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Educational Use Only - Embryology is an educational resource for learning concepts in embryological development, no clinical information is provided and content should not be used for any other purpose.


Newborn infant

There are a variety of prenatal techniques for estimating approximate birth weight that are relevant for preterm, term and prolonged pregnancy. Ultrasound two- and three-dimensional scanning methods are the basis of most current techniques. There are also standard autopsy weight curves that have been developed from second and third trimester fetal and also neonatal autopsy. Low birth weight is accurately defined as a statistical indicator for development. High birthweight definition on the other hand varies in the literature and between countries with a lower cut-off above 4000 gm or 4500 gm.

At birth, infants are generally weighed as soon as possible and may also be monitored during the neonatal period. In Australia, the average birthweight for all babies born in 1991 was 3,350 grams and about the same in 2004 at 3,370 grams.

Birth Links: birth | Lecture - Birth | caesarean | preterm birth | birth weight | macrosomia | Birth Statistics | Australian Birth Data | Developmental Origins of Health and Disease (DOHAD) | Neonatal Diagnosis | Apgar test | Guthrie test | neonatal | stillbirth and perinatal death | ICD-10 Perinatal Period | Category:Birth
Historic Birth links  
1921 USA Birth Mortality

Links: Ultrasound | Fetal Origins Hypothesis | Maternal Diabetes | Macrosomia

Some Recent Findings

  • Ten-Year Review of Major Birth Defects in VLBW Infants[1] "Birth defects (BDs) are an important cause of infant mortality and disproportionately occur among low birth weight infants. We determined the prevalence of BDs in a cohort of very low birth weight (VLBW) infants cared for at the Eunice Kennedy Shriver National Institute of Child Health and Human Development Neonatal Research Network (NRN) centers over a 10-year period and examined the relationship between anomalies, neonatal outcomes, and surgical care. ...Chromosomal and cardiovascular anomalies were most frequent with each occurring in 20% of affected infants. Mortality was higher among infants with BDs and varied by diagnosis. Among those surviving >3 days, more infants with BDs underwent major surgery (48% vs 13%, P < .001). Prevalence of BDs increased during the 10 years studied. BDs remain an important cause of neonatal morbidity and mortality among VLBW infants."

Birth Weight Classifications

The primary causes of VLBW are premature birth (born <37 weeks gestation, and often <30 weeks) and intrauterine growth restriction (IUGR), usually due to problems with placenta, maternal health, or to birth defects. Many VLBW babies with IUGR are preterm and thus are both physically small and physiologically immature.

Human Birth Weight Classifications
no colour
Birth weight (grams) less 500 500 – 999 1,000 – 1,499 1,500 – 1,999 2,000 – 2,499 2,500 – 2,999 3,000 – 3,499 3,500 – 3,999 4,000 – 4,499 4,500 – 4,999 5,000 or more
Extremely Low Birth Weight
Very Low Birth Weight
Low Birth Weight
Normal Birth Weight
High Birth Weight

Extremely Low Birth Weight

  • Less than 500 grams (1 lb 1 oz or less)
  • 500 – 999 grams (1 lb 2 oz – 2 lb 3 oz)

Very Low Birth Weight

  • 1,000 – 1,499 grams (2 lb 4 oz – 3 lb 4 oz)

Low Birth Weight

  • 1,500–1,999 grams (3 lb 5 oz – 4 lb 6 oz)
  • 2,000–2,499 grams (4 lb 7 oz – 5 lb 8 oz)

Normal Birth Weight

  • 2,500–2,999 grams (5 lb 9 oz – 6 lb 9 oz)
  • 3,000–3,499 grams (6 lb 10 oz – 7 lb 11 oz)
  • 3,500–3,999 grams (7 lb 12 oz – 8 lb 13 oz)

High Birth Weight

  • 4,000–4,499 grams (8 lb 14 oz – 9 lb 14 oz)
  • 4,500–4,999 grams (9 lb 15 oz – 11 lb 0 oz)
  • 5,000 grams or more (11 lb 1 oz or more)
  • see also * macrosomia

No Background Version

Human Birth Weight Classifications
Birth weight (grams) less 500 500 – 999 1,000 – 1,499 1,500 – 1,999 2,000 – 2,499 2,500 – 2,999 3,000 – 3,499 3,500 – 3,999 4,000 – 4,499 4,500 – 4,999 5,000 or more
Extremely Low Birth Weight
Very Low Birth Weight
Low Birth Weight
Normal Birth Weight
High Birth Weight

Small for Gestational Age

Gastroschisis Birth Weight Graph

(SGA) Term used for infants as having a birth weight more than 2 standard deviations (SD) below the mean or less than the 10th percentile for the gestational age ((GA}}. WHO birthweight definitions are low birthweight as less than 2,500 grams, very low birthweight is less than 1,500 grams and extremely low birthweight: less than 1,000 grams. Growth restriction can be symmetrical (slow development with limited brain growth) or asymmetrical (head circumference and length are preserved and brain growth is relatively spared).

  • Symmetric SGA (Weight, head circumference and length all below the 10th percentile) can be due to chromosomal abnormalities, intrauterine infection, severe placental insufficiency and or a constitutionally small infant.
  • Asymmetric SGA (Weight below the 10th percentile) can be due to interference with placental function and or interference with maternal health in 3rd trimester.

There are a large number of known relationships between low birth weight and both maternal and fatal abnormalities, a few examples are shown below.

Fetal Gastroschisis

Gastroschisis patients are commonly small for gestational age (SGA, birth weight < 10th centile). Frequency line graphs of the birth weight distribution.[2]

The abnormality is usually situated to the right of the umbilicus and abdominal contents, mainly gastrointestinal, are found outside the anterior body wall. Can occur in isolation and also in association with other gastrointestinal anomalies (intestinal atresia, perforation, necrosis or volvulus). Defects in other organ systems have been reported in up to 35% of children.

Maternal Elevated Testosterone

Maternal elevated testosterone levels is associated with low birth weight in humans. Hyperandrogenism associated with polycystic ovarian syndrome (PCOS) and pre-eclampsia have a higher prevalence of small-for-gestational age newborns. A rat model study suggests that maternal testosterone does not cross the placenta, to directly suppress fetal growth, but affects nutrient delivery to the fetus by down-regulating specific amino acid transporter activity.[3]

High Altitude

Altitude affects growth patterns measured in a a recent Peruvian study of 63,620 healthy infants born at low (150 m) and high (3000-4400 m) altitude were compared. [4] They found that in the third trimester "Mean and median birth weight differences between those born at low and high altitudes reached statistical significance after 35 and 33 weeks, respectively."


Definition: Live births with a birth weight of 4,500 grams or more, expressed as a percentage of all live births with known birth weight.

High birth weight can result in complications for the infant and mother during birth and may be associated with an increased risk of diabetes.

Links: Statistics Canada, Vital Statistics, Birth Database.


  1. <pubmed>23733791</pubmed>
  2. <pubmed>22004141</pubmed>| BMC Pediatr.
  3. <pubmed>21812961</pubmed>| Reprod Biol Endocrinol.
  4. <pubmed>19038011</pubmed>



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Search Pubmed: Low Birth Weight | small for gestational age

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Cite this page: Hill, M.A. (2024, May 20) Embryology Birth-Weight. Retrieved from

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© Dr Mark Hill 2024, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G