A prenatal chromosome analysis for a parental Robertsonian translocation 13/14 is absolutely imperative. In the present case of a twin pregnancy, it was possible to present the chromosome complement of only one foetus. Since this foetus showed the Robertsonian translocation in the same form as the phenotypically healthy mother, and since the ultrasound findings were normal, it was concluded that foetus I was normal. Further measures for the cytogenetic analysis of foetus II were abandoned after the first amniotic sample could not be analysed. At the time of the amniocentesis this child was retarded when compared to foetus I. Since the life expectancy of a possibly chromosomally unbalanced child is slight, it was not necessary for the parents to take further steps. During the rest of the pregnancy however, this difference in the development of the children evened out unexpectedly and two phenotypically healthy children were born. The implications of a delayed growth rate as an indication of abnormal foetal development are discussed.