Birth Weight

This report examines low birth weight and birth weight for gestational age, including small for gestational age (SGA) and large for gestational age (LGA), in the KFL&A area and in Ontario.

Birth weight has been shown to have a range of short- and long-term consequences for children's health and development.1  Public health units in Ontario are mandated through the Ontario Public Health Standards (OPHS)2 to aim to increase the proportion of newborns born within a healthy weight range.  Historically, the low birth weight rate, defined as the number of babies born with a birth weight less than 2500 grams per 1,000 births, has been used as a risk factor for infant morbidity and mortality.  However, low birth weight can result from an infant being small for gestational age (SGA), being born preterm (born before 37 weeks), or from a combination of these factors.  An infant is considered to be small for gestational age when the birth weight is below the 10th percentile of birth weights of the same sex and gestational age (in weeks), and large for gestational age (LGA) when the birth weight is above the 90th percentile, when using Canadian population-based growth standards.3  The term SGA is sometimes used interchangeably with intrauterine growth restriction (IUGR), although there are distinctions between the two; IUGR refers to the occurrence of poor fetal growth that may occur due to a number of mechanisms, and SGA describes an infant's position on growth charts after birth.

As there are differences in the health outcomes and factors associated with an infant who is born SGA or preterm,4 it is increasingly recognized that birth weight should be examined in the context of gestational age.  Rather than only reporting on low birth weight rates, public health agencies are now urged to report rates of small for gestational age (SGA) and large for gestational age (LGA) births.  National reports on perinatal health (for example, from the Public Health Agency of Canada) also examine rates of small-for-gestational age and preterm birth.4;5

One of the reasons for the increasing importance given to SGA births is that maternal tobacco smoking accounts for over one-third of SGA births in Western counties.6  Canadian data indicates that other factors associated with SGA are young maternal age (under 20), being a first time mother, maternal hypertension, low pre-pregnancy weight, low gestational weight gain, and social factors such as neighbourhood income and urban residence.4;6  Infants born SGA are at higher risk of stillbirth, neonatal and infant death, and diabetes and obesity later in life.7 

Large for gestational age births are also of public health interest.  LGA is defined as a birth weight that is above the 90th percentile of birth weights of the same sex and gestational age (in weeks), according to population based growth charts.3  Research on the risk factors associated with LGA is still emerging, although it appears that higher pre-pregnancy weight, higher gestational weight gain, gestational diabetes, and reduced smoking prevalence among pregnant women account for the increase in the proportion of LGA births over the last 30 years in Canada.8;9  Large for gestational age births are associated with higher rates of perinatal mortality and morbidity and with obstetrical complications for the mother.10

Figure 1. Rates for small and large for gestational age births by year, 2005-2013, KFL&A and Ontario¹¹

Interpretation for Figure 1

Figure 1 shows the rates of small, average, and large for gestational age births in KFL&A and Ontario between 2005 and 2013.¹¹  In 2013 in KFL&A, the SGA rate was 8.9 per 100 births, and the LGA rate was 9.3 per 100 births.  KFL&A had slightly lower rates of SGA, but higher rates of LGA, than Ontario for all years.  In both KFL&A and Ontario, the LGA rate was higher than the SGA rate.

Interpretation for Table 1

Most commonly, SGA and LGA are reported as births below the 10^th percentile and above the 90^th percentile of birth weight, respectively (using standard population values).  However, SGA are sometimes reported as the 3^rd or 5^th percentile, and LGA may be reported as the 95^th or 97^th percentile.  Table 1 displays these various SGA and LGA rates for KFL&A.

Figure 2. Rates for small, average and large for gestational age births by maternal age group, KFL&A 2012-2013¹²

Interpretation for Figure 2 and Table 2.

Figure 2 and Table 2 present the rate and average numbers of small, average, and large for gestational age births by maternal age for KFL&A in 2012-2013.¹²  Across all age groups, approximately three quarters of infants were average weight for gestational age (that is, they had a birth weight between the 10^th and 90^th percentile).  SGA rates were higher for younger mothers, particularly those under 25, whereas LGA rates were higher for older mothers, especially those over 40 (Figure 2).  These age-related patterns are consistent with Canadian data⁴ and observations in other Western countries, such as the United Kingdom.¹³

Figure 3. Low birthweight rate, KFL&A, 2012-2013¹²

Interpretation for Figure 3.

Figure 3 presents the low birth weight rate by maternal age in KFL&A.  The low birth weight rate represents the number of infants per 100 that weigh less than 2500 grams at birth.  The highest rate of low birth weight is among mothers aged 40 and over, with one in ten infants born with low birth weight.  

Figure 4 Low birth weight and small for gestational age rates

Interpretation for Figure 4.

Rates of low birth weight and SGA for preterm and full-term infants are shown in Figure 4.  About half of all preterm infants in KFL&A have a low birth weight (<2500 grams).  A low birth weight among preterm infants is common, as preterm infants miss several weeks of growth and development; however, only 14% of preterm infants are small for gestational age, which is usually a result of intrauterine growth restriction (IUGR).⁴  Most preterm infants in KFL&A are of average size for their gestational age.  It is also notable that 23.2% of SGA babies are admitted to the NICU (KFL&A, 2012-2013¹²).

1. United Nations Children's Fund, World Health Organization. Low Birthweight: Country, Regional and Global Estimates.  2004. New York, WHO Publications.
2. Ministry of Health and Long-Term Care. Ontario Public Health Standards.  2008.
3. Kramer MS, Platt RW, Wen SW et al. A new and improved population-based Canadian reference for birth weight for gestational age. Pediatrics 2001;108:E35.
4. Canadian Institute for Health Information. Too early, too small: A profile of small babies across Canada. 2009.
5. Canadian Perinatal Surveillance System. Perinatal Health Indicators for Canada 2013. Health Canada 2013.
6. Kramer MS. Determinants of low birth weight: methodological assessment and meta-analysis. Bull World Health Organ 1987;65:663-737.
7. Joss-Moore LA, Lane RH. The developmental origins of adult disease. Curr Opin Pediatr 2009;21:230-234.