The embryo just needs a food source. Without modern medicine, it would have killed the mother. Now it is a baby.

Wikki ectopic pregnancy:
"An ectopic pregnancy is a complication of pregnancy in which the fertilized ovum is implanted in any tissue other than the uterine wall. Most ectopic pregnancies occur in the Fallopian tube (so-called tubal pregnancies), but implantation can also occur in the cervix, ovaries, and abdomen. The fetus produces enzymes that allow it to implant in varied types of tissues, and thus an embryo implanted elsewhere than the uterus can cause great tissue damage in its efforts to reach a sufficient supply of blood.
The causes of ectopic pregnancy are unknown. After...

The embryo just needs a food source. Without modern medicine, it would have killed the mother. Now it is a baby.

Wikki ectopic pregnancy:
"An ectopic pregnancy is a complication of pregnancy in which the fertilized ovum is implanted in any tissue other than the uterine wall. Most ectopic pregnancies occur in the Fallopian tube (so-called tubal pregnancies), but implantation can also occur in the cervix, ovaries, and abdomen. The fetus produces enzymes that allow it to implant in varied types of tissues, and thus an embryo implanted elsewhere than the uterus can cause great tissue damage in its efforts to reach a sufficient supply of blood.
The causes of ectopic pregnancy are unknown. After fertilization of the oocyte in the peritoneal cavity, the egg takes about nine days to migrate down the tube to the uterine cavity at which time it implants. Wherever the embryo finds itself at that time, it will begin to implant."

Life has a remarkable way of making things that seemed impossible, possible. It is not restricted to the limits we pretend to put on it.