You want to know what is happening this week, whether the numbers on the screen make sense, and how to plan the next appointment without missing a window. A gestational age chart ties all of that together. It shows how far along the pregnancy is, where your baby’s measurements sit compared to peers, and what steps come next. Worried about small or big percentiles, or puzzled by different dating methods, or juggling work forms and class bookings, or all three at once. You are not alone in wondering. Here is a way to read the data with confidence and to use it for calm planning.

What a gestational age chart shows and why it matters

Think of the gestational age chart as a week by week map of fetal growth and key milestones, a shared language for parents and clinicians, and a single place to plot scans, trends, and decision points. It helps time screenings, follow growth velocity, and align care plans. Simple, visual, practical.

Making sense of weeks, months and local terms

Most clinics count from the first day of the last menstrual period, often called LMP. Some charts call this weeks of amenorrhea, it is the same anchor, weeks since the last period.
Conception based dating is usually about two weeks less than LMP. Twelve weeks LMP is about ten weeks from conception.
Months are fuzzy. Nine months spans about 37 to 41 weeks LMP, which is why medical teams stick to weeks.
A helpful script, We will count by weeks so every appointment, screening, and update lines up.

You might ask, do I need to convert months to weeks every time. No. Let the gestational age chart carry the math for you.

Key outputs you will see on the chart

Percentiles, z scores, and trajectories
Percentiles compare your baby to others at the same gestational age. The 50th percentile is average. Below the 10th suggests small, above the 90th suggests large.
Z scores express distance from average using standard deviations. A z score near 0 is typical. Beyond plus or minus 2 is uncommon and deserves attention.
The trend matters most. A steady line that follows the same band is reassuring, a drop across lines over time signals slower growth and triggers follow up.
Biometric measures that populate the chart
CRL or crown rump length in the first trimester, the best way to date early.
BPD or biparietal diameter and HC or head circumference, head size measures used from mid pregnancy onward.
AC or abdominal circumference, very sensitive to nutrition and a strong driver of weight estimates.
FL or femur length, helps with size estimates and proportionality checks.
EFW or estimated fetal weight, calculated from combinations of the above and given a percentile.

To enrich your toolkit, you may also encounter terms like ultrasound fetal biometry, fetal biometrics reference ranges, percentile charts for fetal growth, and fetal growth percentile by gestational age. These labels point to the reference lines your provider uses to decide where a measurement sits.

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Common uses you will encounter

Dating the pregnancy and confirming the due date, often presented as estimated due date (EDD) from a gestational age calculator or a due date calculator.
Scheduling the nuchal translucency window, the 20 week anatomy scan, and glucose screening at the right times.
Deciding who needs growth surveillance, added Doppler, or referral.

When a chart matters most

When LMP and ultrasound diverge by more than a week in early care.
When growth is below the 10th percentile, or above the 90th, or when the curve falls over time.
In twin and higher order pregnancies.
When maternal conditions like hypertension or diabetes are present.

How gestational age is determined

You may wonder which clock to trust. The best approach is to anchor early, then hold steady.

LMP, Naegele’s rule and practical caveats

LMP is a helpful start. Naegele’s rule gives EDD as LMP plus one year, minus three months, plus seven days, roughly 40 weeks from LMP.
Caveats, irregular cycles, uncertain dates, or recent hormonal contraception muddy the waters. When in doubt, favor ultrasound.
Try saying, We will keep LMP as a first estimate, then confirm with the earliest ultrasound.

For clarity in your chart, note whether you used LMP based dating or ultrasound dating, then stick with that anchor for all later visits.

First trimester ultrasound dating and the earliest milestones

CRL between about 6 and 13 weeks is the gold standard for dating. If CRL and LMP differ by 7 days or more, use the CRL date.
Very early signs, a gestational sac can be seen around 4 to 5 weeks LMP, it tends to grow about 1 millimeter per day. A yolk sac appears around 5 to 6 weeks.
An unusually small or large gestational sac size, or a persistently empty sac, prompts closer follow up. Once the embryo is seen, CRL takes the lead.

Parents often look up a crown rump length (CRL) chart or a nuchal translucency measurement chart to visualize early measurements. If you do, remember that ranges are normal and that ultrasound dating accuracy is highest in the first trimester.

Second and third trimester dating limits and assisted reproduction timing

After 14 weeks, dating error widens, about 7 to 14 days in the second trimester, up to 2 to 3 weeks in the third trimester. Later scans are best for growth trends, not for changing the EDD.
For IVF and related care, embryo retrieval or transfer dates are excellent anchors, and ultrasound confirms progress and rules out mismatch.

