The average birth weight of a full-term newborn in the U.S. is 7 to 7 ½ pounds (3.2 to 3.4 kg). A healthy newborn weighs between 5 ½ pounds (2.5 kg) and 8 ¾ pounds (4 kg). Babies born weighing more or less at birth may require additional tests and special growth monitoring to ensure they’re healthy. The pediatric staff at the hospital and pediatrician’s office use standardized growth charts to verify weight percentiles and to monitor early weight gain or loss.
Several factors influence your baby’s birth weight and early weight gain. Using growth charts and growth tracking are simple ways to ensure your baby’s weight gain is healthy.
In this article, we’ll examine what affects birth weight, contributes to baby weight gain, and what’s considered typical according to commonly used growth charts.
Birth Weight Factors
A newborn’s weight is determined by several factors, including prenatal care received, the mother’s health, if substance abuse was involved, if the baby was pre-term or full-term, and other health and developmental conditions.
Before your baby is even born, your OBGYN is conducting growth monitoring of your little one. While pre-natal growth tracking is always an accurate indicator of your baby’s birth weight, it is very accurate in determining if your baby is developing on track. Fetal biometry uses measurements performed via ultrasound and allows providers to gauge the growth and development of the fetus.
Factors Affecting Newborn Weight
- The mother’s diet and weight, both before and during pregnancy:
- Higher weight individuals may have a heavier baby;
- If the mother has poor nutrition while pregnant, the baby may be smaller.
- The mother’s prenatal health, including conditions like depression, diabetes, high blood pressure.
- Alcohol consumption, smoking, or vaping.
- The parents’ birth weight and genetics.
- The mother’s age (teen moms tend to have smaller babies).
- Boy or girl, boys tend to be slightly heavier.
- Firstborn or subsequent pregnancy: firstborns tend to be smaller than siblings.
- How many babies the mother is carrying, multiples tend to be smaller than single births.
Early Weight Gain
Your baby’s weight and height are measured immediately upon birth and recorded. Within the first few days, even before you leave the hospital, it is common for newborns to lose 5-10% of their birth weight. This initial weight change is no cause for concern.
Breastmilk doesn’t arrive until three to four days after birth; initially, your baby, if breastfeeding, is consuming a thick, vitamin-rich fluid called colostrum. Colostrum includes vital antibodies, vitamins, and nutrients your baby needs. So even if you don’t plan on breastfeeding, consider nursing your baby first three or four days so they receive these valuable nutrients from you.
After this initial weight loss, and once your baby consumes more milk, their weight should tick upward. Most babies have weight and health checks at 1, 2, and 4 weeks for development and growth monitoring.
Your pediatrician will monitor early weight gain at each checkup and record the information on a growth chart in their file.
You can use the Baby Daybook to record these numbers so you have the information with you should questions arise. However, don’t rely on home scales to weigh your baby. They aren’t sensitive enough to pick the minuscule weight gains of infants.
When to be Concerned
If your newborn is exclusively breastfed and not gaining weight after the first week or continues to lose weight, they are likely not getting enough milk. Breastfeeding is not as easy as it looks, so if you’re struggling, there’s no reason for shame or embarrassment!
Talk to your pediatrician or schedule a visit with a lactation consultant for help breastfeeding. They will help you determine the problem, such as a poor latch or breastfeeding position, if your baby is tongue-tied, having a low milk supply, not offering frequent enough feedings or other potential issues.
If your baby is formula-fed and losing weight, it might be due to an allergy, not eating enough, illness, or health condition.
Growth Guidelines
Pediatric offices use growth charts to measure your baby’s height and weight percentiles. A baby at the 50% mark means that (in theory) half of all babies the same age weigh less than them, and the other half weigh more.
Weight gain, like other developmental milestones, is a spectrum. However, you should see typical weight gain within the first months of your baby’s life.
Once your baby is back at their birth weight, typically within one week to ten days, they should continue to gain 4-7 ounces (113-200 grams) weekly for the first six months. After the first six months, formula babies usually gain weight faster, and the introduction of solid foods impacts your baby’s weight gain.
Growth Chart for Breastfed Babies
The CDC recommends using the WHO (World Health Organization) growth charts for babies from birth until age two. Different charts for boys and girls can be found on the CDC website.
Takeaway
Babies grow quickly; within a few months, the tiny newborn you brought home will be a rolly-polly baby! It is usual for infants to lose 5-10% of their birth weight the first week, but that should see a steady incline of a few ounces gained per week. If your baby is losing weight, speak with your pediatrician to discover the cause, which is usually a simple fix!
Sources
Centers for Disease Control and Prevention (CDC). WHO Growth Charts. www.cdc.gov [Accessed February 2024]
Cleveland Clinic (2023). Fetal Biometry. my.clevelandclinic.org [Accessed February 2024]
Medical News Today (2023). What is the average baby weight by month? www.medicalnewstoday.com [Accessed February 2024]
Parents (2022). How Your Pregnancy Weight Directly Affects Your Baby’s Size. www.parents.com [Accessed February 2024]
WebMD.com (2022). What Is Colostrum? www.webmd.com/baby [Accessed February 2024]
WhatToExpect.com (2022). Your Newborn’s Weight: What the Average Baby Weighs and Normal Gains and Losses. www.whattoexpect.com [Accessed February 2024]
