Definition:
A rare type of multiple pregnancy involving one viable pregnancy implanted in the uterus and the other implanted elsewhere as an ectopic pregnancy. Heterotopic pregnancies occur very rarely in couples who do not use assisted reproduction to conceive, with rates less than 1 in 30,000 naturally occurring pregnancies. Heterotopic pregnancies can occur in as many as 1 in 100 couples who conceive through assisted reproduction.
Source
Ludwig, M., "Heterotopic pregnancy in a spontaneous cycle: do not forget about it!" European Journal of Obstetrics & Gynecology and Reproductive Biology Nov 1999. Accessed 5 Jun 2008.
