Medical Genetics Biological factors, including illnesses The rubella virus, which causes german measles, poses a serious hazard to the developing baby. It can cross the placenta and attack the baby to cause a range of defects. The proportion of mothers infected with the rubella virus during the first 12 weeks of pregnancy who have an abnormal child can reach 20 per centMedical Genetics Biological factors, including illnesses The rubella virus, which causes German measles, poses a serious hazard to the developing baby. It can cross the placenta and attack the baby to cause a range of defects. The proportion of mothers infected with the rubella virus during the first 12 weeks of pregnancy who have an abnormal child can reach 20 per cent