Women who give birth to at least one child have a 44 per cent lower risk of lung cancer, a National University of Singapore (NUS) study has found, after it analysed records of more than 28,000 women.
A total of 311 women developed lung cancer during the study period of over a year, starting from last year. Of this number, 253 or 81 per cent did not smoke. It mirrored the trend found among the total number of women who took part in the study: More than 93 per cent were non-smokers.
In fact, researchers started this study based on a medical mystery: That most Asian women who died of lung cancer did not smoke.
Dr Tan Min-Han, adjunct assistant professor at NUS' Saw Swee Hock School of Public Health (SSHSPH), who co-authored the study, said: "Eight out of 10 women here with lung cancer do not smoke. No one really knows why they get lung cancer. Is it air pollution or other environmental factors?"
Lung cancer is the second leading cause of cancer among women here, after breast cancer, he noted.
Researchers from the National Cancer Centre Singapore and SSHSPH then set out to find out if female hormones affected lung cancer. The women who were studied had taken part in the Singapore Breast Cancer Screening Project from October 1994 to February 1997 and were aged between 50 and 64 then.
Chinese women made up about 84 per cent of the study cohort.
What they found was that most hormonal factors, such as age at menopause, reproductive period, age at first delivery, breast-feeding, oral contraceptives and hormone therapy, were not significantly associated with a higher risk of cancer.
But they found a link between childbirth and the cancer.
"In this large cohort study, having at least one child is associated with a substantial reduction in lung cancer risk for both smoking and non-smoking women," said Dr Tan.
Admitting that the researchers who did the study did not have a definite answer as to why women who give birth were at lower risk, Dr Tan speculated that the body's immune system - changed by pregnancy - may increase protection against lung cancer. That will be the next stage of research: To find out if there is a definite link.
He said: "I would think of an immune cause. It may be that the maturation of the immune system may increase protection during lung cancer. We have seen previous studies where after childbirth, there were decreased risks of breast cancer."