When one twin gets cancer, the other’s risk of facing it too rises, study shows

A large new study of twins has found that having a twin sibling diagnosed with cancer puts you at high risk of developing any form of cancer. A research team led by the Harvard T.H. Chan School of Public Health looked at more than 200,000 twins, both identical and fraternal, in Denmark, Finland, Norway, and Sweden, who were followed over an average of 32 years between 1943 and 2010. The researchers analysed both familial risk (an individual’s cancer risk) as well as the heritability of cancer, a measure of how much of the variation in cancer risk of populations is due to genetic factors.

Overall, one in three people in the study developed cancer over the course of a lifetime. In 3,316 of the pairs studied, cancer was diagnosed in both twins; the same cancer was diagnosed among 38 per cent of identical twins and 26 per cent of fraternal twins (who are similar genetically to siblings who aren’t twins). The researchers estimated that, when one fraternal twin was diagnosed with any cancer, the co-twin’s risk of getting cancer was 37 per cent; among identical twins, the risk jumped to 46 per cent. The heritability of cancer overall was 33 per cent. Significant heritability was found for skin melanoma (58 per cent), prostate cancer (57 per cent), non-melanoma skin cancer (43 per cent), ovarian cancer (39 per cent), kidney cancer (38 per cent), breast cancer (31 per cent), and uterine cancer (27 per cent).

Oral contraceptive use not associated with increased risk of birth defects

Oral contraceptives taken just before or during pregnancy do not increase the risk of birth defects, according to a new study by researchers from the Harvard T.H. Chan School of Public Health and the Statens Serum Institut in Denmark. They found that the prevalence of major birth defects was consistent (about 25 per 1,000 live births) across all pregnant women in the study population regardless of contraceptive use. The study, which appears in the BMJ, used data collected from multiple Danish health registries between 1997 and 2011 and linked by the unique personal identification number assigned to all residents of Denmark. The researchers looked at 880,694 live-born infants, and the health of these children at one-year follow-up. Among the women in the study, a fifth had never used oral contraceptives before becoming pregnant, and more than two-thirds had stopped using oral contraceptives at least three months before becoming pregnant. Eight per cent had discontinued use within three months of becoming pregnant, and 1 per cent had used oral contraceptives after becoming pregnant. The prevalence of birth defects was consistent across each category of oral contraceptive use, and remained so when the researchers added in pregnancies that ended as stillbirths or induced abortions.