Fetal Weight By Weeks | What Healthy Growth Looks Like

If you study every ultrasound report and wonder whether the numbers look right, you are in good company. Many parents search for fetal weight by weeks charts so they can match each appointment to a clear picture of growth and feel more relaxed between visits.

This guide explains how doctors estimate your baby’s size, what a typical week by week curve looks like, why percentiles matter, and when a reading deserves extra attention. You will also see how health, genetics, and daily habits shape weight over time and which questions to raise at your next visit.

Understanding Fetal Weight And Growth Curves

Fetal weight is an estimate rather than a direct measurement. During an ultrasound, the sonographer measures several body parts, such as head size, abdominal size, and femur length. Those values are fed into formulas built from large research studies that compared many scans with birth weights and then refined the math.

Research groups such as the Eunice Kennedy Shriver National Institute of Child Health and Human Development and the World Health Organization have published growth standards that map estimated fetal weight across pregnancy. These projects produced charts that show the 10th, 50th, and 90th percentiles for each week of gestation, based on healthy pregnancies from different backgrounds.

On the printed chart, the middle line is the 50th percentile. A baby plotted on that line weighs more than half of babies at that week and less than the other half. Lines above and below show the spread of normal size. Your baby’s estimated weight is placed on the chart so the care team can see which percentile band it falls in and how that position changes over time.

Percentiles do not grade your baby. A baby on the 15th percentile can be just as healthy as one on the 70th percentile, especially when the curve stays smooth and other checks look good. What matters most is pattern over time: does the line stay roughly parallel to a percentile band, or does it drop or rise across several lines?

Fetal Weight By Weeks In A Typical Singleton Pregnancy

Most single pregnancies follow a similar shape of growth. Early gains are small, the curve rises more steeply in the middle months, and weekly gains slow again near the due date. Exact numbers vary slightly between charts, but large studies show very similar patterns.

The table below shows rounded average estimated fetal weight at the 50th percentile for selected weeks. Values draw on major growth chart projects such as the NICHD fetal growth studies and World Health Organization standards, along with widely used clinical teaching tables.

Gestational Week	Average Weight (g)	What This Often Shows
12 weeks	14 g	Baby is still tiny; basic organs and limbs are present.
16 weeks	100 g	Skeleton strengthens and early movements appear on scan.
20 weeks	300 g	Anatomy survey is common; many families learn the sex now.
24 weeks	600 g	Lungs and brain mature; baby starts to gain more fat.
28 weeks	1,000 g	Third trimester begins; some pregnancies start regular growth scans.
32 weeks	1,700 g	Baby fills more of the uterus; kicks feel stronger and more frequent.
36 weeks	2,600 g	Size approaches birth weight; head may move down into the pelvis.
40 weeks	3,400 g	Average full term weight; many healthy babies sit above or below this.

These values are midpoints, not targets. A baby who weighs 2,900 grams at 40 weeks can be just as healthy as one at 3,600 grams, especially when the growth curve has stayed smooth and other checks such as amniotic fluid volume and Doppler readings look reassuring.

Ultrasound estimates also carry a margin of error. Near term, many estimates can differ from true weight by 10 to 15 percent in either direction. A reading of 3,200 grams may reflect an actual weight slightly under 3,000 grams or closer to 3,500 grams. Care teams combine the chart position with clinical findings rather than relying on one number alone.

First Trimester: Tiny But Busy Weeks

During the first trimester, fetal weight stays low even though development races ahead. By weeks 10 to 12, limbs, facial features, and basic organs are present, yet the baby still weighs only a few grams. Hospitals such as Cleveland Clinic describe this stage as a time when structure matters more than size.

Second Trimester: Steady Climb In Size

The second trimester brings a steady climb in both weight and length. Around 20 weeks, many babies weigh near 300 grams and measure about 25 to 26 centimeters from head to heel. Organizations such as March of Dimes describe babies at this point as active, with visible nails, clear limb movements, and a heartbeat that is easy to hear with Doppler.

Third Trimester: Rapid Gains Then A Slowdown

From about 28 weeks onward, many babies gain around 150 to 200 grams each week for a while. Muscles strengthen, fat layers thicken under the skin, and brain and lung tissue mature. Near 37 to 40 weeks, the curve flattens and weekly gains shrink, which often means the baby is reaching a comfortable birth weight.

Week By Week Fetal Weight Chart For Your Prenatal Visits

Knowing that growth charts exist is one thing; seeing how they guide routine care is another. At each growth scan, the sonographer measures head, belly, and femur, the machine calculates estimated weight, and the value is plotted on the chosen chart to show where it lands.

