From Conception to Birth: A Week-by-Week Guide to Fetal Development

If you've ever wondered how a microscopic cluster of cells transforms into a crying, kicking newborn, you're in the right place. This week-by-week guide breaks down fetal development from conception to birth, using everyday analogies to make the process clear. We'll cover what's happening inside the womb, what you might feel, and what prenatal care looks like at each stage. No medical degree required—just curiosity about one of nature's most incredible journeys.

Weeks 1–4: The Starting Line — Fertilization and Implantation

Pregnancy is counted from the first day of your last menstrual period, which means you're not technically pregnant during the first two weeks. But by week 3, ovulation happens, and a sperm meets an egg in the fallopian tube. That single fertilized cell—now called a zygote—contains all the genetic material needed to build a person.

From Zygote to Blastocyst

Over the next few days, the zygote divides as it travels toward the uterus. By day 5 or 6, it becomes a blastocyst—a hollow ball of cells with an inner cluster that will become the embryo. Think of it like a seed: the outer shell (trophoblast) will burrow into the uterine lining, while the inner cells start to specialize.

Implantation occurs around week 4, when the blastocyst attaches to the endometrium. Some women notice light spotting called implantation bleeding. This is also when the placenta begins to form, creating the lifeline between mother and baby. At this stage, the embryo is barely visible—about the size of a poppy seed. Yet the foundation for the brain, spinal cord, and heart is already being laid down.

What can you do? Start taking a prenatal vitamin with folic acid if you haven't already. Avoid alcohol, smoking, and certain medications. Most women don't even know they're pregnant yet, so if you're trying to conceive, act as if you are.

Weeks 5–8: The Heart Starts Beating — Organ Formation Begins

By week 5, the embryo is about the size of an orange seed. The neural tube—which will become the brain and spinal cord—is closing. The heart begins to beat around day 22, though you won't hear it on a Doppler until later. Blood vessels are forming, and tiny limb buds appear where arms and legs will grow.

Major Organs Take Shape

Week 6 brings the development of the eyes, ears, and mouth. The heart is now beating at about 100–160 beats per minute—twice as fast as an adult's. The liver starts producing blood cells, and the digestive tract begins to form. By week 7, the embryo has a distinct head and trunk. The arms and legs lengthen, and hands and feet appear as paddle-like structures.

Week 8 is a milestone: the embryo is now about half an inch long—the size of a raspberry. All major organs have started to develop, though they're not fully functional. The heart has four chambers, and the brain is sending signals to the muscles. The embryo can even make small movements, though you won't feel them yet. This is a critical window for development; exposure to toxins or infections can cause birth defects. If you haven't had your first prenatal visit, schedule it now. Your provider will confirm the pregnancy, estimate your due date, and run blood tests.

Common symptoms: morning sickness, fatigue, breast tenderness. These are driven by surging hormones like hCG and progesterone. While unpleasant, they're often a sign that the pregnancy is progressing normally.

Weeks 9–12: From Embryo to Fetus — The First Trimester Ends

At week 9, the embryo officially becomes a fetus. This is a big label change, but the real story is rapid growth. The fetus is about the size of a green olive—1 inch long. The tailbone (remnant of the tail from earlier development) disappears. Fingers and toes are now distinct, and fingernails begin to form.

Movement and Reflexes

By week 10, the fetus can flex its arms and legs. The kidneys start producing urine, which is released into the amniotic fluid. The face is more human-like: the eyes are closed but the eyelids are forming, and the ears are in their final position. The intestines have moved from the umbilical cord into the abdomen. At week 11, the fetus is about 2 inches long—the size of a fig. The genitals are developing, but it's too early to see on ultrasound. The fetus swallows and kicks, though these movements are still too small to feel.

