Head Circumference Data Sources

Last updated: May 2026

The short version

If you set your Default Baseline to CDC, the Head Circumference chart uses CDC data for ages up to 36 months. Above 36 months, it automatically switches to the WHO head-circumference standard. That is the only chart where this happens, and it is by design — it is not a bug or an inconsistency in the app.

The other three charts (Weight, Length, and Weight-for-Length) use your chosen baseline across their full age range.

Why does CDC head circumference stop at 36 months?

The CDC growth charts only publish head-circumference data for children from birth to 36 months. There simply is no CDC head-circumference reference for older children. This is a deliberate choice by the CDC: head circumference is a routine pediatric measurement during infancy and early childhood (when the brain and skull are growing fastest), and it is rarely tracked on a percentile chart after the third birthday.

For children older than 36 months, the CDC explicitly refers clinicians to the World Health Organization (WHO) head-circumference standard, which covers a wider age range.

What does Seedling do above 36 months?

When your Default Baseline is CDC and your child is older than 36 months, Seedling follows the CDC's own guidance: it falls back to the WHO head-circumference standard for that part of the chart. The result is a single, continuous head-circumference curve — CDC data for the months it covers, then WHO data above 36 months — with no visible gap or jump.

This fallback applies only to head circumference. Your CDC baseline still drives the Weight, Length, and Weight-for-Length charts across their entire range.

What you'll see in the app

On the Head Circumference chart, when your Default Baseline is set to CDC, Seedling shows a short note under the chart:

"CDC provides head circumference up to 36 months; beyond that, WHO data is used."

That note is there so you always know which reference body produced the percentile you're reading. It disappears if you switch your Default Baseline back to WHO (because then the whole chart already uses WHO), and it never appears on the Weight, Length, or Weight-for-Length charts.

What about the implausible-value warnings?

Seedling flags measurements that fall far outside the expected range — what clinicians call "biologically implausible values" (BIV). For head circumference, those warnings always use the WHO thresholds, regardless of whether your Default Baseline is WHO or CDC.

This is intentional. The WHO thresholds are the published standard for catching data-entry mistakes (a typo, a unit mix-up), and a measurement that is biologically implausible is implausible no matter which reference curve you happen to be viewing. So the source you pick for the chart does not change which thresholds Seedling uses to warn you.

Which baseline should I use?

Both WHO and CDC are legitimate, widely used references — they were built from different populations and serve slightly different purposes:

• WHO Child Growth Standards describe how children should grow under optimal conditions, based on an international, breastfed-baby cohort. The WHO standard covers head circumference across a wider age range.
• CDC Growth Charts describe how children in a historical United States sample actually grew. The CDC head-circumference data covers 0–36 months only.

If you're tracking head circumference for a child older than three, WHO is the more natural choice because it covers the full range without a source switch. If you prefer CDC for consistency with your pediatrician's other charts, the automatic WHO fallback above 36 months keeps the head-circumference chart usable for older children too.

As always, your pediatrician is the right person to interpret any percentile. Seedling computes the numbers; it does not give medical advice.

FAQ

Does the WHO fallback change my export files?

No. Your CSV and PDF exports still reflect your chosen Default Baseline in the filename and footer. The head-circumference column borrows WHO data above 36 months exactly the way the chart does, but the report's stated source — WHO or CDC — matches the baseline you selected.

Why doesn't this happen on the Weight or Length charts?

Because CDC publishes weight, length/stature, and weight-for-length/stature data well past 36 months. Head circumference is the only measurement where the CDC reference ends earlier than the rest, so it's the only chart that needs a fallback.

Is the curve continuous at the 36-month mark?

Yes. Seedling stitches the CDC data (up to 36 months) and the WHO data (above 36 months) into one curve. The two standards line up closely at the seam, so you won't see a step or a gap.

Can I turn the note off?

The note only appears on the Head Circumference chart under the CDC baseline. Switching your Default Baseline to WHO (Settings → Charts → Default Baseline) hides it, because the whole chart then uses WHO and there's nothing to disclose.

Questions?

Email rodrigocf_92@icloud.com.