Fetus Development Timeline | Week By Week Baby Growth

Knowing what your baby does at each point in pregnancy turns lines on a chart into a real story. A clear timeline also helps you decode ultrasound reports, pregnancy apps, and phrases your care team uses.

This guide walks through pregnancy by weeks and trimesters, then shows how to read growth charts without worry. It cannot replace personal medical advice, yet it can help you spot which questions to raise at prenatal visits.

Pregnancy Weeks And How Timing Is Counted

Health professionals usually count pregnancy from the first day of the last menstrual period, often shortened to LMP. That date is about two weeks before conception for many cycles, but it still counts as week one.

Using this method, a full-term pregnancy lasts about forty weeks. Many medical groups, including the American College of Obstetricians and Gynecologists, group this time into a first trimester from week one through week thirteen, a second trimester from week fourteen through week twenty-seven, and a third trimester from week twenty-eight until birth.

The Eunice Kennedy Shriver National Institute of Child Health and Human Development describes pregnancy as a stretch of up to forty-one weeks with the baby growing inside the uterus. This span includes cell division, organ formation, rapid growth, and the final shift toward life outside the womb.

Due dates are estimates, not promises. Births between thirty-seven and forty-one completed weeks fall in the term range. Earlier births are called preterm, and births after forty-two weeks are called postterm.

Fetus Development Timeline By Trimester

Trimesters give a wide view of fetal development that pairs well with detailed weekly charts. Each trimester has its own feel for both the pregnant person and the baby.

First Trimester: Foundations And Organ Formation

The first trimester runs from week one through week thirteen after the LMP. During the first two weeks, there is no embryo yet; the body prepares for ovulation. Around week three, sperm and egg meet, and a fertilized egg travels down the fallopian tube toward the uterus.

By week four, the tiny ball of cells settles into the uterine lining, and a basic placenta begins to form. Around week five and six, a simple heart tube beats, and early brain and spinal cord structures appear. Limbs start as small buds along the sides of the embryo.

Weeks seven through ten bring rapid change. Fingers and toes separate, facial features appear, and the heart develops chambers. By week ten, the embryo has shifted into what doctors call the fetal stage. Many organs are present in early form, including kidneys, liver, and intestines.

By week twelve, external genitalia start to take shape, bone tissue begins to replace cartilage in some areas, and the baby can make tiny movements. The fetus remains only a few centimeters long yet already has a visible head, body, arms, and legs.

Second Trimester: Growth, Movement, And Senses

The second trimester, from week fourteen through week twenty-seven, is often a calmer stretch for many pregnant people, and it is also when many parents feel the baby move. Around week fourteen, red blood cells start forming in the bone marrow, and facial muscles practice small expressions.

By week sixteen, the eyes move toward the front of the head, the ears reach their final spot, and limb movements grow stronger. Between week eighteen and twenty, many parents feel the first kicks, first as flutters and later as clear jabs. Ultrasound exams during this stretch review the heart, brain, spine, and limbs in detail.

From week twenty-one through twenty-seven, the baby gains weight at a faster pace. Taste buds form, the digestive tract practices tiny swallows of amniotic fluid, and lungs develop branching airways. Fine hair called lanugo covers the skin, and a waxy coating called vernix protects the baby from constant contact with fluid.

Third Trimester: Finishing Touches Before Birth

The third trimester runs from week twenty-eight until birth. During this time, the baby’s brain and lungs mature, body fat increases, and movements grow stronger and more coordinated. Around week thirty-two, the baby often settles into a head-down position, though some stay breech for longer.

Between week thirty-four and thirty-six, the lungs build more air sacs, and the nervous system refines control of breathing and body temperature. Babies born during this late preterm window often do well with modern care yet can still face challenges with feeding or staying warm. Each additional week inside the uterus adds reserves.

By week thirty-seven through forty-one, many babies have a firm grasp, rhythmic breathing motions, and a layer of fat under the skin. Skull bones stay soft and slightly separated, which allows the head to mold during birth.

Gestational Weeks	Approximate Baby Size	Major Changes
4–5	Poppy seed to sesame seed	Implantation, early placenta, start of heart tube
6–7	Lentil to blueberry	Heartbeat on ultrasound, early brain and spine structures
8–9	Grape to olive	Limb buds lengthen, fingers and toes separate
10–12	Plum to lime	Organs present in early form, first small movements
13–16	Lemon to avocado	Bones harden, facial features sharpen, fingerprints start
17–24	Turnip to ear of corn	Stronger kicks, hearing improves, lungs build airways
25–32	Large eggplant to squash	Eyes open, more fat under skin, brain growth surges
33–41	Pineapple to watermelon	Lungs mature, head often moves down, reflexes firm

Fetal Development Timeline Week By Week Overview

A week-by-week tour gives a clearer picture of milestones described in many clinic guides. Each baby grows at a slightly different pace, yet the order of events stays much the same.

Weeks three and four center on implantation and the earliest layers of cells that will form organs, placenta, and membranes. The neural tube, which later becomes the brain and spinal cord, starts to close around week four. This is one reason folic acid before and during early pregnancy matters so much for many pregnant people.

By week six, the heart beats in a steady rhythm, and the head is large compared with the rest of the body because brain tissue grows quickly. Around week eight, eyelid folds appear, and limb buds lengthen. A Mayo Clinic first trimester overview notes that by the end of the first trimester, all major organ systems exist in early form, while fine tuning continues for many months.