Week 12 marks the end of the first trimester. The fetus is now 2.5 inches long—about the size of a lime—and weighs half an ounce. The risk of miscarriage drops significantly after this point. The placenta is fully formed and taking over hormone production. Many women find their energy returns and nausea eases. Your provider may offer a nuchal translucency screening, an ultrasound that checks for chromosomal abnormalities like Down syndrome. This is also a good time to hear the heartbeat with a Doppler—a moment many parents remember vividly.

What to watch for: severe nausea or vomiting (hyperemesis gravidarum) can lead to dehydration. If you can't keep fluids down, call your doctor.

Weeks 13–16: The Second Trimester Begins — Baby Gets Busy

Welcome to the second trimester, often called the 'golden period.' The fetus is now about 3 inches long—the size of a peach. The body is growing faster than the head, so proportions start to look more baby-like. The skin is thin and translucent, and blood vessels are visible underneath.

Bones and Muscles Strengthen

By week 14, the fetus can make facial expressions—squinting, frowning, and even sucking its thumb. The liver and spleen are producing red blood cells. The thyroid gland is maturing, and the kidneys are making urine regularly. At week 15, the fetus is about 4 inches long—the size of an apple. The skeleton is hardening from cartilage to bone. The scalp hair pattern is forming, though it may not be visible yet. The fetus is very active, but you still might not feel it if this is your first pregnancy.

Week 16 is a common time for the 'quickening'—the first fluttering movements. Some women describe it as gas or popcorn popping. The fetus is now 4.5 inches long—the size of an avocado—and weighs about 3 ounces. The heart is pumping about 25 quarts of blood per day. The ears are fully formed, and the fetus can hear your voice and heartbeat. Talking or singing to your belly isn't silly—it's early bonding.

Your body: you may feel the uterus rising out of the pelvis, which can reduce urinary frequency. A belly bump may start to show. Many women have more energy and less nausea. This is a good time for the anatomy scan (usually at 18–20 weeks), but some providers schedule it as early as week 16.

Tip: start thinking about maternity clothes. Your pre-pregnancy jeans might not button by week 18.

Weeks 17–20: Halfway There — The Anatomy Scan

At week 17, the fetus is about 5 inches long—the size of a pomegranate. Fat is beginning to form under the skin, which will help regulate body temperature after birth. The umbilical cord is getting thicker and stronger, carrying nutrients and oxygen. The fetus can now hiccup, which you might feel as rhythmic little jumps.

Growing Senses and Coordination

By week 18, the fetus is about 5.5 inches long—the size of a bell pepper. The bones in the middle ear are hardening, so sound perception improves. The fetus may startle at loud noises. The eyes are still closed, but the retinas are sensitive to light. If you shine a flashlight on your belly, the fetus might turn away. At week 19, the fetus is about 6 inches long—the size of a mango. The skin is covered in a waxy coating called vernix caseosa, which protects it from the amniotic fluid. The brain is developing millions of motor neurons, allowing more coordinated movements.

Week 20 is the halfway mark. The fetus is about 6.5 inches long—the size of a banana—and weighs about 10 ounces. The anatomy scan (level 2 ultrasound) is typically done now. The technician checks every major organ, the spine, the limbs, and the placenta. You can also learn the sex if you want. The heart's four chambers are visible, and the blood flow patterns are assessed. This scan can detect most structural abnormalities, though not all. If everything looks good, it's a huge relief. If something is flagged, you'll be referred to a specialist for follow-up.

Your body: you may feel Braxton-Hicks contractions—practice contractions that are usually painless and irregular. They're normal, but if they become regular or painful, call your provider.

Weeks 21–24: Viability Milestone — The Lungs Begin to Prepare

At week 21, the fetus is about 7 inches long—the size of a carrot. The digestive system is practicing peristalsis—the muscle contractions that move food through the intestines. The fetus swallows amniotic fluid, which is absorbed by the digestive tract and excreted as urine. This practice helps the gut mature. The bone marrow is now producing blood cells, taking over from the liver and spleen.

