Last updated: May 2026
The Trajectory Estimator is a small planning tool inside Seedling. You tell it either a future weight or height (and it tells you roughly when your baby will get there), or a future date (and it tells you roughly what your baby will weigh or measure on that day).
It's meant for everyday planning — should I buy the next size up of diapers now, or wait? Will this winter coat still fit in March? It is not a medical prediction and not a substitute for your pediatrician.
Babies tend to stay on the same growth curve they've been on. The familiar 3rd, 50th, 75th, and 95th “percentile” lines at the doctor's office are reference curves published by WHO and CDC; if your baby has tracked close to the 50th for a few months, a reasonable guess is that they'll keep tracking near it. The estimator looks at your baby's recent measurements, figures out which band they've been on, and walks that band forward (or backward) on the chart.
It needs at least five measurements to do this fairly — one point is a snapshot, two is a line that could go anywhere, and a handful starts to show a pattern. We look at the last five measurements you've logged and pick the typical pattern — the middle one. That way a single unusual reading (after a big feed, or with a wiggly baby on the scale) doesn't throw off the prediction.
Jadzia is 6 months old, and she's been weighing in close to the 50th percentile for girls her age — about 7.3 kg, which is right where the 50th-percentile line sits at 6 months. Her parents want to know when she'll cross 10 kg, because the next stage of her car seat is rated to a different weight range.
The estimator looks at the WHO girls' weight-for-age 50th percentile and reads off where it reaches 10 kg. On that chart, 50th-percentile girls hit roughly 10 kg between 16 and 17 months. So the estimator answers: around 17 months of age — roughly 11 months from now. Jadzia's parents now know there's no rush before they need the next car-seat stage.
Becket is 18 months old and he's been tracking close to the 75th percentile for height (about 84.2 cm at 18 months on the WHO boys' length-for-age chart). His parents are looking at a toddler bed listed online and want to know if Becket will be big enough to use it a year from now.
They open the estimator in reverse mode, pick “one year from today” as the target date, and ask “how tall will he be?” At 30 months on the same chart, the 75th-percentile line sits at about 94.7 cm. So the estimator answers: roughly 95 cm — about 3.1 feet — if he keeps tracking his current curve.
Three situations where the tool politely declines:
The estimator assumes your baby keeps following the same curve they've recently been on, and real babies don't always cooperate. It's common and normal for a baby to shift percentile bands as feeding settles in — a baby tracking the 75th in month one might settle onto the 50th by month four, and that's usually nothing to worry about.
Pediatricians look at far more than a line on a chart: trends, curve shape, where the kid sits relative to siblings and parents, and clinical signs no projection captures — feeding, alertness, milestones, the physical exam. A projection is a single line; a pediatrician is the whole picture. If something about your baby's growth concerns you, talk to your pediatrician.
Seedling uses the WHO Child Growth Standards (default) or the CDC clinical growth charts (selected in Settings → Charts → Default Baseline). The estimator follows whichever you've chosen, so flipping between the two will shift the projection slightly; both describe slightly different populations and either is defensible.
Sources: WHO Child Growth Standards and CDC Clinical Growth Charts.
Email rodrigocf_92@icloud.com.