Chart standards and how to choose one

You may hear your team mention which reference they use. That choice can shift a percentile a little, which is why consistency matters.

International and population references

INTERGROWTH 21st growth chart and WHO standards come from low risk, multinational cohorts. They allow clinics to compare across regions.
NICHD and other local references reflect population specifics like maternal size and ethnicity, which can fit some communities better.

EFW formulas and practical chart selection

Weight estimates usually rely on a Hadlock growth chart family of formulas that combine HC, AC, FL, sometimes BPD. Pick one method and use it every time for apples to apples tracking.
Some centers use customized charts adjusted for maternal height, weight, parity, and fetal sex. These can reduce misclassification when inputs are accurate and governance is strong.

Specialty tools and resource notes

Many teams embed calculators in the medical record so EFW percentiles and z scores are generated automatically.
Parent facing tools like gestational age vs pregnancy weeks, pregnancy timeline by gestational age, and trimester specific GA milestones can be helpful for planning at home.

How to read a gestational age chart step by step

Questions first, What is the dating source, how are the points plotted, and does the line hold steady. Then decisions flow from those answers.

Step 1, establish accurate gestational age

Use early CRL when possible, document the anchor clearly.
Script, We will write down how the pregnancy was dated so every future visit uses the same clock.

Step 2, plot biometric measurements and pick an EFW formula

Plot HC, AC, FL, and BPD if used, then compute EFW with a consistent method.
Example note, 28 plus 0 weeks, HC 230 mm, AC 210 mm, FL 50 mm, EFW about 1100 g using Hadlock, at the 40th percentile.
Script, We use the same equation each time so comparisons make sense.

If you like visual aids, look for a biparietal diameter chart, a head circumference chart, an abdominal circumference chart, and a femur length percentile curve. These help you see single measures next to the weight estimate.

Step 3, interpret percentiles and z scores

SGA is under the 10th percentile, very SGA is under the 3rd. LGA is over the 90th. Z scores beyond plus or minus 2 are uncommon.
Check symmetry, if HC and FL are average but AC lags, that suggests later onset, asymmetric restriction. If all are proportionally small, that suggests earlier, symmetric restriction.
Script, Below 10 means small, above 90 means big, and the pattern over time matters most.

Step 4, assess growth velocity with serial plotting

One point is a snapshot, two or three points show motion. A fall across percentile bands is a signal to add tests.

Step 5, integrate Doppler and amniotic fluid with the chart

Combine growth with blood flow and fluid before changing care. Normal growth, normal Doppler, normal fluid usually means routine follow up. Abnormal combinations call for closer surveillance.

Typical clinical timepoints and tests tied to the chart

First trimester, dating scan and NT window

Dating CRL, about 6 to 13 weeks. Nuchal translucency, 11 plus 0 to 13 plus 6 weeks, often paired with maternal blood tests.
Paperwork often happens after the first official scan near 12 weeks.

Mid pregnancy, anatomy scan and growth baseline

The 20 week anatomy scan assesses structure, placenta location, and collects a growth baseline. You may see anatomy scan findings by gestational age listed in your report.

Late pregnancy, growth scans and surveillance

A first late growth check around 28 to 32 weeks is common, then every 1 to 3 weeks when concerns are present.
For EFW under 10th, growth plus Doppler every two weeks is typical. For EFW above 90th, every 3 to 4 weeks is common, more often if diabetes is present.

Screening triggers that rise from the chart

Abnormal percentiles or falling velocity usually trigger Doppler, NST or BPP, review of maternal blood pressure and glucose, and referral if needed.

Interpreting abnormal findings without alarm

SGA versus FGR

Small for gestational age (SGA) describes a baby under the 10th percentile who may be constitutionally small and healthy.
FGR or intrauterine growth restriction (IUGR) suggests a pathologic process, a small EFW plus slowed velocity, abnormal Doppler, or other signs of placental insufficiency.
Script, Small on the chart is not automatically unhealthy, we look at trends and blood flow to decide next steps.

LGA and macrosomia

Large for gestational age (LGA) means over the 90th percentile. Macrosomia is often defined by birth weight above 4000 g, some centers use 4500 g.
Risks include shoulder dystocia, neonatal hypoglycemia, and maternal complications. Response focuses on glucose management and delivery planning.

Declining percentiles and faltering velocity

A steady fall, for example from the 50th to the 10th over serial scans, calls for Doppler, NST or BPP, and a visit schedule adjustment. Timing of delivery depends on gestational age and severity.

Multiples and discordant growth

Twin specific references often fit better. Discordance, commonly a 20 to 25 percent gap in EFW, prompts specialist monitoring. If you search for multiple gestation dating (twins) GA or twin pregnancy dating, you will find that twins follow different curves.

