Baptism is a physical act that bestows supernatural grace; someone cannot baptize by mere intention alone. That means that the minister of baptism must be able to sprinkle or pour water on the baby, or immerse the baby in water, which cannot be done with amniotic fluid (even if we suppose that amniotic fluid is valid matter for the baptism).
The search to stretch the means by which baptism can validly be bestowed was more understandable in a time when the common opinion of theologians was that unborn babies could not go to heaven without baptism. Now that it is better understood that God is not bound by the sacraments and that he can bestow sanctifying grace outside of the sacraments to those who are innocently incapable of receiving it in any other way, there is no need to try to stretch how baptism may be validly conferred. All that is necessary is that parents do all that they can to have their children baptized within the first few weeks after birth (cf. CIC 867 §1; CCC 1261).