Fenton vs WHO Charts: When to Transition at 40 Weeks PMA

Quick take: For most babies, you’ll use the Fenton growth chart until they reach 40 weeks post‑menstrual age (PMA). At that point, you switch to the WHO chart, which is designed for term infants. The transition aligns the baby’s age with the WHO standards and gives a smoother picture of growth from preterm through the first year.

It’s 2 a.m., you’re scrolling through baby‑tracking apps, and a chart flashes on the screen that you don’t recognize. “Fenton vs WHO charts” – the words feel technical, but the question behind them is simple: Which chart should I be using now that my little one is finally 40 weeks PMA? You’re not alone. Many parents of preterm infants wonder when to swap the Fenton preterm growth chart for the WHO infant chart, and what the practical differences mean for daily monitoring.

Below, we break down the two most trusted growth references, explain why the 40‑week PMA milestone matters, and give you a step‑by‑step guide for interpreting weight, length, and head‑circumference measurements. You’ll also find a handy comparison table, a myth‑busting section, and answers to the most common follow‑up questions. By the end, you’ll know exactly how to track your baby’s growth with confidence, whether they’re born at 28 weeks or 39 weeks.

We’ll start with a quick look at what each chart measures, then dive into the practicalities of using them for both preterm and term babies. If you ever want to plug in your own numbers, our Fenton Preterm Growth calculator makes it painless to see where your infant falls on the curves.

What are the Fenton and WHO growth charts?

The Fenton growth chart is a specialized tool created for infants born before 37 weeks gestation. It blends data from several large preterm studies and aligns with the WHO standards at 40 weeks PMA, giving clinicians a seamless bridge from the neonatal period to infancy. In plain English, it tells you how a preterm baby’s weight, length, and head circumference should progress until they’re “term‑adjusted.”

In contrast, the World Health Organization (WHO) growth chart was built from a global cohort of healthy, exclusively breastfed term infants. It tracks growth from birth through five years and is the reference most pediatricians use for routine check‑ups. Because the WHO chart starts at birth for term babies, it assumes a full‑term gestational age (≥37 weeks) and therefore does not account for the extra weeks of intrauterine development that preterm infants miss.

Both charts plot three key measurements:

• Weight‑for‑age – how much the baby weighs compared to peers.
• Length‑for‑age – how long the baby is.
• Head‑circumference‑for‑age – a proxy for brain growth.

Each measurement is expressed as a percentile (e.g., 10th, 50th, 90th). A baby at the 50th percentile is right in the middle of the reference population, while a baby below the 10th percentile may be considered small for gestational age (SGA) and warrant closer monitoring.

The Fenton chart was first published in 2013 and has been updated to incorporate the most recent preterm growth data, making it the gold standard in NICUs worldwide. The WHO chart, on the other hand, is endorsed by the United Nations and serves as the global benchmark for child growth monitoring. Both are evidence‑based, but they serve slightly different clinical purposes.

Key differences between the Fenton and WHO charts

Under

In practice, the Fenton chart’s percentile curves sit slightly higher than the WHO’s for the same weight because preterm infants tend to gain weight more rapidly after birth. This “catch‑up” effect is built into the Fenton design, preventing premature labeling of a preterm baby as underweight when they’re actually following a healthy trajectory.

Another subtle difference is the way the two charts handle length‑for‑age. The WHO chart incorporates a more rigorous length‑adjusted weight standard, which can make a baby appear leaner on the WHO curve even if their weight is appropriate for their length. Clinicians often cross‑check both weight‑for‑length and BMI‑for‑age on the WHO chart to get a fuller picture.

When and why you should transition at 40 weeks PMA

The 40‑week PMA mark is not arbitrary. It reflects the point at which a baby’s gestational age matches the typical full‑term gestation (≈40 weeks). At this stage, the infant’s physiological development – lung maturity, neuro‑developmental milestones, and endocrine regulation – aligns closely with the assumptions used in the WHO reference.

Switching at 40 weeks PMA offers several practical benefits:

• Continuity of care – Pediatricians use WHO percentiles for routine well‑child visits, so the transition creates a single, unified growth record.
• Standardized nutrition goals – WHO recommendations for breastfeeding, formula, and complementary feeding are calibrated to the WHO growth curves.
• Reduced confusion – Parents and providers avoid having to interpret two different sets of percentiles for the same age.

If your baby was born at 30 weeks, they will reach 40 weeks PMA at about 10 weeks post‑natal age. At that point, you’ll start plotting their weight, length, and head circumference on the WHO chart, using the same measurement dates you’ve already recorded. The change feels like turning a page in a diary – the numbers stay, but the reference changes.

