Babies grow remarkably fast in their first year, gaining about 10 inches (25 cm) in length by their first birthday. That rate is faster than any other period of life outside the womb, and it slows considerably after the first six months.
Growth Rate: Birth to 6 Months
During the first half-year, babies grow about 1 inch (2.5 cm) per month. A baby born at 20 inches long will typically measure around 26 inches by the six-month mark. This pace is roughly consistent month to month, though individual babies will have periods of faster and slower growth rather than a perfectly steady increase.
Most of this length comes from the legs and torso. The head grows too, but its contribution to overall length is proportionally smaller. Babies measured at well-child visits during this period are laid flat on an exam table using a measuring board with a fixed headpiece and a movable footboard pressed gently against the soles of their feet. This recumbent measurement is the standard for any child under 24 months because they can’t reliably stand upright.
Growth Rate: 6 to 12 Months
Growth slows noticeably in the second half of the first year. Most babies gain about half an inch (1.2 cm) per month between 6 and 12 months, bringing the total first-year gain to roughly 10 inches. By their first birthday, many babies are about 50% longer than they were at birth.
This slowdown is normal and coincides with other developmental changes. Babies are spending more energy learning to sit, crawl, and eventually stand, and their caloric intake increasingly goes toward fueling movement rather than pure growth.
Growth Rate: 12 to 24 Months
In the second year, the pace slows further. Toddlers typically grow 3 to 5 inches (7.5 to 12.5 cm) over the entire year, averaging less than half an inch per month. By age two, most children have reached roughly half of their eventual adult height, which is why some pediatricians use the two-year height as a rough predictor.
Growth Spurts and What They Feel Like
Babies don’t grow at a perfectly even rate. They go through growth spurts, short bursts of accelerated growth that typically last a few days. The most common times for these spurts are around 2 to 3 weeks, 6 weeks, 3 months, and 6 months of age.
During a growth spurt, your baby may seem hungrier than usual and want to feed more frequently. This cluster feeding is one of the most reliable signs. Babies may also be fussier, sleep more (or less), and seem generally unsettled. These phases pass quickly. If your baby suddenly wants to eat around the clock for two or three days, a growth spurt is the most likely explanation.
How Bones Actually Get Longer
A baby’s bones aren’t fully solid. At each end of the long bones in the arms and legs, there’s a zone of cartilage called a growth plate. New cartilage cells form on one side of this plate, while the older cartilage on the other side gradually hardens into bone. This conveyor-belt process is what makes bones physically longer, and it continues throughout childhood and adolescence until the growth plates close permanently.
Growth hormone, produced by a small gland at the base of the brain, drives this process. Babies produce growth hormone in pulses throughout the day and night, with the largest surges happening during deep sleep. This is one reason adequate sleep matters so much for infant growth.
What Determines How Fast Your Baby Grows
Genetics is the single biggest factor. Taller parents tend to have longer babies, and maternal height in particular correlates strongly with birth length. But genetics isn’t the whole story, especially in the first year when nutrition plays an outsized role.
Breastfeeding for six months or more, adequate birth weight, and satisfactory weight gain in the early weeks are all protective factors for healthy linear growth. On the other side, low birth weight, prematurity, frequent infections, and anemia can all slow a baby’s growth trajectory. Broader environmental factors matter too: family poverty, food insecurity, and limited access to clean water are well-documented risk factors for stunted growth in young children.
Protein and calorie intake are especially important during the first two years, when the growth rate is highest and the body is least able to compensate for nutritional gaps. Babies who fall behind in length during this window can have difficulty catching up later.
Premature Babies and Catch-Up Growth
Babies born early are typically shorter at birth than their full-term peers, but most close the gap through a period of accelerated “catch-up” growth. This catch-up usually brings premature infants in line with standard growth curves by 12 to 18 months of corrected age (meaning their age calculated from the original due date, not the actual birth date). In some cases, catch-up growth continues more gradually until age 5 to 7.
Pediatricians use corrected age for plotting premature babies on growth charts during the first two years. So if your baby was born two months early and is now four months old, their growth will be compared against the curve for a two-month-old. This adjustment prevents unnecessary concern about a baby who is simply younger, developmentally, than their calendar age suggests.
When Growth Is Too Slow
Pediatricians track your baby’s length on a growth chart at every well-child visit. What matters most isn’t a single measurement but the pattern over time. A baby who consistently follows the 15th percentile curve is growing normally, just on the smaller side. A baby who drops from the 75th percentile to the 25th percentile over several visits is a different story.
Clinically, a sustained drop that crosses two major percentile lines (for example, falling from the 50th to the 10th) raises concern. Length-for-age or weight-for-length below the 5th percentile is another red flag. These patterns can signal nutritional problems, hormonal issues, or underlying medical conditions that need evaluation. A single low measurement rarely means anything on its own, especially if the baby is otherwise feeding well and meeting developmental milestones.
Getting Accurate Measurements at Home
Measuring a squirming baby accurately is harder than it looks. Clinics use a rigid measuring board for a reason: a tape measure against a wiggling infant can easily be off by an inch or more. If you want to track length at home, lay your baby flat on a firm surface, gently straighten their legs, and mark the positions of the top of the head and the bottom of the heel. Measure between the marks after you move the baby. Even with this technique, expect some variation between measurements. The numbers your pediatrician records on a proper measuring board are the ones to trust for tracking growth over time.