Fetal Growth By Weeks | What Happens Inside Pregnancy

Parents hear a lot of numbers during pregnancy, from weeks and due dates to ultrasound measurements and fundal height. Understanding fetal growth week by week helps those numbers feel less abstract and gives a clearer sense of how a baby changes from the first missed period to delivery day and beyond.

What Fetal Growth Actually Means

Fetal growth describes how a baby’s size, organs, and systems change from conception until birth. In early weeks, development centers on forming major structures like the brain, heart, and spine. Later on, the focus shifts toward weight gain, lung maturity, and fine details such as fingernails and eyelashes.

Clinicians use several tools to follow this process. Ultrasound measurements show length and weight estimates. Fundal height gives a quick sense of growth between scans. Ongoing prenatal care lets the team track patterns over time instead of reacting to a single number on one day.

Fetal Growth By Weeks: Big Picture Timeline

Pregnancy is usually counted from the first day of the last menstrual period, not from the day of conception. That means the earliest “weeks” on a chart span the time when ovulation and fertilization occur. By around week 5 or 6, ultrasound can often see a gestational sac, and soon after, a tiny heartbeat may appear.

First Trimester: Foundation Weeks (Weeks 1–13)

During the first trimester, the focus is building the basic layout of the body. The neural tube forms, which later becomes the brain and spinal cord. The heart starts to beat, and limb buds appear. By the end of this phase, the baby has a recognizably human shape, while size is still small.

By about week 10, the fetus measures only a few centimeters from crown to rump. Yet major organs already exist, and fingers and toes have formed. Health information sites such as Mayo Clinic’s first trimester fetal development overview describe this period as one of rapid structural change more than weight gain.

Second Trimester: Growth And Movement (Weeks 14–27)

In the second trimester, bones harden, muscles strengthen, and many parents start to feel kicks. Facial features become clearer, and the baby can move arms and legs in coordinated ways. Vernix (a creamy coating) and lanugo (fine hair) appear on the skin to protect it in the womb.

By around week 20, an anatomy ultrasound usually checks the brain, heart, spine, kidneys, limbs, and placenta in detail. March of Dimes and other groups note that many babies measure about 6.5 inches long and around 300 grams at this point, with steady growth over the weeks that follow.

Third Trimester: Finishing Touches (Weeks 28–40)

Once the third trimester begins, the baby spends more time putting on weight and refining organ function. The lungs mature, fat stores increase, and brain growth continues at a brisk pace. Movements may feel stronger and more rolling as space becomes tighter.

By weeks 37 to 40, most babies fall in a birth weight range of roughly 6 to 9 pounds and a length of about 18 to 20 inches. Organizations such as March of Dimes stress that staying pregnant until at least 39 weeks, when possible, gives the brain and lungs extra time to mature.

Week-By-Week Fetal Growth Stages And Milestones

Charts that show fetal growth by weeks usually group changes into blocks, since daily variation is normal. The outline below uses common benchmarks drawn from major health sources while keeping language clear for parents who are not reading dense medical textbooks.

Weeks Of Pregnancy	Approximate Size	Main Growth Changes
4–6	Few millimeters	Gestational sac and yolk sac visible; tiny heartbeat may appear.
7–8	About 1.3–2 cm	Head and limb buds form; eyes and ears begin to take shape.
9–10	About 3–4 cm	Fingers and toes separate; elbows bend; organs continue forming.
11–13	About 5–8 cm	Fetus moves freely; external sex organs start to differentiate.
14–20	About 9–16 cm	Skeleton hardens; movements grow stronger; anatomy scan done.
21–28	About 26–35 cm	Weight gain speeds up; baby can hear sounds; lungs develop air sacs.
29–36	About 36–47 cm	Subcutaneous fat builds; brain and nervous system mature.
37–40	About 48–53 cm	Organs reach functional maturity; baby prepares for birth.

How Clinicians Measure Fetal Growth

From the first trimester onward, ultrasound is the main tool to capture fetal size. Early on, crown–rump length gives the best estimate of gestational age. Later, measurements such as head circumference, abdominal circumference, and femur length feed into formulas that estimate weight.

Authoritative medical sites such as the ACOG guide on how the fetus grows explain that these measurements have margins of error, so they should not be read as exact predictions.

How Prenatal Care Helps Fetal Growth

Regular prenatal care allows clinicians to keep an eye on fetal growth by weeks and to respond when patterns change. At early visits, the team reviews medical history, checks blood pressure, orders lab tests, and confirms dating with ultrasound when needed.

