By week 22, the fetus can hear its mother’s heartbeat, breathing, and stomach sounds. Around week 29, the fetus can see and hear most stimuli from the outside world.
Bone and Body Changes
Bones begin hardening around week 11, though at that point the skin is still translucent. This hardening process continues throughout pregnancy, with one important exception: the skull bones around the brain remain soft and flexible so the head can pass through the birth canal during delivery. These skull bones don’t fully fuse until well after birth.
Fat accumulation follows a clear timeline. The fetus starts adding its first thin layer of body fat around week 17. By week 23, fat storage accelerates rapidly. This fat serves several purposes after birth, including temperature regulation and energy reserves. Throughout the third trimester, the fetus gains weight quickly, filling out its skin and developing a plumper appearance.
Lung Maturation
The lungs are among the last organs to mature. Specialized cells in the lungs begin producing surfactant, a substance that keeps the air sacs from collapsing, at around 24 weeks of gestation. Surfactant production increases steadily from that point, but the lungs typically aren’t fully ready for independent breathing until closer to 36 weeks. This is one of the main reasons premature birth carries serious risks.
Fetal Movement
The fetus begins moving its limbs well before a pregnant person can feel it. Most women first perceive fetal movement, called quickening, between 16 and 20 weeks. Women who have been pregnant before often notice it closer to 16 weeks, while first-time mothers typically feel it around 20 weeks. Early movements feel like flutters or bubbles, becoming stronger and more recognizable as the fetus grows.
Viability Outside the Womb
The age at which a fetus could potentially survive outside the womb falls within the periviable period, generally defined as weeks 20 through 25. Survival rates vary dramatically depending on the week of delivery. Before 23 weeks, survival is only about 5 to 6 percent, and nearly all survivors experience significant health complications. At 23 weeks, survival ranges from 23 to 27 percent. At 24 weeks, it rises to 42 to 59 percent. By 25 weeks, survival reaches 67 to 76 percent.
These numbers reflect the reality that organ systems, particularly the lungs and brain, need time to mature. Each additional week in the womb during this critical window substantially improves outcomes. By the time a pregnancy reaches full term (39 to 40 weeks), all systems are typically prepared for life outside the uterus, and the fetus has completed the remarkable transformation from a single cell to a fully formed newborn.