by Olaide Akande

Created on: August 18, 2009

One of the common questions a woman is asked after having a baby is what did he or she weigh, probably second only to what is the sex. Your baby's birth weight is also probably something you will remember for a long time. This is especially true if they were either low birth weight or high birth weight.

The normal birth weight for term babies is 2.5kg to 4kg (about 5.5 to 9pounds). Babies falling outside of this range are classed as either low or high birth weight. The medical term for a high birth weight baby is a macrosomic baby. About 7% of babies are born with a LBW while macrosomic babies account for about 10% of all deliveries.

LBW and macrosomic babies often have specific medical risks associated with them.LBW babies account for a high proportion of neonatal mortality rates, while big babies are at risk of birth trauma. So what are the causes of deviations from the normal birth weight? There are two main reasons why a baby may be born low birth weight. These are prematurity and intrauterine growth restriction (IUGR)

A baby born before term will most likely be LBW especially if they are born several weeks earlier. Prematurity is caused by fetal factors such as fetal distress, multiple gestation, and congenital malformations. It may also arise from placental problems like placenta previa and abruptio placenta. Abnormalities of the uterus such as a bicornuate uterus (the uterus is split in two compartments) may also result in a premature birth.

Some other conditions which may cause a woman to deliver earlier than expected include rupture of the membranes, excessive amniotic fluid (polyhydramnios). It may also be iatrogenic such as a caesarian section done without knowing the mother's date was inaccurate.

IUGR is a failure of a fetus to achieve its genetic growth potential. This means factors either in the fetus, mother or placenta prevented the baby from growing to its full weight. Chromosomal abnormalities, donor of a twin- twin transfusion syndrome, and placental insufficiency are some specific causes of IUGR. Other factors associated with both prematurity and IUGR are:

Black race

Low socioeconomic status

Low pre pregnancy weight(less than 45kg)

Poor pregnancy weight gain(less than 10pounds)

Lack of antenatal care

Young or old maternal age (<16, >35)

Cigarette smoking, cocaine, and heroin use

Low level of maternal educational achievement

Maternal hypertension, diabetes, or heart disease,

Infections such as bacterial vaginosis, toxoplasmosis, syphilis, herpes, and rubella.

The causes of a high birth weight are:

Post maturity

Infants born to diabetic mothers

Excessive pre pregnancy weight

High pregnancy weight gain

Multiparity (having had three or more pregnancies)

Previous high birth weight

Congenital malformation syndromes e.g. beckwith-wiedemann

Genetics

Racial factors

Just as prematurity results in a LBW, in a similar fashion, babies who are post term i.e. born after 42 weeks of pregnancy are likely to be high birth weight. This is however no longer a common cause, as a woman will not usually be allowed to carry her pregnancy this far.

Factors such as genetics and race also have an effect on birth weight. Children of tall, big parents are likely to be born macrosomic. High birth weight has also been more commonly observed among Hispanic women than women of other races.

Despite the above mentioned causes of a macrosomic baby, a large proportion of babies are born with a high birth weight without having any of these factors. Low birth weight babies on the other hand most often have an identifiable cause, with prematurity accounting for most cases of LBW.

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