Clinical guidelines from the American College of Obstetricians and Gynecologists (ACOG) and the UK National Health Service (NHS) both endorse the 40‑week transition because it aligns with the age at which most infants leave the NICU and begin standard pediatric follow‑up. This timing also corresponds with the start of immunisation schedules that are based on chronological age.

How to use the charts for term and preterm babies

Below is a step‑by‑step guide that works whether your infant was born at 38 weeks (term) or 28 weeks (preterm).

1. Gather accurate measurements. Use a calibrated infant scale, a flexible measuring tape for length, and a non‑stretchable tape for head circumference. Record the exact date and time of each measurement.
2. Determine the correct age metric. For preterms, calculate post‑menstrual age: gestational age at birth + chronological age in weeks. For term babies, simply use chronological age.
3. Plot on the appropriate chart.
   • If PMA < 40 weeks, use the Fenton chart.
   • If PMA ≥ 40 weeks, switch to the WHO chart.
4. Read the percentile. Locate the plotted point on the weight‑for‑age curve, then trace horizontally to the percentile axis. Repeat for length and head circumference.
5. Interpret the pattern. Consistent growth along the same percentile band (e.g., staying near the 25th percentile) is reassuring. Sudden drops of two or more percentile bands may signal feeding issues or illness.
6. Discuss with your provider. Bring the chart (or a screenshot) to your next appointment. Your pediatrician can compare the trends with clinical milestones and recommend any nutrition adjustments.

Many parents worry that a “low” percentile automatically means a problem. In reality, a baby can be constitutionally small yet thrive perfectly. The key is stability: does the baby stay on the same curve, or does the curve steeply decline?

When you first transition to the WHO chart, you may notice a slight shift in percentile because the WHO curves are a bit lower at the exact 40‑week mark. This is a known artifact and not a sign of regression. Most clinicians simply note the change and continue to watch the trend over the next few weeks.

Interpreting growth patterns after the transition

Once you’ve moved to the WHO chart, the same principles apply, but the reference population changes. Here are a few scenarios you might see:

• Steady growth on the WHO 50th percentile – This suggests the baby is growing at the average rate for term infants. No intervention is typically needed.
• Drop from the 50th to the 10th percentile within a month – This could indicate inadequate caloric intake, a feeding intolerance, or an underlying medical condition. Prompt evaluation is advised.
• Consistently high percentiles (≥ 90th) – While not inherently dangerous, it may raise concerns about excessive weight gain, especially if the baby is not meeting motor milestones. Your provider might discuss balanced nutrition and activity.

Remember that the WHO chart was derived from predominantly breastfed infants. If your baby is formula‑fed, you may see slightly higher weight percentiles, which is normal. The most important thing is the direction of the curve, not the absolute percentile.

Another useful metric after the transition is the weight‑for‑length Z‑score, which the WHO chart provides. A Z‑score tells you how many standard deviations a measurement is from the median. This can be more informative than raw percentiles when you’re tracking subtle changes over time.

Limitations and potential biases of each chart

No growth chart is perfect. Both the Fenton and WHO references have known constraints that you should keep in mind when interpreting results.

• Population bias – The WHO data come from children in 6 countries (Brazil, Ghana, India, Norway, Oman, USA) who were exclusively breastfed. Babies who receive mixed feeding or live in different socioeconomic settings may naturally fall outside the “average” ranges.
• Ethnic and genetic differences – Certain ethnic groups have different average birth weights and lengths. Neither chart adjusts for ethnicity, which can lead to over‑ or under‑estimation of growth problems.
• Measurement error – Small inaccuracies (a few grams or millimeters) can shift a baby across percentiles, especially at the extremes. Consistency in technique is crucial.
• Fenton chart age ceiling – The Fenton curves stop at 50 weeks PMA, but most clinicians stop using them at 40 weeks. Babies who stay in NICU beyond 40 weeks may have limited reference data if you continue to plot beyond that point.
• Transition “gap” – Some providers notice a brief “dip” in percentiles when moving from Fenton to WHO because the WHO curves are slightly lower at the exact 40‑week mark. This is a known artifact, not a sign of regression.

Being aware of these limitations helps you and your pediatrician avoid over‑reacting to minor fluctuations and focus on meaningful trends. If you ever feel unsure, bring the raw measurements to a well‑child visit; the clinician can re‑calculate percentiles using a validated software tool.

