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What Determines the Size of My Baby?

By Rose Welton ; Updated June 13, 2017

The size of your baby is likely to change constantly. According to Kids Health, your baby will nearly triple his birth weight and increase his length by 50 percent by the time he is 1 year old. Because of all of these changes, you may find yourself wondering what contributes to your baby’s size at birth and during his growth.

Birth Timing

The time that your baby was born may affect her size. According to Kids Health, an average full term baby born between 37 and 40 weeks gestation weighs between 6 pounds, 2 ounces and 9 pounds, 2 ounces. The average length of a full term baby is between 19 and 21 inches. If your baby was born earlier than 37 weeks gestation, she may be smaller with a low birth weight. If your baby comes after her due date, she may be bigger than average.

Prenatal Factors

Prenatal care during pregnancy can also affect your baby’s size. Poor nutrition, drinking, smoking or using drugs during pregnancy can cause him to be small. Gestational diabetes can cause your baby to be big. If he is more than 9 pounds, 15 ounces, he is considered to be larger than average. If your baby is born smaller or bigger than average, it does not necessarily mean that anything is wrong with him. He will be checked by a doctor after birth to determine if he is healthy.


Genetic factors also play a role in your baby’s size. At her birth and as she grows older, her size is likely to reflect that of her parents. If her parents are tall, she may also be tall, and if they are short she may also be short. Gender is a factor as well. According to Kids Health, girls tend to be smaller than boys.


As your baby grows older, his nutrition intake is an important factor in his size. He may experience failure to thrive he does not eat enough. To make sure he is getting the nutrition he needs, feed him whenever he seems hungry. Your baby’s doctor will keep track of his length and weight at appointments to make sure that he is growing consistently.

Baby's Health

Your baby’s health can cause her to be on the smaller side as she gets older. If she is sick, has an infection or birth defect, she may have trouble eating or gaining weight. If you are concerned about her weight gain, be sure to talk to her doctor about ways to make sure she grows properly.

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