The sequential cohort study included data from 749 monozygotic twin pairs and 687 dizygotic twin pairs between the age of 8 and 17 and their parents.
From the sciencedaily.com
Heterotopic pregnancy is an exceedingly rare type of dizygotic twinning in which one twin implants in the uterus as normal and the other remains in the fallopian tube as an ectopic pregnancy.