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Low birth weight

A baby is said to have a low birth weight if they weigh less than 2,500g (2.5kg) when born.

A baby may be a low birth weight at full term, or if born prematurely.

Around 7% of all live births are low birth weight. The weight of a baby may not be a cause for concern. Some babies are naturally smaller than others.

Babies are classed as being very low birth weight (VLBW) if they weigh less than 1,500g at birth and extremely low birth weight (ELBW) below 1,000g.

What causes low birth weight?

Factors which may cause a baby to have a low birth weight include the mother being a smoker and the baby being a twin or other multiple birth sibling.

Other factors, both medical and statistical, include:

  • Mother being short (height)
  • Mothers being older when having children
  • Low pre- pregnancy body mass index ( BMI)
  • Low gestational (pregnancy) weight gain
  • Substance abuse
  • Prolonged standing and activity, strenuous work
  • Having bacterial vaginosis infection
  • Stress
  • Being less well off
  • Some ethnic minority groups, including black and Asian women, are statistically more likely to have low birth weight babies

Why is low birth weight a concern?

Being a low weight at birth increases the risk of a baby being stillborn.

Low birth weight babies are also weaker and find it harder to keep warm. They have a greater risk of contracting diseases which could affect their hearing or eyesight. They also have a greater risk of sudden infant death syndrome (SIDS or cot death), having asthma, learning difficulties and cerebral palsy.

Although more research is needed, low birth weight has been linked with children developing autistic spectrum disorders and depression later in life.

Looking after a low birth weight baby

Doctors may decide a low birth weight baby needs extra nutrition in addition to mother's breast milk or special preterm formula.

If a baby starts life smaller than those born at the same time, it is important that all routine growth and development checks are carried out.

The midwife, health visitor or GP can help with concerns about the baby.

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WebMD Medical Reference

Medically Reviewed by Dr Rob Hicks on October 26, 2016

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