Answer

One can think of various arguments either way, and I’m sure you can find them with an online search. But there is no fallacy here, because in everyday language, in many cases we state the facts without stating the bridging principle, because it seems obvious to us. It’s like saying that murder is forbidden because if you murder someone, he dies. There is no fallacy here, because this formulation is not a precise philosophical formulation but an everyday one. It hides behind it another assumption: that death is a bad thing. The same is true regarding motherhood. Someone who argues in favor of the egg or the womb is assuming some bridging premise, even if he does not always spell it out. I assume that generally he will spell it out as well.