Seasonal variation studies on chromosomal abnormalities, especially Down syndrome, have been previously reported (Kallen and Masback, 1988). Different hypotheses have been made including hormonal imbalance and exposure to ionizing radiation. A very comprehensive review article on seasonal variation at birth concluded that there is no real evidence of seasonality in the birth prevalence of Down syndrome (Stolwijk et al., 1997). One of the various criticisms of the reviewed studies was that they did not take into account those fetuses with chromosomal anomalies that would be lost before term. It was, therefore, suggested that data obtained from prenatal diagnosis closer to the month of conception should be studied.2025-01-01