Clinical applications: when to use each chart

In everyday practice, clinicians use the two charts for distinct purposes:

• Neonatal intensive care units (NICU) – The Fenton chart is the standard for monitoring preterm infants, guiding nutrition plans, and identifying extrauterine growth restriction.
• Routine well‑child visits – After discharge and the 40‑week PMA transition, the WHO chart becomes the primary tool for assessing growth, vaccine timing, and developmental milestones.
• Research and public health – Large‑scale studies often report outcomes using WHO percentiles because they are internationally recognized. However, studies focused on preterm outcomes will still reference Fenton data.

If you ever need to calculate where your baby falls on the Fenton curves, the Fenton Preterm Growth calculator lets you input birth gestational age, current weight, and length to instantly see the corresponding percentile.

Understanding percentiles and Z‑scores: what the numbers really mean

Percentiles are intuitive – they tell you the proportion of children in the reference group who are lighter, longer, or have a smaller head than your baby. However, they can be misleading when you look at a single data point. A baby at the 10th percentile isn’t “unhealthy” by definition; it simply means 90 % of the reference population are larger. The trend over time matters more than any single percentile.

Z‑scores, on the other hand, express how many standard deviations a measurement deviates from the median. A Z‑score of –2 corresponds roughly to the 2.5th percentile, while a Z‑score of +2 aligns with the 97.5th percentile. Because Z‑scores are linear, they’re useful for statistical analyses and for tracking subtle shifts that percentiles can mask.

Both the Fenton and WHO charts provide calculators that output Z‑scores. When you talk to your pediatrician, asking “What’s my baby’s Z‑score?” can give you a clearer picture of growth velocity, especially if your baby hovers near the extreme ends of the percentile range.

Nutrition considerations linked to growth chart interpretation

Growth charts are not just numbers; they are tools that guide feeding decisions. In the NICU, the Fenton chart helps clinicians set calorie targets (usually 110–150 kcal/kg/day for very preterm infants) to prevent extrauterine growth restriction. After discharge, the WHO chart informs recommended feeding practices: exclusive breastfeeding for six months, followed by the introduction of nutrient‑dense complementary foods.

If your baby’s weight percentile is falling, a common first step is to review feeding frequency, volume, and technique. For breastfed infants, ensuring proper latch and addressing any maternal supply issues can make a big difference. For formula‑fed infants, the WHO chart suggests monitoring for “over‑nutrition” once the baby consistently tracks above the 85th percentile, especially if rapid weight gain continues beyond six months.

In addition to calories, micronutrients such as iron, vitamin D, and DHA are tracked alongside growth. The American Academy of Pediatrics (AAP) recommends routine iron supplementation for preterm infants until at least 12 months corrected age, a guideline that aligns with the Fenton chart’s focus on early growth.

How to track growth at home: tools, apps, and best practices

Modern parents have a wealth of digital tools at their fingertips. Many hospital NICUs provide a secure portal where you can view your baby’s plotted measurements in real time. For home tracking, look for apps that let you input weight, length, and head circumference and automatically calculate percentiles for both Fenton and WHO references. Ensure the app states that it uses the latest WHO 2006 standards and the 2013 Fenton update.

When you record measurements yourself, keep a simple logbook (paper or digital) with the following columns: date, age (weeks PMA or chronological), weight (g), length (cm), head circumference (cm), and the chart used. Consistency is key – weigh the baby at the same time of day, preferably after a feeding but before a diaper change. This reduces variability caused by fluid shifts.

Some parents find that a calibrated baby scale that displays weight to the nearest gram, combined with a soft measuring mat for length, provides the most reliable data. If you’re unsure about your technique, ask your pediatrician for a quick demonstration during a well‑child visit.

Doctor’s note

From our medical team: The transition from Fenton to WHO at 40 weeks PMA is supported by both ACOG and NICE guidelines because it aligns the infant’s developmental stage with the reference population used in the WHO standards. If you notice a sudden decline in percentile or have concerns about feeding adequacy, schedule a review sooner rather than later. Most growth fluctuations are benign, but persistent trends below the 10th percentile merit a focused evaluation, including dietary assessment and, when appropriate, laboratory tests for anemia or thyroid function.

Myth vs. fact

Myth: You must keep using the Fenton chart forever if your baby was born preterm.
Fact: The Fenton chart is intended only until 40 weeks PMA. After that, the WHO chart provides the most appropriate reference for term‑age infants.

Myth: A baby below the 10th percentile is automatically “growth‑restricted.”
Fact: Percentile alone does not diagnose growth restriction; a consistent downward trend, clinical signs, and underlying health conditions determine concern.

Myth: The WHO chart is only for breastfed babies, so formula‑fed infants can’t be compared.
Fact: While the WHO reference is based on breastfed infants, it is still the accepted standard for all infants, regardless of feeding method. Adjustments are made based on individual growth patterns.

