I noticed that there are babies that are under 5th percentile. The baby is about a year old and the height and weight under 5th percentile. What does this mean? Does this mean the baby will be small forever?
No, it does not mean the baby will be small forever. Some children are small because their genes will lead them to be small adults. Others start out small because of issues during the pregnancy, and eventually become more typical. Others slow their rate of growth because something is wrong, such as an illness or a problem with nutrition. That's why growth is measured so frequently - it's a way to know if everything is still ok.
This article from the Canadian Pediatric Society says:
While children usually follow the same percentile for weight and height (or length) for most of childhood, children growing normally may also change percentiles in their first two or three years, to adjust toward their genetic potential . Using the National Center for Health Statistics (CDC) growth charts (showing the third, 10th, 25th, 50th, 75th, 90th and 97th percentile), DW Smith showed that as many as 30% of normal children crossed one major percentile line and 23% crossed two in the first two years of life . Smith’s estimates cannot be applied to the WHO growth charts because the major percentiles charted are different (the 0.1 percentile, 3rd, 15th, 50th, 85th, 97th, 99.9th percentile). Crossing two major channels on the WHO growth charts would represent a greater change, and one that cannot be considered ‘normal’. Birth weight and length are strong predictors of subsequent growth , but do not always reflect a child’s genetic potential. Intrauterine growth may be affected by external factors (eg, maternal malnutrition or smoking, gestational diabetes, placental insufficiency). After birth, there may be some ‘catch-up’ if an infant was born smaller than her/his genetic potential, or a ‘catch-down’ if the child was born larger than his/her genetic potential.
Growth patterns also depend on feeding, with breastfed infants often growing faster than formula-fed infants in the first six months of life, and formula-fed babies growing faster after six months.
Children with a constitutional growth delay will start showing retarded linear growth in the first three years of life. After that time, growth resumes at a normal rate but parallel to or under the growth curve, or along the lower growth percentiles during the prepubertal years. After the age of three, there should be no more change in growth percentile until puberty.
Two children I know well enough to know their growth chart positions were in the 10th percentile throughout childhood. One of them is a girl and started her period quite a bit later than normal. Both are regular sized adults and completely healthy. They were just slow growers.
My understanding is that moving down the growth charts is something to look into, but just starting out small is not something to worry about. After all, 10% of babies are under the 10% mark - that's the definition of it.
The percentile system doesn't mean anything from one measure only. The criteria behind it, is that the organism you are checking is growing up, so is mandatory to follow up his grow process.
This leads us to measure babies multiple times during the infant life (but also during the fetal life!!), and this allows us to create a curve.
Usually a baby under the 10th percentile is definite IUGR or SGA, differently. Or, if he's above the 90th percentile, he's big for gestational age.
For the same criteria I've explained before, being between the 10th and the 90th percentile with one measure, doesn't mean the baby is ok. Infact, in the medical field, if a baby is between the range above, but lose in a month more than 40 percentile points, this is considered like a baby under the 10th percentile. From this, you can understand really well the importance of the following up and constant measurements during his growing up life.
Coming back to your question, no, this doesn't mean your baby will be forever small. Think about premature birth, for example: they grew up normally, but usually are small when born.
This of course is a case that require follow up, not because he will have problems, but to monitor his grow and act on it, maybe with supplies or other.