Clinical pathways that the chart prompts

Follow up for suspected FGR

Serial growth scans every 1 to 3 weeks, umbilical artery and middle cerebral artery Doppler, sometimes ductus venosus.
Maternal evaluation for hypertension or diabetes.
Delivery timing guided by gestational age, Doppler changes like absent or reversed end diastolic flow, and fetal testing.

Management when the chart suggests LGA or macrosomia

Optimize glucose, plan interval growth checks, discuss induction or expectant management using shared decision making. Induction is individualized, not automatic.

When to involve maternal fetal medicine

Severe or worsening FGR, EFW under the 3rd percentile, absent or reversed flow on umbilical artery, marked discordance in twins, or uncertainty about optimal delivery timing.

Measuring and calculating estimated fetal weight

Common formulas and how they appear on charts

Most centers use Hadlock formulas that combine head size, abdominal size, and femur length, sometimes biparietal diameter. You may see this plotted on a Hadlock growth chart with a fetal growth percentile and a z score.

Limits, error margins and practical example

Typical error is about 10 to 15 percent, wider in late pregnancy or with challenging positions.
Example, at 28 weeks, HC 230 mm, AC 210 mm, FL 50 mm, EFW about 1100 g, mid range percentile for that age. If that stays stable, routine care. If it falls across visits, add Doppler and possibly refer.

Parents sometimes ask, can I rely on a single EFW number. Better to follow the fetal growth trajectory across time.

Doppler, amniotic fluid and integrated monitoring

Umbilical artery Doppler shows placental resistance. Absent or reversed end diastolic flow needs prompt escalation with close monitoring and delivery planning.
Middle cerebral artery Doppler and the cerebroplacental ratio add context. A low MCA pulsatility index suggests brain sparing, a sign of reduced reserve.
Amniotic fluid, AFI under 5 cm or maximum vertical pocket under 2 cm suggests oligohydramnios, AFI over 24 cm or pocket over 8 cm suggests polyhydramnios. Oligohydramnios plus slow growth or abnormal Doppler strengthens the case for early delivery.

Viability, survival and practical counseling

Viability usually begins around 23 to 24 weeks in well resourced settings, with improving survival week by week.
Always consider local NICU capacity and outcomes. Conversations that include neonatology align expectations with reality and help with time sensitive choices. This is where pregnancy outcomes by gestational age and neonatal outcomes by gestational age data shape decisions about steroids, magnesium, and timing.

Everyday use at home, a compass not a rulebook

Many parents like a simple week by week pregnancy chart on the phone or fridge. It links what you feel to what the baby is doing each week.
Expect normal variability, a 10 to 15 percent swing around average is common.
Use the gestational age chart to prepare questions for visits, to plan classes, and to spot changes that deserve a call.

If you enjoy tools, printable or digital, you might appreciate a gestational age chart printable or a gestational age chart week by week view. Some websites also share fetal biometric reference standards and fetal biometrics reference ranges for curiosity. These are references, not verdicts.

Trimester snapshots that bring the chart to life

First trimester, weeks 1 to 13 counted from LMP

Weeks 1 to 2, groundwork, ovulation and fertilization.
Week 3, implantation begins, early fatigue or mild nausea may appear.
Weeks 4 to 7, organ formation starts, cardiac activity appears.
Weeks 8 to 10, limbs form, rapid length growth.
Weeks 11 to 13, transition from embryo to fetus, nausea often eases.

Early visual markers like yolk sac size and gestational sac size change quickly here, which is why a CRL measurement chart helps anchor timing.

Second trimester, weeks 14 to 27

Weeks 14 to 16, facial features sharpen, energy often returns.
Weeks 17 to 19, first movements are common.
Week 20, about 19 cm and 300 g on average, sex may be visible on ultrasound.
Weeks 21 to 24, senses develop, growth accelerates.
Weeks 25 to 27, lungs mature, the baby builds reserves.

Third trimester, weeks 28 to 40

Weeks 28 to 31, stronger responses to outside stimuli.
Weeks 32 to 34, marked weight gain, movements may feel different as space tightens.
Weeks 35 to 37, head often settles into the pelvis, practice contractions are common.
Weeks 38 to 40, term is considered from about 37 weeks. Typical size near birth is around 3 to 4 kg and 50 cm, with healthy variation.

These bands link directly to clinical labels like early term, full term, late term, and post term, which your provider may use when planning delivery.

Practical planning the chart can support

Paperwork often happens after the first scan near 12 weeks.
Use your week anchor to schedule classes, leave, and family logistics. Policies vary by region, so match your timeline to local guidance and your clinic schedule.
Script, We will map forms and classes to your gestational weeks so deadlines do not sneak up.

