Low Birthweight

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Babies born with a weight of under 2500 grams are described as having a low birthweight (LBW). Throughout the world, the proportion of LBW births is recognized as an important indicator of the health and well-being of a population since there is a close relationship between low birthweight and perinatal and infant mortality. Low birthweight is associated with preterm birth or babies that are small for their gestational age.

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To provide some background information about low birthweight, the following questions are addressed:

• Is low birthweight important?
• What are the trends in low birthweight proportions in Canada?

A number of provinces/territories have either explicit or implicit targets for LBW rates. In Ontario, for example, the Ministry of Health and Long-Term Care (1997) has set the objective of lowering the low birthweight rate to 4% by the year 2010. The four per cent figure is also employed in other parts of Canada and elsewhere in the world. Given the temporal trends of the past two decades, that target may be difficult to reach. Figure 1 shows that in 1996 only the LBW rates in the Yukon were close to that objective. On the other hand, there are many sub-provincial areas where this objective is met or exceeded.

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Figure 1. Provincial/Territorial Low Birthweight Rates, 1996

Interpretative mapping of low birthweight proportions has been undertaken employing various age groupings of mothers. The maps were prepared using 1996 data only. One-year data may be subject to significant variability when showing sub-provincial patterns. Consequently, census divisions with fewer than 25 births in the respective mothers' age categories are indicated as having insufficient data for reporting purposes.

Is low birthweight important?

In addition to the extra costs associated with caring for low birthweight infants, the weight at birth is considered a key determinant of the chances for survival and good health (Federal Provincial and Territorial Advisory Committee on Population Health). It is estimated that 75% of all newborn deaths and illnesses occur in LBW babies; and those that survive are more likely to experience learning disorders, visual impairment, respiratory illnesses, and cerebral palsy (Best Start 1998).

The list below identifies the major modifiable factors that have been identified as increasing the risk of LBW in terms of social and personal risk factors.

Social Risk Factors

• poverty
• single parent
• teenage parent
• little or no prenatal care
• living with a violent partner
• generally stressful life
• workplace conditions
• types and amount of work

Personal Risk Factors

• smoking
• alcohol and other drug use
• poor nutrition before and during pregnancy
• limited stress-relief strategies

Not all specialists dealing with low birthweight infants would necessarily agree with the inclusion of all of these factors. Direct birth outcome relationships for some have not been fully documented. It is also important to recognize that none of these risk factors can be singled out as the sole cause of low birthweight.

Being a teenager and smoking are certainly recognized as primary risk factors. "From 1991 to 1996, the rate of low birth weight increased among teenage mothers from 6.0% to 7.0%. This increase occurred at the same time as smoking rates among teenagers increased from 23% to 29%. Such an increase in smoking prevalence could account for one-half to two-thirds of the increase in low-birth-weight babies" (Statistics Canada, The Daily, Friday, March 31, 2000).

Trends in Low Birthweight Proportions in Canada

Canada has an enviable record when it comes to measures of health such as infant mortality. By 1997, the infant mortality rate had fallen to 5.5 per 1000 births (Statistics Canada). Unfortunately, this decrease has slowed in the 1990s. The recent rise in the incidence of LBW in part explains the slowing of the decline in the infant mortality rate (Nault (1997)).

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Figure 2. Infant Mortality and Low Birthweight, 1986 to 1996

The recent increase in LBW rates may be partly explained by the increasing proportion of multiple births and an increase in reporting very small and premature babies, who later die, as live births rather than stillbirths. Other contributing factors are the relatively high birth rates for younger and for older mothers. The birth rate for teens has declined slightly in recent years but still represents a fairly large proportion of total births. Birth rates for women over 30 years of age continue to rise steadily.

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Figure 3. Birth Rates, 1986 to 1996

LBW rates are currently higher among both younger (less than 20 years of age) and older (especially 35 years of age and older) mothers. The mean age at which Canadian women first give birth has increased from 27.0 years in 1986 to 28.4 years in 1996 (Statistics Canada).