Different clinics may use slightly different charts. Some prefer standards based on low risk pregnancies only, while others use references designed for specific regions or populations. The method matters less than consistency. The main goal is to follow how your baby moves along one chart over time.

In the second half of pregnancy, your midwife or doctor may also measure fundal height with a tape measure from the top of the pubic bone to the top of the uterus. After about 24 weeks, that distance in centimeters usually matches gestational age within two to three centimeters. If the measurement falls outside that range, a growth scan is often arranged.

How Doctors Judge Whether Growth Is On Track

For many pregnancies, growth is considered reassuring when several pieces line up:

• Estimated weight stays between roughly the 10th and 90th percentile on a consistent chart.
• Growth points follow a smooth line without sudden drops across several percentile bands.
• Amniotic fluid, Doppler blood flow readings, and baby’s movements look normal.
• The pregnant person feels well and has no uncontrolled conditions such as severe high blood pressure.

Guidance from the American College of Obstetricians and Gynecologists describes fetal growth restriction when estimated weight or abdominal size falls below the 10th percentile on a reliable chart together with other warning signs. In that setting, extra scans, closer monitoring, and sometimes earlier birth are planned.

What Affects Fetal Weight Week By Week

Healthy babies come in many sizes. Some families simply tend to have smaller or larger babies based on genetics. Others see weight shift up or down because of health conditions, placenta function, or external influences such as smoking or certain infections.

Medical research groups such as the Eunice Kennedy Shriver National Institute of Child Health and Human Development report that fetal weight reflects a mix of parental build, blood sugar control, blood pressure, nutrition, multiple pregnancy, and access to steady prenatal care. Charts help put this mix into context rather than replacing clinical judgment.

Factor	Effect On Growth	Typical Response In Care
Parental genetics	Parents who were smaller or larger at birth often have babies of similar build.	Growth is judged in light of family size and background.
Placenta function	A placenta that delivers less oxygen or nutrients can slow weight gain.	Extra scans, Doppler checks, and planning around timing of birth.
Blood sugar levels	Gestational or pre existing diabetes can lead to higher fetal weight.	Advice on food and activity, plus medication if needed to keep glucose in range.
Blood pressure and vascular disease	Conditions such as preeclampsia may reduce blood flow to the baby.	Frequent monitoring, medication, and sometimes earlier delivery.
Smoking and substance use	Tobacco and some drugs are linked with lower birth weight.	Stopping exposure and linking to services that help with quitting.
Multiple pregnancy	Twins and triplets often have lower weight by weeks than singletons.	Use of twin specific charts and more frequent growth checks.
Infections or chronic illness	Some infections and long term illnesses can slow growth.	Treatment of the condition and planned fetal surveillance.

Small For Gestational Age Versus Growth Restriction

A baby below the 10th percentile may be described as small for gestational age. Some babies in this band are healthy and simply petite. Growth restriction is more likely when the chart shows a drop in percentile over time or when other signs such as low fluid or abnormal Doppler studies appear.

Using high quality fetal growth charts helps doctors separate healthy small babies from those who are not getting enough through the placenta. Large projects such as the NICHD fetal growth studies were designed to give better reference points across different populations so that growth problems can be found earlier.

Large For Gestational Age And Why It Matters

Babies whose estimated weight sits above the 90th percentile may be described as large for gestational age. This pattern appears more often when birth parents have diabetes, higher body mass, or a history of large babies. Extra size can raise the chance of birth complications such as shoulder dystocia or the need for cesarean birth.

Growth scans help care teams balance those risks. If weight estimates and other factors suggest a high chance of birth injury, your doctor may talk through induction of labor or planned cesarean, with timing chosen to protect both baby and parent while keeping the pregnancy as close to term as is safe.

Talking With Your Care Team About Growth Charts

Growth charts are tools, not verdicts. The most helpful part of any report is the conversation it starts. When you receive a scan, you can ask where your baby sits on the chart, how that position compares with earlier visits, and whether the pattern raises any concern.

If your baby measures smaller or larger than expected, ask what follow up is planned. That might mean a repeat scan in a few weeks, extra monitoring such as non stress tests, or review of health factors like blood pressure or blood sugar. Each plan is shaped by both the growth curve and your overall health.

When you read about fetal weight ranges on trusted sites such as March of Dimes or Cleveland Clinic, those charts show averages and ranges rather than strict goals. Professional groups like the American College of Obstetricians and Gynecologists keep updating guidance so that growth concerns are handled in ways that are safer for both parent and baby.

Your baby’s movements, your symptoms, and your medical story matter just as much as any number on the screen. When you place all of those pieces alongside fetal weight by weeks, the chart becomes a helpful guide instead of a source of worry.