For proper classification, the birth weight and gestational age (GA) of all newborns must be plotted according to an appropriate standard.¹ This has been the practice in most newborn units over the last 40 years. As was observed by Thomas et al.,² this practice has been based on the recognition that variance in GA (small or large for gestational age) predicts short-term morbidity and mortality.

Anthropometric parameters, particularly birth weight in relation to GA distribution, may vary from one population to another. Some of the reasons for this variability include socioeconomic factors, race, altitude, and incidence of environmental factors, such as smoking and time of year, which affect the birth weight.^3-6

The most widely available intrauterine growth charts are those constructed over three decades ago.^7,8 There are, however, important limitations to the use of these charts in Nigeria. This is because they were either derived mainly from a population of Caucasian babies or at high altitudes;⁷ neither are typical characteristics of the Nigerian newborn. Baby birth weight has been reported from various parts of Nigeria but a standard growth chart has not been evolved for use in this country.^9-11 In 1981, Olowe developed an intrauterine chart from a small population of Nigeria children, largely using a sample of only 436 patients from the country’s commercial city of Lagos.¹² This chart has, therefore, not been widely used in the country and most newborn units continue to use the Lubchenco chart.⁷

Various studies have, however, shown that failure to account for racial differences in rates of intrauterine growth when these charts derived from Caucasians are used, leads to inaccurate diagnosis of fetal growth abnormalities in those non-Caucasian newborn.^13-16 These studies have observed that, on average, non-white infants are smaller at birth than white infants. These differences persist after the data are controlled for such variables as nutrition, pre-natal care and socio-economic status.¹⁷ Growth charts are now available for various countries and populations within the same country.^17-19 These charts are usually population-based and produced according to gender, race and type of delivery.^20,21

The objective of this study was to generate birth weight for GA curves in a population of Nigerian newborn to guide the appropriate classification of these babies. It will also provide a basis for comparison of birth weights with results from other parts of the world in order to determine the appropriateness or otherwise of growth curves developed in other populations.

A total of 5275 babies were recruited to the study with birth weights ranging from 400 to 4300 g. GA ranged from 24 to 45 weeks. One baby had GA of 24 weeks, and one had GA of 45 weeks; both of these babies were excluded from the data analysis. Of the 5273 analyzed, 844 (16%) were pre-term, 4334 (82.2%) were term while 95 (1.8%) were post-term. There were 2690 (51%) males and 2583 (49%) females. There was insufficient representation of newborns with GA under 26 weeks.

Tables 1-3 show the distribution of the babies according to GA and the percentile intervals of their weight, length and head circumference. Figure 1 reflects the smoothed curves for the 10^th, 50^th and 90^th of the weights for the respective GA.

Figure 2 shows the comparison of the mean birth weights for our study and that of Lubchenco et al.⁷ The comparison of the average growth curves with that of the Lubchenco chart showed that the babies in these studies were generally lighter at the early stages of gestation while this difference was virtually nil at term. Thereafter, the average growth curve had a steep decline for the post-term category but this was not measured in the Colorado study and, therefore, no comparison could be made. With respect to gender, the data showered a fairly similar growth curve at the early weeks of gestation from 26-36 weeks. However, beyond this point there was a consistent increase in the average weight of the male babies compared to the females (Figure 3).

The importance of the intrauterine growth curve described by Lubchencho^, et al.,⁷ Usher and McLean over three decades ago is proved by the fact that they are still used in neonatal units today.⁸ They serve as standard references to classify newborns as small for GA, appropriate for GA and large for GA. Incorrect classification may then lead to failure in recognizing those at risk or lead to unnecessary tests and treatment in normal babies.

This study revealed two important characteristics in relation to the Lubchenco chart. The first is that for GA of less than 37 weeks, the babies tended to weigh less. Secondly, there was a sharp decline in the weight for GA immediately pregnancy went beyond 42 completed weeks of gestation. The Lubchenco chart did not report the weight of babies born after 42 weeks. Babies weighed less despite the study having been conducted practically at sea level compared with Denver, Colorado (5000 ft above sea level), and this may be in agreement with observations by Thomas et al.² In their study by the Pediatrix medical group, these authors found altitude as an independent variable might not significantly affect birth weight. This was a multicenter study carried out in 85 neonatal units at various altitudes (below or over 4000 feet) in the United States. The group also found that race is important as black babies were found to weigh less than white babies. It was suggested that this reflected either a biological or a pathological effect as black mothers were more prone to pregnancy-associated hypertension. However, the current study did not consider the effect of hypertension and only included black babies. It is, however, unlikely that hypertension would be the basis of the observed decline in birth weight. It would have been expected that the impact of hypertension in pregnancy on the fetal weight should increase with GA.^23,24 This was not found in the current study. Instead, the authors found a more divergent mean birth weight with younger fetal age while hardly any divergence was seen from 37 through to 42 weeks of gestation. It is, therefore, more probable that this is a reflection of racial or other socio-biological differences in the early fetal growth pattern. With respect to the marked reduction in the weight percentiles after 42-weeks gestation, there were no data with which to compare this category. There were also very few babies in this category with the possibility of underrepresentation. However, the decline in weight for this GA category is not totally unexpected. The sharp decline in birth weight of post-term babies is largely associated with placental insufficiency.²⁵ This justifies the 42^nd week expectative management of pregnancies with GA beyond 40 weeks.²⁶

An intrauterine growth curve for Nigerian babies has been developed. Comparison with curves developed in Denver, Colorado, showed that Nigerian pre-term babies weighed slightly less than the Caucasian babies, the implication being that use of the Lubchenco chart in classifying Nigerian newborn may result in some babies being wrongly classified with consequent implications. However, the impact of these differences in the mean birth weight on the newborn classification requires further evaluation.