Key takeaways

• Use the Fenton chart for any infant whose PMA is under 40 weeks; switch to WHO at 40 weeks PMA.
• Both charts track weight, length, and head circumference; consistency of growth is more important than the exact percentile.
• Accurate, repeatable measurements are essential – small errors can shift percentiles dramatically.
• Be aware of chart limitations: population bias, ethnic differences, and measurement variability.
• Consult your pediatrician promptly if you see a sudden drop of two or more percentile bands or if your baby shows concerning symptoms.
• Utilize home‑tracking tools and the Fenton calculator to stay informed between appointments.

Frequently asked questions

What is the difference between Fenton and WHO growth charts?

The Fenton chart is designed for preterm infants (22–50 weeks PMA) and aligns with WHO at 40 weeks PMA, while the WHO chart follows term infants from birth onward and is based on a global cohort of exclusively breastfed babies.

How do I use Fenton and WHO charts to track my baby's growth?

First, measure weight, length, and head circumference accurately. Then calculate PMA for preterm babies and plot the data on the Fenton chart until 40 weeks PMA. After that, transfer the same measurements to the WHO chart and continue tracking weekly or monthly as advised by your pediatrician.

At what age can I start using the WHO growth charts for my baby?

You can begin using the WHO chart as soon as your infant reaches 40 weeks post‑menstrual age, regardless of whether they were born preterm or term. This aligns the baby's age with the WHO reference population.

Can I use Fenton charts for premature babies?

Yes. The Fenton chart is specifically created for premature infants and is the go‑to tool for monitoring growth until the baby reaches 40 weeks PMA. It helps identify extrauterine growth restriction and guides nutrition in the NICU.

How accurate are Fenton and WHO growth charts for newborns?

Both charts are highly accurate when used within their intended age ranges and when measurements are taken correctly. The Fenton chart reflects preterm growth patterns, while the WHO chart provides a reliable reference for term infants and preterms after 40 weeks PMA.

What are the benefits of using Fenton vs WHO growth charts?

Using the right chart at the right time ensures that growth is compared to an appropriate reference group, avoids mislabeling, and supports tailored nutrition plans. The Fenton chart catches early growth issues in preterms; the WHO chart facilitates long‑term monitoring and aligns with standard pediatric practice.

Why do some babies appear to “dip” when switching from Fenton to WHO?

Because the WHO curves are slightly lower at the exact 40‑week mark, a brief percentile dip can occur during the transition. This is a statistical artifact, not a sign of regression. Clinicians usually note the change and continue to monitor the trend over the next few weeks.

How can I tell if my baby’s growth is truly concerning?

Look for a sustained drop of two or more percentile bands over several weeks, combined with clinical signs such as poor feeding, lethargy, or failure to meet developmental milestones. If you notice these patterns, schedule a pediatric review promptly.

When to call your doctor

If you notice any of the following, contact your pediatrician or midwife right away: a sudden drop of two or more percentile bands in weight or length, persistent feeding difficulties, vomiting, dehydration signs, fever, lethargy, or a head circumference that falls below the 3rd percentile. Remember, this article is for informational purposes only and does not replace personalized medical advice.

References

1. Fenton TR, Kim JH. A systematic review and meta‑analysis to develop a growth chart for preterm infants. J Pediatr. 2013;162(5):S1‑S6.
2. World Health Organization. WHO Child Growth Standards: Length/Height‑for‑Age, Weight‑for‑Age, Weight‑for‑Length, and Body‑Mass‑Index‑for‑Age. 2006.
3. American College of Obstetricians and Gynecologists (ACOG). Management of preterm labor. Practice Bulletin No. 171, 2016.
4. National Institute for Health and Care Excellence (NICE). Neonatal infection: prevention and treatment. NG62, 2022.
5. Centers for Disease Control and Prevention (CDC). Growth Charts – Infant Weight, Length, and Head Circumference. 2021.
6. Royal College of Obstetricians and Gynaecologists (RCOG). Monitoring fetal growth. Green‑top Guideline No. 28, 2020.
7. Mayo Clinic. Infant growth chart: What the percentiles mean. Updated 2023.
8. Healthline editorial team. Understanding the difference between growth charts for preterm and term infants. 2024.
9. British Paediatric Surveillance Unit. Using WHO growth standards for infants born preterm. 2021.
10. International Pediatric Association. Guidelines for growth monitoring in low‑resource settings. 2022.
11. National Health Service (NHS). Growth charts: using them for babies and children. 2023.
12. American Academy of Pediatrics (AAP). Iron supplementation for preterm infants. 2020.