A heterotopic pregnancy happens when one embryo (or more) grows inside the uterus at the same time as one embryo (or more) grows outside of the uterus. Like any ectopic pregnancy, an embryo growing in the cervix, a fallopian tube, or against the outside wall of the uterus can be dangerous and must be removed.
Early in a heterotopic pregnancy, the ectopic embryo(s) can be easily overlooked, because there are clear signs of pregnancy in the uterus. That is why it is common for the first sign to be pain from a rupture of a fallopian tube or the cervix.
Overall, heterotopic pregnancies are not common. They are more common when assisted reproductive technology (ART) is used to become pregnant.