Before the shailo I'll give a short introduction (based on what I saw on Wikipedia -- click the word for the link) on what a Ectopic Pregnancy (in Modern Hebrew, "הריון חוץ רחמי") is.
This is where the vlad is conceived outside of the rechem either in the Fallopian tubes, cervix, ovaries or abdomen of the isha.
An interesting shailo was asked by the Beis Horah today. What's the din by an isha that had an Ectopic Pregnancy and the vlad was removed from her guf through a certain procedure. If this was her first pregnancy and after this she become pregnant again and gave birth normally to a baby boy, what's the din in regards to Pidyon HaBen? Does the first pregnancy make it that a Pidyon HaBen cannot be made now?
This shailo is of course discussed in the Sefer Nishmas Avraham (see here). He brings there from the Shevat HaLevi that a Pidyon HaBen can yes be made for the baby boy born after this first pregnancy. See here for the Shevat HaLevi inside (Chelek 8 Siman 239).