Compared to previous studies in China, this study had larger sample size to estimate prevalence rate and to explore the associations between possible risk factors and birth defects by controlling for other predictive variables respectively through multivariate analysis. The prevalence rate of birth defects in Inner Mongolia was 156.1 per 10000 births (95%CI: 146.3-165.8 per 10000), which is consistent with the prevalence rate in Gansu Province, western China (154.0 per 10000 births)
]. The prevalence rate of neural tube defect was found to be the highest (20.1 per 10000) in this study, which was shown to be higher than that in other provinces of China (15.9 per 10000) and even higher than the prevalence in Korea (5.1 per 10000)
]. The prevalence of cleft lip and palate together and congenital hydrocephalus in Inner Mongolia (16.2 per 10000 and 9.9 per 10000, respectively) were also higher than the prevalence in Korea (10.3 per 10000 and 3.6 per 10000, respectively) and some provinces of China (13.6 per 10000 and 6.5 per 10000, respectively)
]. The prevalence of congenital heart disease in this study (17.1 per 10000) was lower than in Belgium (83.0 per 10000)
], but similar to the prevalence in Colombia (12.0 per 10000)
]. One reason for the lower prevalence of congenital heart disease may due to the diagnostic criteria which only included severe congenital heart disease in this study, which is consistent with the criteria used in a study in Taiwan (14.2 per 10000)
]. The difference of prevalence rates among different regions either in China or other countries may due to the period children were observed after birth, the types of birth defects, different data collection method, method of statistical analysis
There might be several possible reasons for the higher risk of birth defects we observed in families living in rural areas. First, living, and especially working, in rural areas may bring people into contact with substances related to birth defects that are not present in urban areas. The results of one study indicated an association between birth defects and maternal agricultural work
]. Second, different lifestyles and habits of mothers or their living conditions or taking nutrition supplements may differ between rural and urban areas, for example, whether the use of multivitamin supplements is promoted during pregnancy or not in rural areas. A study showed that multivitamin and folic acid supplementation before the pregnancy can reduce the overall occurrence of congenital abnormalities and neural-tube defects
]. The RR of birth defects among mothers with prenatal alcohol exposure was 3.22 (95% CI: 2.33-4.32) in this study. The result is slightly differed from a study which reported an adjusted odds ratio of 4.6 (95% CI: 1.5-14.3) for mothers with heavy prenatal alcohol exposure in the first trimester of pregnancy
In this study, we found no association between prenatal smoking habits of mothers and birth defects. The result is similar to a study showed that the offspring exposed to prenatal tobacco smoke had no increase in the prevalence of congenital malformations compared with non-exposed offspring in both crude and adjusted analyses
This study found that younger mothers aged <25 years had a greatly increased risk of birth defects in their offspring, which is consistent with a research in China
]. The authors found a statistically significant difference in prevalence of birth defect among different maternal education levels. This difference may indirectly duo to mothers’ socioeconomic status, dietary habits, neighborhood conditions, and access to health care and appropriate foods, as well as the mother’s knowledge about the importance of folic acid in the diet
]. Previous studies showed that offspring with parental consanguinity had an increased risk for major birth defects
], but parental consanguinity was not predictive in this study. Given a history of similar defects in first-degree relatives, the relative risk of birth defects increased from a study of Denmark
]. Besides, another study of Xinjiang Autonomous Region showed that the risk of birth defects was higher among ethnic Han Chinese than other ethnicities
]. The results in this study also indicated that familial inheritance and ethnicity were related to birth defects.
The following is the limitations of the study: 1) it may could not include all cases with birth defect due to the limitations of diagnostic methods, for example, defects with insignificant symptoms are not likely to be found in early years of childhood; 2) perinatal death prior to diagnosis was also not included in this study.
Through this study, it found lower risk of birth defects in Mongolian than Han Chinese and other ethnic groups, which need further research on the influence of genetics or cultural and environmental factors on birth defects among Mongols. This study also found that maternal age less than 25, alcohol drinking, familiar inheritance, lower education level of mothers, times of pregnancies and living in rural areas may increase the risk of birth defects. Moreover, some environmental factors such as underground water contamination due to mining activities, agriculture chemical exposure such as overdose using of pesticides etc. should be explored in the future. Based on the results which was obtained in this study, a few intervention activities are recommend as follows: 1) regular screening tests among pregnant women are needed at local health care sectors to decrease the rate of birth defects in Inner Mongolia; 2) in rural areas, health education programs among females at reproductive age and among pregnant women are also necessary to avoid exposures to preventable risk factors of birth defects; 3) health education for pregnant women at younger age both in rural and urban areas should be taken into consideration as part of the interventions at their first visit to the hospitals.