The U.S. Centers for Disease Control and Prevention notes that healthy habits during pregnancy, such as taking folic acid, avoiding tobacco and alcohol, and following vaccine guidance, help protect both parent and baby during these growth phases.

Visit Patterns Across Pregnancy

Traditionally, average-risk pregnancies follow a schedule of visits every four weeks in early and mid pregnancy, then every two weeks, then weekly near term. Professional bodies now encourage more flexible schedules that still include timely ultrasounds and lab checks while adapting to patient needs.

Typical Ultrasound Measurements By Week

Ultrasound reports can feel technical, with abbreviations and numbers on every line. Understanding a few core terms makes it easier to follow how fetal growth by weeks appears in those results.

Gestational Age	Common Measurement	What It Tells You
8–12 weeks	Crown–rump length	Helps confirm dating and due date within a few days.
18–22 weeks	Head and abdominal circumference	Checks brain and abdominal growth at the anatomy scan.
24–28 weeks	Estimated fetal weight	Combines several measurements to follow growth trend.
30–34 weeks	Growth scan with Doppler	Looks at blood flow through cord and placenta if growth concern appears.
35–38 weeks	Repeat weight estimate	Helps plan birth if unusually small or unusually large size is suspected.

Factors That Influence Fetal Growth

Many influences can shift growth above or below the average line. Some relate to the placenta, others to the parent’s health, and others to the baby’s own genetics.

Conditions such as high blood pressure, preeclampsia, or chronic kidney disease can limit blood flow through the placenta, which may slow growth. On the other side, gestational diabetes can raise blood sugar levels, which sometimes leads to larger babies.

Nutrition and lifestyle matter too. Guidance from the CDC pregnancy pages encourages a balanced diet, regular movement cleared by the care team, and avoidance of substances that may harm a developing baby.

Small, Average, And Large For Gestational Age

Clinicians use labels such as small for gestational age (SGA), appropriate for gestational age (AGA), and large for gestational age (LGA) when they compare estimated fetal weight with population-based charts.

SGA usually refers to a baby below the 10th percentile for its gestational age. Some SGA babies are constitutionally small because their parents are small. Others have growth restriction due to placental or medical factors and need closer monitoring.

LGA typically means a baby above the 90th percentile. This can happen in families with a taller build or when conditions such as diabetes push growth higher. In both directions, the care team looks beyond the label to understand why size falls where it does.

When Fetal Growth Raises Concern

Clues that prompt more testing include a fundal height that lags by more than two centimeters compared with gestational age, a growth scan that shows weight dropping across percentiles, or ultrasound signs of poor blood flow through the placenta.

In these situations, clinicians may order more frequent ultrasounds, Doppler studies of the umbilical cord, or nonstress tests to track heart rate patterns. The goal is to decide whether the baby can stay in the womb longer with close watching or whether early delivery gives better odds.

Talking With Your Care Team About Growth

Parents sometimes leave a scan feeling worried because they heard a single number in isolation. It helps to ask how that number compares with earlier visits and with the wider picture of the pregnancy.

Tracking Fetal Growth Between Visits

Parents can keep a gentle eye on fetal growth between appointments. The most practical tool is a daily or near-daily kick check once movements are well established, usually after around 24 to 28 weeks.

Many clinicians suggest picking a time of day when the baby tends to be active, then counting how long it takes to feel 10 movements. A sudden drop in movement pattern, especially later in pregnancy, deserves prompt attention.

When To Seek Medical Help About Growth

Pregnancy always comes with some level of uncertainty, and fetal growth by weeks will never follow a perfectly straight line. Still, some signs should lead to urgent contact with the care team or emergency services.

Concerning symptoms include no fetal movements at a time of day when movement is usually felt, heavy vaginal bleeding, severe abdominal pain, or sudden swelling of hands, face, or feet along with headache or vision changes.

Bringing Week-By-Week Fetal Growth Into Daily Life

Week-by-week fetal growth charts can give parents a sense of progress and connection as they picture each new stage. The numbers on those charts are best used as reference points, not strict goals, since every baby and body follows its own path.

Combining basic knowledge of fetal growth by weeks with regular prenatal visits, a balanced lifestyle, and open communication with clinicians gives a solid foundation for making decisions. When questions arise, bringing them to the next appointment helps turn raw measurements into plans that feel suited to the family.