Viability and Lung Development

By week 22, the fetus is about 7.5 inches long—the size of a spaghetti squash. The eyes are fully formed but still lack pigment in the iris. The eyebrows and eyelashes are visible. The lungs are beginning to produce surfactant, a substance that helps air sacs stay open after birth. This is a critical step for survival outside the womb. At week 23, the fetus is about 8 inches long—the size of a grapefruit. The skin is still thin and wrinkled, but fat is accumulating. The fetus can hear a range of sounds, including your heartbeat, digestion, and external noises like music or conversation.

Week 24 is a major milestone: viability. If born now, the fetus has a chance of survival with intensive medical care. The lungs are still immature, but surfactant production is ramping up. The fetus is about 8.5 inches long—the size of a cantaloupe—and weighs about 1.3 pounds. The brain is growing rapidly, and the sense of balance is developing. The fetus can respond to touch and sound. Some babies born at 24 weeks survive, but they face long NICU stays and potential complications. The goal is to keep cooking as long as possible.

Your body: you may experience leg cramps, heartburn, or swelling. Stay hydrated and elevate your feet when possible. If you have a history of preterm labor, your provider may start monitoring cervical length.

What to avoid: lying flat on your back for long periods—the weight of the uterus can compress blood vessels and reduce blood flow to the baby. Sleep on your side, preferably the left side.

Weeks 25–28: The Third Trimester — Rapid Weight Gain and Brain Growth

The third trimester starts at week 25. The fetus is about 9 inches long—the size of a cauliflower—and weighs about 1.5 pounds. The lungs are still developing, but the air sacs (alveoli) are forming. The nostrils begin to open. The fetus can blink its eyes, though they're still sealed shut. The brain is forming billions of neurons, and the sleep-wake cycle becomes more regular.

Brain and Sensory Development

By week 26, the fetus is about 9.5 inches long—the size of a head of lettuce. The eyes are opening and closing, and the retina is sensitive to light. The fetus can distinguish between light and dark. The taste buds are functional, and the fetus can taste sweet and bitter flavors in the amniotic fluid. Studies suggest that flavors from your diet—like garlic or vanilla—can be detected. At week 27, the fetus is about 10 inches long—the size of a rutabaga—and weighs about 2 pounds. The lungs are producing more surfactant, and the respiratory system is practicing breathing movements. The fetus can hiccup, which you may feel as rhythmic jerks.

Week 28 is the start of the third trimester. The fetus is about 10.5 inches long—the size of a large eggplant—and weighs about 2.5 pounds. The brain wave patterns are similar to those of a newborn, with periods of REM sleep. The eyes can move in their sockets. The bones are fully developed but still soft. The fetus may settle into a head-down position, though many babies flip later. Your provider may check your blood sugar for gestational diabetes and screen for anemia. This is also when you'll start counting kicks—a way to monitor fetal well-being.

Your body: you may feel more tired as carrying the extra weight takes a toll. Back pain, shortness of breath, and frequent urination are common. The baby's movements should be strong and regular. If you notice a decrease in movement, call your provider immediately.

Weeks 29–32: Preparing for Birth — Lungs Mature, Position Matters

At week 29, the fetus is about 11 inches long—the size of a butternut squash—and weighs about 3 pounds. The brain is growing rapidly, and the skull bones are still soft to allow passage through the birth canal. The lungs are capable of breathing air, but they're still maturing. The fetus can turn its head and respond to sound. The bone marrow is producing red blood cells.

Position and Practice

By week 30, the fetus is about 11.5 inches long—the size of a large cabbage—and weighs about 3.5 pounds. The eyes are fully open when awake. The fetus can track a light source if one is moved across your belly. The lungs are producing more surfactant, and the respiratory system is almost ready. The fetus may practice breathing by inhaling and exhaling amniotic fluid. At week 31, the fetus is about 12 inches long—the size of a coconut—and weighs about 4 pounds. The brain is developing billions of connections. The fetus can store iron, calcium, and phosphorus for use after birth. The skin is less wrinkled as fat accumulates underneath.