Between week thirteen and twenty, bone hardening progresses, and the fetus can swallow small amounts of amniotic fluid. Around week eighteen to twenty, ultrasound exams often map out the brain, face, spine, heart, abdomen, and limbs in detail. The American College of Obstetricians and Gynecologists fetal growth FAQ describes this visit as a time to check growth, placenta location, and organ structure.

Weeks twenty-one through twenty-four mark a sensitive window for lung and brain development. Tiny air sacs form inside the lungs, and nerve networks in the brain build more connections. The MotherToBaby review of critical periods describes mid-pregnancy as a time when some organs remain vulnerable to alcohol, tobacco, some infections, and certain medicines.

By week twenty-eight, the eyelids can open and close, and the baby can sense light changes through the uterus. Around week thirty-two, breathing motions grow more rhythmic, and body fat builds under the skin. Many parents notice more pronounced patterns of sleep and wakefulness during this stage.

From week thirty-four through week thirty-six, the baby rehearses sucking and swallowing, which prepares for feeding after birth. The nervous system fine tunes control of temperature and breathing. By week thirty-seven and beyond, the baby is considered early term or full term, and growth shifts more toward weight and fat storage.

Factors That Shape Individual Growth Patterns

Timelines show the usual pattern, yet real babies do not all match the charts. Growth curves from large research projects, such as the NICHD Fetal Growth Studies, show wide natural variation in size at each week among pregnancies without major complications.

Several elements influence how a baby grows inside the uterus. Genetics, placenta health, the amount of amniotic fluid, and the pregnant person’s health habits all contribute. Conditions such as high blood pressure, diabetes, or infections can slow or speed growth, which is why regular prenatal visits and recommended screening tests matter.

The NICHD pregnancy fact sheet and public health agencies such as the Centers for Disease Control and Prevention encourage steps that lower the chances of growth problems and some birth defects. These steps include taking folic acid before conception when possible, avoiding alcohol and tobacco, staying active as advised by a clinician, and keeping chronic conditions under control.

Even with careful habits and solid medical care, some pregnancies still run into growth concerns. In those cases, health professionals may watch growth with repeated ultrasound measurements, track amniotic fluid, and follow the baby’s heart rate pattern with specialized testing.

Why Ultrasound Measurements Can Differ From Dates

Many parents worry when ultrasound estimates of fetal size do not match the due date exactly. In practice, a small difference is common. Early pregnancy ultrasounds that measure crown-rump length give a more precise estimate of gestational age than measurements taken later.

As pregnancy advances, genetics and other factors influence size more strongly, so estimates carry a wider margin of error. Providers often look for trends, not a single number. A baby who measures a bit smaller than average yet stays on a similar curve from scan to scan may be healthy, while a baby whose growth slows sharply may prompt closer monitoring.

How To Use A Fetal Timeline Without Stress

A detailed chart of weeks and milestones can be reassuring, yet it can also stir anxiety if your experience does not match every line. Growth charts and a fetus development timeline work best as tools for conversation, not strict scorecards.

Many parents like to read week-by-week updates from clinic websites, pregnancy books, or reputable apps. When you compare those notes with your own pregnancy, watch for patterns instead of focusing on exact sizes. Knowing when limbs form, when senses wake up, and when lungs mature can make each kick feel more meaningful.

During prenatal visits, you can keep a mental checklist of topics that line up with the current stage. Early in pregnancy, you might ask about folic acid, nausea, and screening tests. During the middle stretch, you might ask about the anatomy scan, signs of preterm labor, and how to track movement. In the last weeks, it often helps to ask about birth planning, feeding plans, and newborn care.

Weeks	Visit Focus	Helpful Questions
4–8	Confirm pregnancy, review history, start prenatal vitamins	Which supplements do you recommend, and how much folic acid do they contain?
9–13	Early ultrasound, review screening choices	Which screening tests fit my situation, and what can they show or miss?
14–20	Anatomy scan, check growth and placenta	How did the heart, brain, and spine look on the scan?
21–27	Monitor blood pressure, track movement patterns	What symptoms mean I should call the clinic right away?
28–32	Screen for diabetes, talk about birth place and care partners	What should I know about glucose testing and my facility’s birth routines?
33–36	Check baby’s position, review labor signs	What happens if the baby stays breech or labor does not start by itself?
37–41	Close monitoring of well-being, plan for induction if needed	How do you decide whether to recommend induction or continued waiting?

When The Fetal Timeline Takes A Different Turn

Many pregnancies track charts closely, yet some take a different path. Twins and other multiples often grow at a different rate than singletons. Babies can also measure smaller or larger than average even when they are healthy.

In some pregnancies, the placenta does not deliver enough oxygen or nutrients, or the baby has a medical condition that affects size. Clinicians may use the label “fetal growth restriction” when an ultrasound shows that a baby measures well below the expected range and other signs suggest that growth has slowed. Care plans in that setting depend on how early the growth problem appears and how the baby handles life inside the uterus.

Preterm birth also reshapes the picture. A baby born at thirty-two weeks has organs that work differently than a baby born at thirty-eight weeks, even if both weighed the same inside the womb. Neonatal teams watch breathing, feeding, and temperature closely in preterm babies, since lung and brain tissue are still maturing.

If you learn that your baby’s growth pattern does not match the usual timeline, it is natural to feel worried. Asking your clinician to explain what the numbers mean, what the plan is for monitoring, and what steps you can take at home can restore a sense of partnership.

Whether your pregnancy tracks every milestone on the chart or takes unexpected turns, the core idea stays steady. Inside the uterus, a new person forms over months through steady change: cells divide, organs appear, senses wake up, and the body practices for life outside. Understanding that story week by week can make each appointment, scan, and kick feel less mysterious and more connected.