A combination of an extra-uterine and an intra-uterine pregnancy is defined as heterotopic twin pregnancy. A case is reported where the 2 infants were born alive and have had a normal psychomotor development at 3 years of age. It is a case where the first observation taken by ultrasound made the diagnosis possible at 27 weeks of the pregnancy and therefore gave rise to the crucial problem of how to manage the case. Because of this case history we have discussed the frequency (one in every 30,000 pregnancies), the aetiological factors, the clinical factors, the prognostic factors and what is to be done. The discussion takes note of the modern ways of diagnosing it (ultrasound), of monitoring it and of handling in a specialised unit this exceptional kind of "high risk" pregnancy.