Week 32: the fetus is about 12.5 inches long—the size of a large jicama—and weighs about 4.5 pounds. The fingernails have grown to the tips of the fingers. The lungs are close to full maturity. Most babies are now in a head-down position. If your baby is breech (feet or bottom first), your provider may discuss options like external cephalic version (ECV) to turn the baby. The immune system is building antibodies from your blood. This is a good time to finalize your birth plan and pack your hospital bag.

Your body: you may experience more Braxton-Hicks contractions, shortness of breath as the uterus pushes up on the diaphragm, and swelling in the feet and ankles. Put your feet up when you can. If you have a history of preterm labor, your provider may monitor you more closely.

Weeks 33–36: The Final Stretch — Full Term Approaches

At week 33, the fetus is about 13 inches long—the size of a pineapple—and weighs about 5 pounds. The bones are hardening, but the skull remains flexible. The pupils can constrict and dilate in response to light. The fetus can recognize your voice and may calm down when you speak. The sleep-wake cycle is more established, and you may notice patterns of activity.

Weight Gain and Lung Surge

By week 34, the fetus is about 13.5 inches long—the size of a cantaloupe—and weighs about 5.5 pounds. The central nervous system is maturing, and the lungs are almost fully developed. The fetus can produce surfactant in sufficient amounts to breathe air. The immune system continues to build. At week 35, the fetus is about 14 inches long—the size of a honeydew melon—and weighs about 6 pounds. The kidneys are fully developed, and the liver can process waste. The fetus is getting crowded and may move less vigorously, but you should still feel regular movements. The skin is pink and smooth as fat fills out the body.

Week 36: the fetus is about 14.5 inches long—the size of a head of romaine lettuce—and weighs about 6.5 pounds. The lungs are considered mature. The baby is considered 'early term' at 37 weeks, but waiting until 39 weeks is ideal for brain and lung development. The baby may drop into the pelvis (lightening), which can ease your breathing but increase pressure on your bladder. Your provider may check your cervix for dilation and effacement during appointments. Group B strep screening is typically done this week.

Your body: you may feel more pelvic pressure, back pain, and fatigue. Contractions may become stronger and more regular. If you have any signs of preterm labor—regular contractions, fluid leakage, or bleeding—call your provider immediately.

Weeks 37–40: Ready or Not — Labor and Delivery

At week 37, the baby is considered early term. The average weight is about 6.5–7 pounds, and length is about 19–20 inches. The lungs, brain, and liver are fully mature. The baby is gaining about half a pound per week. The lanugo (fine hair covering the body) has mostly disappeared. The nails may be long enough to scratch the face after birth.

Signs of Labor

By week 38, the baby is full term. The average weight is about 7 pounds. The baby may have a full head of hair, or be bald—both are normal. The brain continues to develop rapidly, and the baby can respond to external stimuli. At week 39, the baby is about 20 inches long and weighs about 7.5 pounds. The lungs produce the most surfactant now. The baby is ready for life outside the womb. The immune system has received antibodies from you, which will protect against infections for the first few months.

Week 40 is the due date, but only about 5% of babies are born exactly on their due date. Most arrive between 39 and 41 weeks. Signs of labor include regular contractions, water breaking, and bloody show (mucus plug). If you haven't gone into labor by 41 weeks, your provider may discuss induction. The baby's position and your health will guide the decision.

Your body: you may feel a mix of excitement and anxiety. Contractions may start and stop. Rest as much as possible, stay hydrated, and keep your provider informed. Once labor begins, trust your body and your birth team. You've spent 40 weeks growing a human—you're ready.

Final note: every pregnancy is different. This guide covers typical development, but variations are normal. Always consult your healthcare provider for personalized advice and care.