The presence of a rib on the seventh cervical vertebra (a cervical rib) represents one of the most common intraspecific variations of the number of cervical vertebrae in mammals. Cervical ribs are highly associated with stillbirths, congenital abnormalities and embryonal tumours. These associations indicate strong stabilising selection against such a change to the highly conserved number of cervical vertebrae in humans. We propose, therefore, that the presence of variation for this highly conserved trait can be used as an indicator of medical risks. We have tested for prolonged effects of controlled ovarian hyperstimulation treatments (OHS) in mice by analysing the frequency of cervical ribs in the offspring of females that had received OHS treatment. We found that OHS treatment in mice had several significant effects on the offspring after adjusting for multiple pregnancy: these included an increase in cervical rib incidence, gestational period and nest size, and a decrease in birth weight and ossification, indicating growth retardation.The high incidence of cervical ribs in the OHS group compared to the control group (39.5% vs. 4.7%) indicates that the OHS treatment affects embryogenesis during a period that is highly sensitive to disturbance, the early organogenesis stage (phylotypic stage). This implies that in mice OHS treatment of the mother has a prolonged effect and continues during early pregnancy.