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Results

1. Coverage attained by the database

Table 1 shows the population coverage attained by the database relative to studies identified as nationally representative conducted between 1985 and 1995. All data together are representing 90% of the estimated total number of live births in developing countries in 1995. Overall, there are data available for 106 out of 146 developing countries. Although the data refer to the period 1985 to 1995, the majority of the information corresponds to the first half of this period, with only 50 (37%) out of the total 136 data points based on studies from 1990 onward.

Table 1. Population coverage of LBW data identified as nationally representative in the WHO Database on Low Birth Weight (1985-1995)

^a Total live births in 1995 based on the UN World Population Prospects, 1995.
^b Includes Europe, North America, Australia, New Zealand and Japan.

Coverage in Africa

Information is available for 75% of the total live births in that region in 1995, with data for 40 out of 53 countries. Overall the coverage is high for Middle (89%) and Western Africa (93%), but low for Eastern (66%), Northern (64%) and Southern Africa (15%). Data are lacking from Burundi, Kenya, Somalia, Uganda and Zambia (in the east); Chad, Congo and Equatorial Guinea (in the middle); Egypt (in the north); South Africa (in the south); and Cape Verde, Liberia and Mali (in the west).

Coverage in Asia

Although only 33 out of 47 countries in Asia have information on the incidence of LBW between 1985 and 1995, the region is highly represented in the database, covering 95% of the total live births. Data are available for the most populous countries in this region such as Bangladesh, China, India, Indonesia and Pakistan. Looking at the coverage in the subregions, Western Asia has 80%, while Eastern, South-central and South-eastern Asia have a coverage rate of 95% or above. Countries still with no information are mostly in South-central Asia (Bhutan, Kazakstan, Kyrgyzstan, Nepal, Turkmenistan, and Uzbekistan) and Western Asia (Armenia, Azerbaijan, Georgia, Kuwait, Palestinian self-rule areas, and Yemen). Data are also lacking from the Democratic People's Republic of Korea (in Eastern Asia) and Lao People's Democratic Republic (in South-eastern Asia).

Coverage in Latin America

Countries from Latin America are well represented in the database (28 out of 33), reaching a coverage of almost 100% of the total number of live births. Those countries still not included in the database are mostly in the Caribbean (Antigua and Barbuda, Barbados, Grenada, and St Vincent and the Grenadines), but include also Belize in Central America.

Coverage in Oceania

Although only half of the countries in the region (8 out of 16) are represented in the database, the overall coverage for Oceania is very high (~ 100%), as it includes the most populous countries such as Australia, New Zealand, Papua New Guinea and Western Samoa. Micronesia is the only subregion with low coverage (16%). The following countries are still not included in the database: Kiribati, Marshall Islands, Federated States of Micronesia and Nauru (in Micronesia), and Cook Islands, Nine, Tonga and Tuvalu (in Polynesia).

2. Validation regression model to estimate incidence of IUGR-LBW

The IUGR-LBW rate estimated using the predictive equation was compared with the observed values in 17 selected data sets to test the agreement between the observed and the estimated rates. Table 2 presents for each of the 17 data sets the incidence of IUGR-LBW observed in the study compared to that derived from the regression model. Overall, the model underestimates the incidence of IUGR-LBW by a mean difference of -1.46% (95% confidence intervals of this mean difference: - 2.514% to 0.403%). Figure 3 presents a graph of the agreement between the observed and the estimated IUGR-LBW rates by plotting their difference against their mean. In 14 out of the 17 data sets, the regression model estimated a lower incidence of IUGR-LBW. The underestimation could be as low as 2.5% (lower bound of the 95% confidence interval) and seems fairly constant at all levels of the IUGR-LBW, although the distribution is wider at higher LBW rates more likely reflecting small number of observations.

3. Worldwide distribution IUGR-LBW

Figure 4 shows the distribution of developing countries according to the incidence of IUGR-LBW. Rates are grouped into four categories (< 5%, 5-10%, 10-15%, and ³ 15%), referred to as (relatively) low, moderate, high, and very high. Most developing countries in Latin America and the Caribbean show low to moderate prevalences of IUGR-LBW. The only exceptions are Colombia and Honduras with rates between 10 and 15%.

