Question: Where does the blastocyst implant to become an embryo?
Disambiguated questions and answers: question: In what lining does the blastocyst implant, in order to become an embryo?
In what organ does the blastocyst implant, in order to become an embryo?
Where does the blastocyst implant when it's an ectopic pregnancy?
answer: endometrium
uterus
fallopian tube
Natural Questions answer: against the endometrium (from the Wikipedia article "Implantation (human embryo)")