Prevalence of birth defects  
among working and non-working women in Singapore

Introduction

The incidence of birth defects may vary from country to country. The observed differences may be due to ethnic differences in the community and to the varying socioeconomic status of pregnant women (1). On the other hand, the impact of environmental factors and genetic susceptiblity may also be factors contributing to birth defects (2). Workers in certain occupational and industrial groups have been reported to have a higher incidence of birth defects (3). There are three major ethnic groups in Singapore, but the socioeconomic differences between these groups are not large. The aims of this paper are: (i) to study the relationship between the prevalence of birth defects among working and non-working women in Singapore and (ii) to identify possible risk factors associated with birth defects.

Methods

Information on live births and cases of birth defects obtained from the relevant government agencies included the mother's date of birth, ethnic group and highest educational qualification, the occupation of the mother and father (4) and, if the mother was employed, the industrial sector involved (5).

All live births in Singapore are registered with the Ministry of Home Affairs' Registry of Births under the Registration of Births and Deaths Act. Since 1 January 1993, the Ministry of Health has maintained a National Birth Defects Register (NBDR) containing information on all children with major birth defects. The situation revealed conforms with the listing of the International Clearing House for Birth Defects Monitoring Systems. The registry obtains information on in-patient cases through arrangements with both government and private hospitals, while notifications are received from government and private outpatient clinics through standard notification forms.

The analyses in this paper cover births from 1 January 1994 to 31 December 1995.

Results

There were a total of 98,189 live births in Singapore between 1 January 1994 and 31 December 1995. Over the same period, 1,533 cases of birth defects were reported to the NBDR.

The mean ages of mothers whose babies birth defects and those whose babies were normal were 30.6 years and 30.1 years, respectively. This age difference was significant (p<0.000).

Among the ethnic groups, the lowest rate of birth defects (0.6%) was among the "Others" group, which includes Eurasians and Caucasians, although the number involved is small. The Indians appeared to have the highest rate - 1.9% (Table 1). This finding has also been reported in other, overseas studies.

Table 1. Prevalence of birth defects by ethnic group

Note: 120 cases did not have complete data on ethnic group

Educational level was connected with birth outcome. Mothers with lower educational levels had higher birth defect rates (Table 2). This trend was significant (p<0.01). However, the ethnic and age distributions may have been factors confounding this observation.

Table 2. Prevalence of birth defects by educational level

There were no significant correlations between the birth defect rates and fathers' occupations or industries in which they worked.

There was no significant difference between the birth defect rates for working (1.4%) and non-working mothers (1.5%). The highest birth defect rate, 2.2%, was observed among "Cleaners, labourers and related workers", while the "Legislators, senior officials and managers" group, at 1.0%, had the lowest birth defect rate (Table 3).

Table 3. Prevalence of birth defects by maternal occupational group

No significant association was noted between the rates of birth defects and the industries in which the mothers were working (Table 4).

Table 4. Prevalence of birth defects by the mother's employment sector

In Table 5, we look at the possible risk factors which may be associated with birth defects. The adjusted odds ratios (OD) for birth defects, using the Chinese group for comparison, were significantly greater for the Malay and Indian groups, which had ratios of 1.22 (1.07-1.40) and 1.43 (1.20-1.72), respectively. Compared with the Chinese, the "Others" ethnic group had significantly lower odds for birth defects (Table 5). When compared with non-working mothers, working mothers do not have a significantly higher risk of birth defects across the occupational spectrum. Although the risk was higher for the "Cleaners, labourers and related workers" (1.29) and lower for the "Legislators, senior officials and managers" (0.71) groups, neither association was significant (Table 5).

Table 5. Risk factors associated with birth defects

* Adjusted for age, educational status, ethnic groups and occupations of the mother by multiple logistic regression.

Discussion

In addition to the ethnicity factor mentioned above, consanguinity, high fertility and childbearing late in the reproductive period have been identified as factors contributing to higher birth defects rates (1,2).

The fact that workplace exposures do not appear to constitute a risk factor for women in Singapore may be related to the generally low chemical exposure levels indicated by the environmental and biological monitoring data of the Ministry of Manpower's Department of Industrial Health.

References

1. Terry PB, Bissenden JG, Condie RG, Mathew FM. Ethnic differences in congenital malformations. Arch Dis Childhood 1985;60:866-879.
2. Balarajan R, Raleigh VS, Botting B. Morality from congenital malformations in England and Wales: variations by mother's country of birth. Arch Dis Childhood 1989;64:1457-1462.
3. Koh D, Chia SE. Health Concerns of women at work in: Jeyaratnam J & Chia KS, eds. Occupational Health in National Development, Chapter 6 pp. 74-101.
4. Department of Statistics, Singapore. Singapore Standard Occupational Classification 1990. Singapore: Singapore National Printers, 1990.
5. Department of Statistics, Singapore. Singapore Standard Industrial Classification 1990. Singapore: Singapore National Printers, 1990.

Sin-Eng Chia
Department of Community, Occupational & Family Medicine,
National University of Singapore

Magdalene Chan
Department of Industrial Health, Ministry of Manpower ´

Bok Huay Foong
Ministry of Health, Singapore

Asian-Pacific Newsletter 2/1999 p.42-43