Are you looking for a tool to estimate your baby's weight at birth, regardless of their gestational age? Use this birth weight percentile calculator to find out where among their peers your child places. It not only shows the average weight of a newborn baby, but this calculator also adjusts their weight according to the week of pregnancy in which the baby was delivered.

In the article below, you can find some information on the average newborn weight, the average newborn weight classification, and an explanation of how to use this birth weight percentile calculator.

Average baby weight by a week of delivery ⏳

The weight of a baby at birth differs depending on their age when they are delivered, which causes their adjusted age to differ. The age from which a baby can survive outside the uterus is 24 weeks old.

On the other side are babies that are 41 weeks old. If they're not born before that time, induction of delivery is necessary to prevent complications, such as meconium aspiration.

Because of such a wide range of possible pregnancy length, the average weight of a newborn baby may also vary.

Average weight of newborn baby 👶

The average birth weight for newborns born full-term is between 5.5 lb (2.5 kg) and 8.8 lb (4 kg).

What is the classification?

  1. Extremely low birth weight (ELBW) is a term used for infants who weigh less than 1,000 g at birth (2.2 lb).
  2. Very low birth weight (VLBW) describes babies born weighing less than 1,500 grams (3.3 lb).
  3. Low birth weight (LBW) is when a baby's weight is less than 2,500 g (5.5 lb), no matter the gestational age.
  4. Normal weight (NBW) is if the baby weight is between 2,500 and 4,000 g (5.5 - 8.8 lb)
  5. High birth weight (HBW) is for babies born weighing more than 4 kg (8.8 lb).

There are more babies every year born at both extremes - with HBW and LBW. What are the reasons? For HBW mostly the increasing quality of life and the number of metabolic and chronic diseases among mothers, such as diabetes. For babies with a low birth weight, developments in medicine and healthcare mean that they are now more likely to survive in utero. Also, children from multiple pregnancies, such as twins, triplets, etc. tend to be delivered preterm and with a weight lower than the average.

Infants with ELBW, VLBW, or LBW are at a high risk of complications such as hypothermia, hypoglycemia, perinatal asphyxia, respiratory distress, anemia, impaired nutrition 🍼, infection, neurological trouble, and hearing problems.

Abnormal newborn weight causes

A preterm birth (before week 38) or slow prenatal growth may cause low birth weight. There are however other causes, such as genetic factors, maternal health issues, or problems with the placenta. 👩

A weight over 4 kg is usually due to a big baby, or the mother gaining more weight than average during the whole pregnancy. Again, the cause may be genetic or due to maternal issues (such as diabetes).

Birth weight percentile calculator ⚖️

To estimate the newborn weight percentile, you only need to know two values. The first one is the gestational age of the baby at birth, given in weeks. The second one is their weight.

Once you put in the data, the result - the exact percentile - will appear. The result is estimated based on the fetal weight equation proposed by Hadlock et al.

Below the calculator you can see a birth weight percentile chart with percentiles marked in different colors. The 50th percentile is the average weight of a newborn baby. It means that 50% of babies born at that age weigh more than this value and 50% weigh less. Your result is a dark dot, found somewhere between the percentile lines. The X values are the gestational age at which the baby may be born (between 24 and 41 weeks), and the Y values are weight. The color lines are percentile lines to help you read where your result is.

Newborn weight percentile calculator in practice

Let's work on an example of a newborn weight percentile calculation. Lucy, born yesterday, was born in the 35th week of pregnancy, a bit too early. The neonatologist wants to calculate the percentile she's in according to her weight. The physician knows that preterm birth is a cause of low weight relative to children born on their due date. She uses this birth weight percentile calculator to estimate how Lucy's weight compares with the other children born at week 35.

She weighs 2350 g (5.181 lb). According to our calculator, that puts her between the 25th and 50th percentile (to be precise, the 43.42th percentile). This means that a little more than 50% of the children born at 35 week of pregnancy weight more, while a little more than 40% of infants have a lower weight than her. She has a perfectly normal body mass!

Dominika Miszewska, PhD candidate
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