A baby grows from a single fertilized cell into a fully formed newborn over roughly 40 weeks, progressing through dramatic stages of development. In the first eight weeks, nearly every major organ system takes shape. Over the remaining months, those organs mature, the baby gains weight, and its senses come online one by one. Here’s what happens at each stage.
How the Placenta Keeps a Baby Alive
Before diving into the weekly milestones, it helps to understand the organ that makes all of it possible. The placenta is a temporary organ that attaches to the wall of the uterus and connects to the baby through the umbilical cord. It acts as the baby’s lungs, kidneys, and digestive system all at once, delivering oxygen and nutrients from the mother’s blood while carrying away carbon dioxide and waste.
Maternal and fetal blood never actually mix. Instead, they flow extremely close together, separated by a membrane only 2 to 3 micrometers thick. Oxygen crosses that barrier by diffusing from higher concentration in the mother’s blood to lower concentration in the baby’s blood. The baby’s blood is especially good at grabbing oxygen because fetal hemoglobin has a higher affinity for it than adult hemoglobin does. A clever chemical exchange happens simultaneously: as the mother’s blood picks up carbon dioxide from the baby, it becomes slightly more acidic, which causes it to release even more oxygen to the fetus.
Weeks 1 Through 8: Organs Take Shape
The first eight weeks are called the embryonic period, and this is when the basic blueprint of the body is laid down. Growth is astonishingly fast. By week 5, the neural tube, which will become the brain and spinal cord, has formed. Cells that will become the heart begin clustering together around the same time, and by the end of week 5, that tiny tube of cardiac cells is already pulsing about 110 times per minute.
By week 6, small buds appear that will eventually become arms and legs. Facial features start to emerge, with early structures for the eyes, ears, and jaw taking shape. By the end of week 8, the embryo is officially called a fetus. It’s only about an inch long, but nearly every major organ has at least begun to form.
The Brain’s Explosive Growth
The brain deserves its own discussion because its development is staggering in scale. To reach the more than 100 billion neurons a newborn is born with, the brain must produce roughly 250,000 new nerve cells per minute, on average, throughout the entire pregnancy.
Between about 15 and 20 weeks after conception, neuron production in the outer brain accelerates dramatically. By the seventh month, the fetus is generating its own brain waves, detectable through the mother’s abdomen. This rapid brain growth continues through the third trimester and well after birth, which is one reason nutrition and a healthy environment matter so much during pregnancy.
Weeks 9 Through 12: Recognizably Human
During these weeks, the fetus grows quickly and starts to look like a tiny person. Fingers and toes separate, bones begin to harden, and the reproductive organs start developing. The fetus begins making small movements, though it’s far too early for the mother to feel them.
The heartbeat is now strong enough to be detected on a standard prenatal ultrasound. Research on early cardiac activity shows the heart may begin beating as early as 20 days after fertilization, though ultrasound imaging typically picks it up starting around the beginning of the sixth gestational week.
The Role of Amniotic Fluid
The fetus floats in amniotic fluid throughout pregnancy, and this liquid does far more than provide cushioning. It gives the baby room to move, which is essential for muscle and bone development. The baby breathes it in and swallows it, which helps the lungs and digestive system mature. It also regulates the baby’s body temperature, protects against infection through antibodies, and prevents the umbilical cord from getting compressed.
Amniotic fluid volume increases steadily throughout pregnancy, peaking at 34 to 36 weeks at just under 1 liter (about 4 cups). After that, levels slowly decrease until delivery.
Weeks 13 Through 27: Senses and Movement
The second trimester is when pregnancy starts to feel real for many parents, largely because of one milestone: quickening. This is the first time a mother feels the baby move, and it typically happens between 16 and 20 weeks. Women who have been pregnant before often notice it closer to 16 weeks, while first-time mothers may not feel anything until around 20 weeks. The baby has actually been moving for weeks before this, but it’s finally large enough for those movements to register.
Around 18 weeks, the baby’s ears begin standing out from the head and it may start to hear sounds. This is why many parents begin talking or playing music to the baby during the second trimester. By 23 weeks, ridges form on the palms and soles of the feet, creating the foundation for unique fingerprints and footprints. The baby also begins having rapid eye movements around this time.
By the end of the second trimester, the fetus weighs roughly 2 pounds. It can hiccup, respond to light and sound, and cycle between periods of sleep and wakefulness.
Weeks 28 Through 40: Final Maturation
The third trimester is all about growth and refinement. The baby gains significant weight, building up fat stores that will help regulate body temperature after birth. The brain, which has been growing rapidly since mid-pregnancy, continues forming complex connections.
One of the most critical developments is lung maturation. The lungs begin producing surfactant, a substance that keeps the tiny air sacs from collapsing, at around 24 weeks. But adequate amounts for independent breathing aren’t produced until about 32 weeks. This is a major reason why babies born very early often need breathing support.
During the final weeks, the baby typically settles into a head-down position in preparation for delivery. The bones of the skull remain slightly flexible and separated, allowing the head to fit through the birth canal. By 40 weeks, the average baby is about 19 to 21 inches long and weighs between 6 and 9 pounds.
What Happens if a Baby Is Born Early
Understanding fetal development helps explain why timing matters so much for premature babies. Survival rates vary dramatically depending on gestational age. For babies born before 24 weeks, only about 24% survive. At 24 completed weeks, that number jumps to roughly 72%. By 25 weeks, survival ranges from 59% to 86% depending on the hospital and the care available.
The biggest challenges for premature infants involve the organs that mature latest: the lungs, brain, and digestive system. A baby born at 26 weeks has a much better chance than one born at 23 weeks, precisely because those extra few weeks allow surfactant production to ramp up and brain development to progress. Each additional day in the womb during the critical periviable period (roughly 22 to 26 weeks) meaningfully improves outcomes.