Average Baby Weight at Birth and By Month
One of the best parts of visiting your baby’s pediatrician for wellness visits is finding out how your little one is progressing. Baby growth charts can track how your infant is developing physically, and you’ll learn where they fall in terms of weight and height compared to other babies their age. But what is the average baby weight at birth, and how many pounds will they pack on in that first eventful year of life? Here’s what you need to know.
Average Baby Weight at Birth
Wouldn’t it be nice if you could know exactly how much your little one will weigh at birth? You could stock up on appropriately sized diapers and clothing, and prepare yourself to have either a mini linebacker or an itty-bitty thing. While you won’t be able to get a precise answer before they’re born, your doctor should be able to give you a rough ballpark estimate. What’s more, you can do some educated guessing based on what’s considered the average baby weight at birth. “The average US baby weighs around 7 pounds at birth,” says Katie K. Lockwood, MD, an attending physician at CHOP Primary Care in South Philadelphia. She adds that a birth weight between 5.5 and 8.5 pounds is considered normal.
What Factors Affect Baby Weight at Birth?
Size, weight and height vary from newborn to newborn, and there are a lot of factors that affect baby’s weight at birth, including:
- Genetics. The apple doesn’t fall far from the tree. “Parents that are bigger tend to have big babies, and smaller parents tend to have small babies,” says Gary Reschak, MD, a pediatrician at Northwestern Medicine Woodstock Hospital.
- Environment and conditions during pregnancy. Baby’s size is “significantly affected” by the environment in your womb, Reschak says. For example, mothers with elevated blood sugars or gestational diabetes expose their unborn babies to higher levels of sugar, which stimulates growth.
- Blood flow during pregnancy. “Prenatally, if there is compromise to the baby’s blood supply—whether it is a too-narrow umbilical cord or an issue with the placenta—the baby can be born on the small side,” Reschak says.
- Gestational age. Babies that are born before their due date tend to be smaller than those born at or after their due date, Reschak says.
- Multiples. Babies who are part of what’s known as multiple gestation pregnancies (i.e., twins, triplets or more) typically have lower birth weights, Lockwood says.
Why Baby’s Weight at Birth Matters
Doctors don’t just jot down baby’s birth weight for fun and memory-making. Rather, a birth weight that’s abnormally high or low can have significant implications for your little one’s health. “In the short term, it may impact temperature regulation, feeding, blood sugar levels, calcium levels and breathing,” Lockwood says. “In the long term, it may contribute to conditions like diabetes and heart disease.”
Baby’s birth weight is also a jumping-off point for your child’s pediatrician to track their growth and development, Reschak notes. “Babies who don’t grow and gain weight appropriately after birth can have poorer developmental outcomes,” he adds.
Average Baby Weight By Month
Baby’s birth weight is a memorable measurement, but it’s normal for newborns to shed some ounces and pounds in the immediate days following their big debut. “A baby may lose up to 10 percent of their birth weight in the days or week after birth,” Lockwood says. By the time they’re 2 weeks old, though, they should be back up to their birth weight.
Beyond that point, your little one should start to pack on the pounds rather consistently, at least for a while. “Babies grow rapidly after birth and the rate will decrease over time,” Reschak says. He adds that in the first three months of life, infants will average about one ounce of weight gain per day. The growth rate will then decrease to about 0.67 ounces of weight gain daily. At the six-month mark, the rate drops to about 0.33 ounces a day. “As a general rule, infants should be at double their birth weight at 4 months and triple their birth weight by 12 months,” he says.
The American Academy of Pediatrics (AAP) recommends using the World Health Organization’s (WHO) growth chart for babies from age birth to 24 months. After baby turns 2, your child’s pediatrician may continue to use the WHO’s growth chart or switch to the growth chart from the Centers for Disease Control and Prevention’s (CDC).
Here’s a breakdown of 50th percentile weights for boys by age:
And here’s what you can expect for 50th percentile weights for girls by age:
What Baby Weight Percentiles Mean
Baby’s doctor will often tell you what percentile your little one is in for weight (along with height and length), and they may show you where baby lands on growth charts. They’ll also often talk about weight in terms of percentiles.
“Percentiles help pediatricians track an infant’s growth compared to other infants of the same sex and age around the world,” Lockwood explains. “A larger percentile means that an infant is bigger in either weight, height or head circumference than same-aged peers of the same sex.”
Reschak breaks it down this way: “If you took a random group of 100 babies, a baby at the 50th percentile for weight would weigh more than 50 of the other babies. A baby at the 95th percentile would weigh more than 95 of the other babies. A baby at the 25th percentile would weigh more than 25 of the other babies.”
So when should you be concerned about baby’s weight percentile? “We worry if a baby is off the chart, below the 5th percentile or above the 95th percentile—or if they’ve crossed over percentiles on the chart,” Lockwood says. Typically, a baby should follow along their percentile curve; when they deviate from that pattern, it’s noteworthy.
When to Talk to the Doctor About Baby’s Weight
Your doctor should be keeping tabs on baby’s weight at every wellness appointment. But if baby isn’t feeding well or has trouble feeding, Lockwood recommends talking to your pediatrician to make sure their growth is on target for their age.
If baby isn’t gaining enough weight, your doctor will want to make sure they’re getting enough calories. “Your pediatrician will take a close look at baby’s diet with you to ensure it’s meeting their growth needs,” Lockwood says. “If they’re eating well and still not gaining weight well, further evaluation will be done to ensure there aren’t other metabolic demands or illnesses contributing to poor weight gain, such as heart, thyroid and metabolic conditions.”
There isn’t necessarily a problem if baby is gaining more weight than expected. “If they have a healthy diet and they’re developing symmetrically and normally, your pediatrician may not do anything about weight gain during infancy,” says Lockwood. That said, if baby is growing in some body parts or dimensions faster than others—or if they’re eating unhealthy foods—your pediatrician will likely evaluate further.
Ultimately, if you have concerns about baby’s weight, bring them up to your pediatrician. They should be able to work with you to find answers. In the meantime, start working on your arm strength. You’re going to be doing some heavy lifting soon!
Katie Lockwood, MD, MEd, is an attending physician at CHOP Primary Care in South Philadelphia and an assistant professor of pediatrics at the Perelman School of Medicine at the University of Pennsylvania. She received her medical degree from Jefferson Medical College in Philadelphia.
Gary Reschak, MD, is a pediatrician at Northwestern Medicine Woodstock Hospital. He received his medical degree from St. George’s University School of Medicine.
Please note: The Bump and the materials and information it contains are not intended to, and do not constitute, medical or other health advice or diagnosis and should not be used as such. You should always consult with a qualified physician or health professional about your specific circumstances.