Sreeramareddy et al. BMC Public Health 2011, 11:403 http://www.biomedcentral.com/1471-2458/11/403 Page 5 of 10 Table 1 Distribution of sample of birth weights (from recall and card) according to main exposure variable and selected socio-demographic factors Not weighed at birth Do not know Total births 22981(24.4) 2404 (60.9) 15445 (79.5) 3965 (58.5) 5790 (33.3) 244 (6.9) 5647 (31.3) 19798 (68.1) 25444 (54.0) Variable Type of fuel High pollution Low Pollution Educational status No education Primary Secondary Higher Urban Rural Total births Type of residence Weighed at birth From card 1441 (66.9) 1672 (19.3) 340 (1.7) 298 (4.4) 1858 (10.7) 617 (17.4) 1958 (10.9) 1155 (4.0) 3113 (6.6) From recall 8387 (4.2) 7779 (13.1) 2798 (14.4) 2117 (31.2) 8721(50.2) 2530 (71.2) 9203 (51.0) 6963 (23.9) 16166 (34.3) * Figures in parenthesis are row percentages. gender of the baby and birth order were statistically controlled, the adjusted OR for the association i.e. effect of cooking fuel on birth weight was the highest (adjusted OR 1.41; 95% CI 1.29, 1.57). In model-1, the female gender has a protective effect for LBW or females babies tended to have a higher birth weight and similarly higher birth order showed a protective effect for LBW i.e. higher birth order babies had a higher birth weight. In model-2, when maternal characteristics like age at child birth, maternal smoking, educational status, BMI and haemoglobin were added, the effect of cooking fuel on birth weight remained significant but effect size decreased (adjusted OR 1.21 95% CI 1.06, 1.32). This decreased effect may be have been due to positive effect shown by BMI (p = 0.0065) and haemo- globin level (p = 0.0019) for birth weight. Further, the effect of cooking fuel on birth weight may have been confounded by maternal education which was protective for LBW. The reason for this could be that women of higher educational status belonged to households using cleaner fuels. Baby girls were likely to have higher birth Table 2 Distribution of size at birth according to type of fuels for births during five years of prior to the survey Size at birth Type of fuel Very large Larger than average Average Very small Total Smaller than average 5288 (15.38) High pollution Low Pollution 1181(3.44) 6280 (18.27) 19270 (56.07) 2346 (6.82) 34365 (100) 590 (4.62) 2676 (20.95) 7492 (58.65) 1466 (11.47) 550 (4.30) 12774 (100) Chi square = 276.35 degrees of freedom = 4, p = < 0.001. Total (%) 1771 (3.76) 8956 (19.0) 26762 (56.77) 6754 (14.33) 2896 (6.14) 47139 (100) 1556 (4.5) 859 (6.7) 948(4.4) 396 (5.8) 1008 (5.8) 163 (4.6) 1288 (6.9) 1177 (4.0) 2415 (5.1) 34365 (100) 12774 (100) 19431 (100) 6776 (100) 17377 (100) 3554 (100) 18046 (100) 29093 (100) 47139 (100) weight and higher birth order was positively associated with birth weight, in model-2 as well. Maternal smoking and mother’s age at child birth were not statistically sig- nificant. In full model (model-3) in addition to child and maternal characteristics, demographic variables like reli- gion, wealth index, type of residence (urban/rural) were also statistically controlled. In model-3, there was no association (p = 0.26) between cooking with biomass fuels and birth weight (adjusted OR 1.07, 95% 0.94, 1.22). The type of residence was not associated with birth weight (p = 0.68). However, wealth index had a positive association with birth weight i.e. higher wealth index had higher birth weight (p = 0.0011, adjusted OR 0.93 95% CI 0.89, 0.97). Wealth index might have masked the association between cooking fuel and birth weight in model-3. This may imply that people with higher income could afford to buy cleaner fuels which are usually more expensive. The effect of child’s gender, birth order, mother’s education, BMI, and haemoglobin level on birth weight remained significant. Age at child birth and maternal smoking which was not significant in models one and two were not significant in the final model as well. Table 3 Test of concordance between birth weight of child (from recall and health card) and size at birth Birth weight of the child Total < 2500 grams 1823 (46.9) 2066 (53.1) 3889 ≥ 2500 grams 13990 (90.9) 1388 (9.1) 15389 15813 3454 19267 Greater than or equal to average Less than average Total