There may be ways to reduce the number of unwanted pregnancies, and that would obviously be a good thing, but unwanted pregnancies will happen anyway, and you have to figure out what to do about them. If, *hypothetically*, we could objectively show that a fetus is only human after three months, then we can allow abortions up to that point. Since abortion is a valuable service to women suffering from an unwanted pregnancy, it's important to think about it. Or consider the morning-after pill: I think it's pretty clear that the fetus is just an insignificant lump of cells at that point. Banning that pill on the grounds that it might kill a "human" is asinine. Saying "you made a mistake, deal with it" solves nothing. You want to know what mistakes are okay to "correct", and let people correct them, not unilaterally declare that all mistakes of a certain type cannot be undone.
Because not knowing where a threshold is exactly does not mean we can set it to a value that's obviously incorrect. Consider the concept of a "large tomato" for example. You can pick a tomato and tell yourself that okay, that's a normal tomato. And you can pick some other tomato and tell yourself, man, that is a BIG tomato. Now, at what exact size is a tomato "big"? We don't really know that, and it's even somewhat subjective.
Your argument is akin to saying that because we don't know at what point a tomato is big, then we should say that all tomatoes are big. That is just plain false! If the embryo is not valuable after 30 seconds, then we can't decide that it is human before that, that would just be plainly incorrect. We don't know the exact point, but we do know it is AFTER that, so it's senseless to set it before.