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Assess newborn weight status based on gestational age and gender
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For gestational age and gender
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50th percentile at 40 weeks
Full-term (37-42 weeks):
Average: 7.5 lbs (3400g) • Range: 5.5-10 lbs (2500-4500g)
Preterm (36 weeks):
Average: 5.7 lbs (2600g) • Higher NICU admission risk
Very Preterm (32 weeks):
Average: 3.7 lbs (1700g) • Requires specialized care
Post-term (42 weeks):
Average: 8.0 lbs (3600g) • May have complications
A birth weight percentile calculator is a neonatal assessment tool that evaluates whether a newborn's weight is appropriate for their gestational age and gender. Birth weight is one of the most important indicators of newborn health and predictor of short-term and long-term outcomes. The calculator compares a baby's actual birth weight to standardized growth curves based on data from thousands of births at various gestational ages. This comparison yields a percentile ranking that shows what percentage of babies born at the same gestational age and gender weigh less than the baby being assessed. Birth weight percentiles are crucial for identifying babies who may be at increased risk for complications. Babies are classified into three categories: small for gestational age (SGA) if below the 10th percentile, appropriate for gestational age (AGA) between the 10th and 90th percentiles, and large for gestational age (LGA) above the 90th percentile. These classifications help healthcare providers anticipate potential problems and provide appropriate care. SGA babies may have experienced intrauterine growth restriction and are at higher risk for hypoglycemia, hypothermia, and developmental issues. LGA babies, often born to mothers with gestational diabetes, face increased risks of birth injuries, breathing problems, and metabolic complications. Understanding birth weight percentiles helps guide immediate newborn care and long-term follow-up planning.
Calculating birth weight percentiles requires three key pieces of information: the baby's weight at birth, the exact gestational age at delivery, and the baby's gender. Birth weight should be measured on a calibrated infant scale within the first hour after birth, before significant fluid loss or feeding. The measurement is typically taken in grams or pounds and ounces. Gestational age is determined from the mother's last menstrual period and confirmed or adjusted by early pregnancy ultrasound, recorded in completed weeks (for example, a baby born at 39 weeks and 5 days is considered 39 weeks gestational age). Once these data points are collected, the calculator references standardized growth curves specific to gestational age and gender. These curves are based on large population studies and show the distribution of birth weights at each week of pregnancy. The most commonly used standards are from Fenton growth charts for preterm infants and WHO or CDC charts for term infants. The calculator uses statistical methods to determine where the baby's weight falls within this distribution. For example, if a baby is at the 50th percentile, half of babies born at that gestational age weigh less and half weigh more. Birth weight percentiles must be interpreted carefully, considering factors like maternal health conditions, multiple gestation, and ethnic background, as these can affect normal birth weight ranges. Serial measurements during infancy help determine if the baby is following an appropriate growth trajectory after birth.
| Gestational Age | Boys (Average) | Girls (Average) | Classification |
|---|---|---|---|
| 28 weeks (Very Preterm) | 2.2 lbs (1000g) | 2.1 lbs (950g) | Extremely premature |
| 32 weeks (Moderate Preterm) | 3.7 lbs (1700g) | 3.5 lbs (1600g) | Premature |
| 36 weeks (Late Preterm) | 5.7 lbs (2600g) | 5.5 lbs (2500g) | Late preterm |
| 37-38 weeks (Early Term) | 6.8 lbs (3100g) | 6.6 lbs (3000g) | Early term |
| 39-40 weeks (Full Term) | 7.5 lbs (3400g) | 7.3 lbs (3300g) | Full term |
| 41-42 weeks (Late/Post Term) | 7.9 lbs (3600g) | 7.7 lbs (3500g) | Late/post term |
The average birth weight for full-term babies (37-40 weeks) is approximately 7.5 pounds (3400 grams), with a normal range of 5.5 to 10 pounds (2500-4500 grams). Boys tend to weigh slightly more than girls on average.
SGA means the baby's birth weight is below the 10th percentile for their gestational age. These babies may have experienced growth restriction in the womb and require closer monitoring for hypoglycemia, temperature instability, and feeding difficulties after birth.
LGA babies (above 90th percentile) are often born to mothers with gestational diabetes, obesity, or excessive weight gain during pregnancy. Genetic factors also play a role - large parents often have larger babies. LGA babies face increased risks of birth injuries and metabolic problems.
Newborns typically lose 5-10% of their birth weight in the first few days due to fluid loss and limited intake before milk comes in. This is normal. Babies should regain their birth weight by 10-14 days of age.
Birth weight has some correlation with childhood size, but many factors influence long-term growth including genetics, nutrition, and overall health. Many SGA babies experience catch-up growth in the first 2 years, while some LGA babies level off to normal percentiles.
Low birth weight (LBW) is defined as less than 2500 grams (5.5 lbs) regardless of gestational age. Very low birth weight (VLBW) is under 1500 grams (3.3 lbs), and extremely low birth weight (ELBW) is under 1000 grams (2.2 lbs).
Third-trimester ultrasounds can estimate fetal weight, but these estimates have a margin of error of ±10-15% (about ±1 pound at term). Clinical examination (fundal height, Leopold maneuvers) provides rough estimates but is less accurate than ultrasound.
Yes, multiples have their own growth charts as they typically weigh less than singletons at the same gestational age. Twin-specific percentile charts should be used to assess whether each twin's weight is appropriate for their gestational age.
This calculator is for educational purposes only and does not replace professional medical assessment. Birth weight percentiles should be interpreted by qualified healthcare providers in the context of the complete clinical picture including gestational age accuracy, maternal health history, and newborn condition. Seek immediate medical attention for any concerns about your newborn's health or feeding.