Physiological Development – What Are the Stages in Infant Growth?

The physiology of infants is fundamentally disparate than the development of older children and adults. Newborns grow and change more in the first year of their life than during any time, it can be both a thrilling and nerve-wracking stage for parents. 

During the first 12 months, parents will be able to notice rapid infant growth and physiological development. 

Infant development involves different physiological growth stages, which relate to changes and skill developments of the body, including the development of muscles and senses. Understanding these changes is an integral part of parenting. Read on to learn about these stages in infant development

Physiological Development - What Are the Stages in Infant Growth?


Babies usually catch up and go beyond their birth weight in the first month of life, and then steadily continue to gain weight. In the early two to three days after birth, a weight loss of approximately 10 percent of birth weight is normal. 

By about two weeks, the infant should have gained this back and be at their birth weight. Although babies grow at a different pace, they should gain about 1 ounce each day after the first two weeks as per average weight (up to 1 month of age). 

The average length of babies at birth is 20 inches (boys) and 19.75 inches (girls). The head size increases slightly less than 1 inch more than birth measurement by the end of the first month. 

Many newborns’ movements and activities are not purposeful; they are mere reflexes and involuntary. Besides having unique reflexes, you can also notice that their eyes are sometimes uncoordinated and may look cross-eyed. 

1 to 3 Months

Babies should have an average weight gain of about 1½ to 2 pounds and an average growth of over 1 inch each month. The average head size growth is about ½ inch each month. At this age, they begin to relax their tight muscle tone and extend their arms and legs more. 

Some of the newborn protective reflexes begin to disappear, and neck muscles become stronger. They can now open and close hands, and active leg movements are noticeable. At the end of 3 months, they begin to reach hands to objects and imitate some sounds (coos, vowel sounds).

4 to 6 Months

There should be an average weight gain of 1 to 1¼ pounds each month, and by 4 to 5 months, they should have doubled their birth weight. At this stage, there is an average growth of ½ to 1 inch and average head size growth of about ½ inch each month. 

The reflexes commonly apparent in young infants disappear, and they begin to support the body with legs when in a standing position. They have a full-color vision and can see longer distances

Babies may start to drool (not always a sign of teething), sleep longer at night, and make “swimming” motions with arms and legs when placed on abdomen.

7 to 9 Months

There should be an average weight gain of 1 pound each month; boys usually weigh about ½ pound more than girls, two times the birthweight by 4 to 5 months, and three times the birthweight by 1 year. 

Babies at 7 to 9 months have an average growth of about ½ inch, and an average head size growth of about ¼ inch each month are also distinguishable. At this age, they are rapidly developing their physical activities and becoming more mobile for the first time. 

They begin teething, which usually starts with the two center front teeth in the lower jaw, then the two center front teeth in the upper jaw. Additionally, they may creep, scoot, and crawl (backward, then forward). 

10-12 Months

Physiological Development - What Are the Stages in Infant Growth?

Birth weight is doubled at about 4-5 months and tripled at one year. There is an average weight gain of about 13 ounces and average growth of just ½ inch per month, as most infants grow 10 inches in the first year. The average head size growth is about ½ inch each month. 

Some new and exciting abilities include standing next to furniture without holding on and sitting back down from standing position. They begin to take steps and walk on their own. New teeth continue to come in, and they may have four to six teeth and their first back molar at one year old. 


Keep in mind that the physiological changes above may be affected by different factors. Still, many infants experience developmental milestones at similar times. Through this guide, you can recognize and acknowledge developmental delays for early intervention.