Africa, in turn, shows the greatest variability among countries, with prevalences going from as low as 1% in the Libyan Arab Jamahiriya or 1.5% in Zimbabwe to 18.1% in Zaire and Guinea. Overall, infants born in sub-Saharan countries are more likely to suffer from IUGR-LBW than their counterparts in the northern countries of Africa. Very high rates, i.e. ³ 15%, are found in Zaire, Angola, Guinea, and The Gambia. High prevalences (between 10 and 14%) are apparent in Tanzania, Rwanda, Niger, Nigeria, Togo, and Guinea-Bissau. However, as shown in Table 1, data are still missing for 13 out of the 53 countries in the region.

South-central Asian countries - which include Afghanistan, Bangladesh, India, Iran, Pakistan, Maldives, Sri Lanka and Tajikistan - show the highest rates worldwide. According to our estimates, Bangladesh presents the highest rate of all countries, with an IUGR-LBW incidence rate of 39.4%. In South-eastern Asia, the highest prevalences (in the range of 10-14%) are found in Myanmar and Cambodia. Eastern Asia shows prevalences of IUGR-LBW ranging from 4 to 7%, that is low and moderate levels, whereas in Western Asia the range goes from 1 to 6%. In this subregion the only exception is Bahrain for which a very high rate of IUGR-LBW (above 25%) has been estimated.

4. Global and regional estimates

Table 3 shows the global, regional and subregional estimates for the incidence of IUGR-LBW and LBW, together with the estimated absolute numbers of newborns affected. Estimates have been calculated only for those regions and subregions that are reasonably well covered (> 80%) by the database (see Table 1). Appendix I presents the incidence rates of LBW and the estimated IUGR-LBW rates used in the calculations for the 136 countries with data available on LBW for the period from 1985 to 1995. Information is also included on the year and source of data for each of the country data points.

In developing countries, one in nine live births (more than 13.5 million) IUGR-LBW newborns were born in 1995, accounting for 11% of the total newborn population that year. Regional data show that the incidence of IUGR-LBW is highest in Asia (12.3%), followed by Oceania (9.8%) (excluding Australia and New Zealand), and Latin America and the Caribbean (6.5%).

Although estimates for Africa could not be calculated because of insufficient data coverage, for the two subregions of Africa for which coverage reached > 80% - Middle and Western Africa the incidence of IUGR-LBW was 14.9% and 11.4%, respectively (Table 3). It would thus seem reasonable to assume that the overall IUGR-LBW rate for Africa as a whole probably follows that of Asia. In South-central Asia, the region which accounts for almost a third of the world's births, it is estimated that 21% of the newborns suffer from IUGR-LBW, i.e., more than 8.5 million each year or approximately one in five live newborns.

As previously mentioned, these global and regional levels refer exclusively to the incidence of IUGR infants that are also LBW (area labeled as A in Figure 2). However, the incidence rates of IUGR and IUGR-LBW observed in the 17 data sets included in the WHO Collaborative Study on Maternal Anthropometry and Pregnancy Outcomes (WHO, 1995b) give an estimate of the magnitude of the difference between the two rates. The definition of IUGR was birth weight below the 10th percentile of the birth-weight-for-gestational-age curve recommended for international use (de Onis and Habicht, 1996). Table 2 shows that the incidence rate of IUGR is consistently higher than that of IUGR-LBW in all data sets by a mean difference of 14.5% (95% confidence intervals of this mean difference: 10.9% to 18.1%). The mean IUGR rate is 23.8%, ranging from 9.4 in China to 54.2 in India (WHO, 1995b).

Table 2. Observed IUGR-LBW rate in 17 datasets from developing countries compared to the incidence of IUGR-LBW estimated using the regression model

* Y = - 3.2452 + 0.852 X (Villar et al, 1994).

** Difference between observed and estimated IUGR-LBW rates [mean difference = - 1.46 (95% CI: - 2.51 to -0.40)].

Source 17 data sets: WHO Collaborative Study on Maternal Anthropometry and Pregnancy Outcomes, 1995 (WHO, 1995b).

LMP = last menstrual period.

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