In S09E22, "The One with the Donor", we see Monica and Chandler go to the doctor where they find out their best options for conceiving are surrogacy, insemination and adoption.
I'm not the most knowledgeable about all these conceiving techniques, but wouldn't it have been an option for them to have an embryo created in vitro with Chandler's sperm and Monica's eggs and implanted in Monica's uterus?
We know she could get pregnant, because otherwise insemination wouldn't have been an option.
We also know that Chandler's problem is that his sperm has low motility, but I don't think that would come in the way of an in vitro conception
to recap, why didn't they consider in vitro fertilization?