Choosing and implementing a chart in practice

Decision guide, population versus customized

Population based charts like WHO or INTERGROWTH fit many settings. Customized charts can help when maternal size, parity, or fetal sex meaningfully shift expected size.

Integrating the chart into clinic workflows

Store the dating source, use a single calculation engine for EFW and z scores, color code abnormal results, and offer suggested next steps inline.

Parent facing communication

Keep visuals simple, emphasize trend lines over single points, and present clear next steps such as repeat scan, Doppler, or referral.

If you like digital support, parent friendly tools often include week by week pregnancy timeline, gestational age chart PDF download, and pregnancy week list.

Limits, sources of error and common pitfalls

Measurement variation happens. Operator skill, fetal position, probe angle, and machine settings can nudge numbers. Early CRL is the most reliable, later measures vary more.
Population differences matter. Charts built from one population can misclassify another. Customized or locally validated references can help.
Single point thinking misleads. Serial data reveal whether the line holds steady or drifts.

For context, many families also look at preterm birth by gestational age rates when planning for backup support or hospital selection.

Practical tools, visuals and widgets that help

Visuals, a CRL dating graphic, biometry percentile plots, an EFW curve, and a weeks to months conversion graphic.
Interactive tools, a gestational age calculator that accepts LMP or CRL, a percentiles calculator for EFW or single measures, and printable checklists.
Quick references, fetal biometric reference standards, red flag thresholds, and recommended scan intervals.

Clinical pearls and quick reference

Quick tip, when to trust the chart

When early CRL and LMP align and serial growth stays consistent, your gestational age chart is a reliable guide.

Red flag box, when to act quickly

Severe growth faltering, absent or reversed end diastolic flow on umbilical artery, vasa previa, or major discordance in twins, escalate to maternal fetal medicine and neonatal teams.

Key takeaways

A gestational age chart turns weeks into action, from dating to screening windows to growth tracking.
Early ultrasound sets the most reliable clock, later measures must be read with a 10 to 15 percent margin and in sequence.
The choice of reference, such as fetal biometrics reference ranges or an INTERGROWTH 21st growth chart, can shift percentiles slightly, so consistency across visits matters more than the exact line you pick.
Abnormal patterns guide the next step, Doppler, NST or BPP, maternal checks, sometimes specialist referral or delivery planning.
Trends trump single points. A steady path is reassuring, a falling path deserves more checks.
There is support at every step. Your care team, local resources, and parent friendly tools are ready to help. For personalized tips and free child health questionnaires, you can download the application Heloa.

Before you close the tab, ask yourself, What is my current week, what is the dating anchor, when is the next measurement, and what will we do if the percentile shifts. With those answers on the calendar, your gestational age chart becomes a calm, practical compass.

Questions Parents Ask

How do I use a gestational age chart calculator and what should I enter?

Most calculators accept either your LMP (first day of your last period) or an early ultrasound measurement (CRL). For later scans they ask for biometric measurements such as BPD, HC, AC and FL to estimate fetal age, EDD, EFW and percentiles.
Quick tips: enter whichever dating anchor is most reliable (early CRL if available), choose which reference the tool uses (Hadlock, INTERGROWTH, WHO) if that option appears, and treat single outputs as estimates rather than absolutes. If numbers look surprising, ask your caregiver which reference and formula were used — consistency matters more than the exact engine. Using a calculator can help plan appointments, but rely on serial measurements and your clinical team for decisions.

Can femur length (FL) alone tell me the gestational age or health of the baby?

Femur length gives useful information about growth and proportions, but by itself it’s limited. In early second trimester FL correlates reasonably with gestational age, yet variability rises later and between populations. Clinicians use FL alongside HC and AC to check proportionality and to calculate estimated fetal weight.
If FL seems much shorter or longer than expected, common next steps are: repeat the measurement to rule out technical error, confirm the dating anchor, and look at other measures before drawing conclusions. Significant isolated differences may prompt specialist input, but often they simply lead to closer monitoring.

Are fetal weight charts in grams or kilograms reliable to predict birth weight?

Charts that show estimated fetal weight (EFW) in grams or kg are useful for context and planning, but they are estimates with a usual error of about 10–15% (larger late in pregnancy). Those charts are best used to compare the baby’s EFW to percentiles for that gestational week rather than to predict an exact birthweight.
Use them to track trend—stable percentiles are reassuring; falling percentiles prompt follow up. If preparing practical details (e.g., neonatal care or parental leave), discuss ranges with your provider rather than relying on a single gram value.

Gestational age chart, a clear roadmap for your pregnancy