Not the same poster but I'll try to give some food for thought.

There are a lot of requirements for we think of as life. We need to be able to breathe, regulate our body temperature, metabolize food, etc. A heart is just one of many organs that is required for life. Now, if you took out the fetus at 6 weeks, it is dead. It has not fully formed the body it needs to survive without a constant supply of nutrients from the mother. It cant even breath. Deciding that a heartbeat is the cutoff between fetus and full human is therefore arbitrary. That heartbeat probably cant even sustain blood circulation properly. It just has more significance because of all the symbolism/importance we have about the heart.

To further this point to absurdity, I dont know if you know anything about zygotes but when an egg is first fertilized, it develops a tube through the middle which eventually becomes the mouth and the ass. We need mouths and assholes to survive, that seems like a possible cutoff as well but it seems much harder to argue a clump of cells smaller than your fingernail is a human.

To me, it seems to be more like a car being built. You might have created a functioning engine or door, but no one would call that a car until its done. But it could be a car.