Birth order and number of siblings
Testicular cancer risk is 8% higher in firstborns, and risk decreases with increasing birth order, a meta-analysis showed. It is also 25% higher in males with no siblings versus those with 4+ siblings, and risk decreases with increasing number of siblings. Testicular cancer risk is 22-31% higher in males with a twin, meta-analyses have shown.[2,3] These attributes are associated with higher oestrogen levels in utero, but also with delayed childhood exposure to infections (linked with some other cancers), and parental subfertility, so the root cause remains unclear.
Events during pregnancy and birth
Testicular cancer risk is 33% higher in men whose mother had bleeding during pregnancy, a meta-analysis showed. It is 31% higher in men born prematurely, a meta-analysis showed; and may be around fourfold higher in those born extremely prematurely (22-29 weeks gestational age), a large cohort study showed. But testicular cancer risk is not associated with maternal nausea (though one study suggests an association with severe vomiting during pregnancy), hypertension or pre-eclampsia during pregnancy.[1,5] Nor is it associated with delivery (breech or caesarean), maternal age or body mass index (BMI) [1,6] Hormonal factors are implicated in some of these events/conditions.
In utero exposure to synthetic oestrogen diethylstilbestrol (DES) is classified by The International Agency for Research on Cancer (IARC) as a possible cause of testicular cancer, based on limited evidence. Testicular cancer risk may be around doubled in males exposed to DES in utero, though in meta-analysis the association was not significant. Cryptorchidism is around twice as common in DES-exposed men, a cohort study showed.