The Complete Baby Growth Guide — Birth to 5 Years

A growth percentile compares your child's measurement to a reference population of children the same age and sex — it is not a grade or a ranking. A baby at the 40th percentile for weight is heavier than 40% of babies their age and sex in the reference data, and lighter than 60%. There is no "best" percentile to be at; a healthy baby can be anywhere on the chart. What matters more than the exact number is that your child follows a fairly consistent curve over time.

In the US, pediatricians typically use World Health Organization (WHO) growth standards for babies from birth to 2 years, and Centers for Disease Control and Prevention (CDC) growth charts from age 2 onward. The WHO charts describe how healthy, breastfed babies typically grow under optimal conditions, while the CDC charts are based on a broader reference population of US children. This is routine, standard practice.

Typical weight gain slows steadily over the first year: roughly 5–7 ounces (140–200g) per week for the first 3 months, slowing to about 3–5 ounces (85–140g) per week from 3–6 months, and around 2–4 ounces (55–115g) per week from 6–12 months. Most babies double their birth weight by around 5 months and triple it by around 12 months. These are general averages — a healthy baby's actual pattern can vary while still being entirely normal.

Some movement across percentile lines is common and often not a concern, especially in the first few months as a baby settles into their own genetic growth pattern, sometimes called "catch-up" or "catch-down" growth. A shift of one percentile band is usually considered unremarkable. A larger or more sudden shift — crossing two or more major percentile lines in a short period — is something your pediatrician will typically want to look at more closely.

Growth spurts are short bursts of accelerated physical growth. In the first year, they commonly cluster around 7–10 days, 3 weeks, 6 weeks, 3 months, 6 months, and 9 months, though timing varies between babies. The WHO doesn't publish an official list of growth-spurt ages — these are widely used clinical and parenting reference points — but a growth spurt often shows up on a WHO growth chart as a brief upward shift in percentile before the curve settles again. During a growth spurt, babies often feed more frequently, seem fussier than usual, and sleep somewhat more or wake more often for feeds. Growth spurts typically last a few days to about a week before settling back to the previous pattern.

Genetics — particularly parental height — is the largest single influence on a child's eventual size, alongside nutrition, sleep, general health, and for some children, birth history such as prematurity. Chronic illness or certain medical conditions can also affect growth, which is one reason routine growth checks are part of well-child visits.

For babies born before 37 weeks, pediatricians typically use "adjusted age" (also called corrected age) for growth tracking — your baby's chronological age minus the number of weeks born early. This is usually used until around 2 years of age, since premature babies often need this extra time to catch up to full-term growth patterns.

Head circumference is measured at well-child visits because it roughly reflects brain growth, particularly in the first two years when brain growth is fastest. Like weight and length, head circumference is tracked as a percentile and curve over time rather than a single number. A measurement that's unusually small (microcephaly) or large (macrocephaly), or one that shifts suddenly, is something your pediatrician will want to evaluate further.

Faltering growth (sometimes called failure to thrive) describes a pattern where a baby or child isn't gaining weight as expected over time, often reflected in a significant drop across percentile lines. It has many possible causes, including feeding difficulties, increased calorie needs, or an underlying medical condition, and is diagnosed and managed by a pediatrician based on the full growth history — not from a single low weight check.

Growth is typically measured at every well-child visit — usually several times in the first year, then roughly annually after age 2 or 3, alongside any sick visits where it's relevant. These routine measurements are what let your pediatrician see the overall trend rather than judging growth from any single visit.

Yes, to a significant degree — parental height is one of the strongest predictors of a child's eventual adult height, alongside nutrition and overall health. Pediatricians sometimes use a rough mid-parental height calculation as one data point among several when a child's growth pattern is being evaluated, though it's only ever one part of the picture.

Growth rate slows substantially after the first year — a toddler gains weight much more gradually than an infant does — and appetite typically drops to match. A toddler eating noticeably less than they did as a one-year-old is usually a normal reflection of a slower growth rate, not a feeding problem, as long as they're growing steadily and have good energy.

Worth discussing with your pediatrician: a sudden or significant drop across two or more percentile lines, very slow or no weight gain over an extended period, a head circumference that's unusually small, large, or shifting quickly, or any noticeable change in appetite, energy, or development alongside a growth concern. A single lower measurement rarely means much on its own — it's the overall trend your pediatrician is watching for.

Lunara's growth tracker logs weight, length or height, and head circumference over time and plots them against standard growth curves, making percentile trends easy to see at a glance between pediatrician visits. Weekly AI summaries highlight meaningful changes worth noting, and having an organised growth history on hand makes well-child visits easier to prepare for. It's free to start.