Asking this question requires you to make arbitrary choices.

Are you going to use the most common current definition of life that biologists use? Because that definition isn't the only one that has been used, and is a source of some debate among biologists. That very definition itself is arbitrary.

Even if we go with that definition, and most people choose to, we still have different scientists saying different things. Each of the following points has been supported by multiple scientists:

Conception (genetically distinct, but still far from being biologically distinct/viable. Also, fewer than 30% of fertilized eggs ever become fetuses. For these and other reasons, few scientists support this anymore)

Gastrulation (about 14 days)

(It should be noted that sex isn't determined until about 6 weeks in. Since this is one of the most important and personal aspects of being a human, some argue that we can't be considered independent beings until some time after this point. )

Start of a recognizable EEG pattern (about 24-27 weeks in)

(Some scientists have aegued that the heartbeat would be the point, but since this is an autonomous muscle function many argue that this is excessively arbitrary)

Viability (the ability to carry on all normal functions outside of the womb without assistance. Usually believed to be about 25 weeks, although more gestation time is seriously recommended and preferred when possible.)

Birth

So first you've got to decide which is the most valid. This will be arbitrary and personal. If even biologists (people who spend their lives studying life) can't come to a uniform consensus of when life starts or what life even truly is, your decision can't be taken as fact. I know it sounds counter-intuitive to saw "when life starts" is an opinion question. But for all intents and purposes, it is, at least at this point.

The other problem is every fetus gestates differently. Most of the points I listed above exist on a continuum instead of at a specific day in the pregnancy. And there are recorded cases reaching that point far earlier or far later than normal for almost every category.

This whole thing would be easier if we could highlight a soul or some other "essence of life". But after all these thousands of years, billions of births and more research than almost any other aspect of human existence, such a thing still can't be found. If we can't even prove that such a thing exists, how can we say when it entered the body?

Again, our choice would be arbitrary, and in this case, we don't even have any evidence or tests we can do to make a more informed decision.

The way I see it, it is a gradual process, one that varies from pregnancy to pregnancy, and life itself is practically an artificial concept. There is no "point when life begins". But one thing we can be sure of, if that baby is alive at birth, it is independent and generally viable. That is the ONE point when we can be 100% certain that it is a life form.

When the spirit enters the vessel.

Uhhhh.... I think it already has, like, a long time ago. Otherwise, who are we?

That depends somewhat on ones terminology, and entirely upon ones moral sentiment.

It ultimately depends on what you mean by the term 'human life,' which is a pretty ambiguous statement.

In terms of being biologically alive, fertilization. It's true that the sperm and egg cells are themselves alive prior to fertilization, but these aren't organisms themselves rather than part of the tissues of the parents bodies; it only becomes a distinct organism at fertilization.

Of course, that's only one definition. True, a zygote is biologically alive, but so is a brain dead individual, and so is the follicle from a hair that has just been removed. I turn, at this point, to the characteristics of life

1-3, cells/organization/energy use, are present from the moment of fertilization. What about others?

4, response to the environment, is reliant on the nervous system in our family of organisms. This begins developing in the first trimester, but does not develop the capacity to actually receive or respond to even the most basic of stimuli until well into the second trimester in most cases; a fetus does not meet this criterion prior to this.

5, growth, begins almost immediately after fertilization.

6, reproduction, is possible from fertilization on in the form of cellular division. While this isn't generally considered reproduction given that we are multi-cellular organisms, twinning can occur as a result of cellular division, and twinning is a form of reproduction, strictly speaking. "Normal" reproduction isn't possible in most cases prior to the onset of puberty, but I'll give the benefit of the doubt and mark this down at fertilization.

7, adaptation, is similarly reliant on information from the nervous system, and as such can't really occur prior to the second trimester.

So, we've got two working definitions now.

A newly fertilized zygote is biologically alive in a technical sense, just as a brain dead individual, or a recently shed drop of blood is.

A second semester fetus is just beginning to meet all of the criteria we use to define life. It is alive in the same sense that a spider is alive, or grass is alive.

Remember, though, that we're talking about human life here- the second trimester fetus meets the same criteria that a blade of grass or a spider meets, and neither of these are considered to be comparable to human life by most. Even vegans still consume plants, which are considered alive by the same criteria.

The final piece of the puzzle is labelled differently by different people; I usually go with 'personhood' as a catch-all term for self-awareness, personality, and those various traits that differentiate an individual. These are quite important; if these are completely overlooked, identical twins could be considered as the same person, and we would similarly be compelled to keep brain dead individuals on life support indefinitely. There is no clear point where these qualities emerge, though. The process of their development begins mostly from birth, as the brain requires stimulation from external stimuli that aren't present in the womb (or at least, not in a manner conducive to such developments). Self awareness CAN develop in an infant as young as 6 months, but it generally doesn't; this usually develops somewhere between the 1 and 2 year mark, typically at around 18 months. Personality is a lifelong developmental process, but is similarly sufficiently developed to call a personality at around 2-3 years of age, earlier on very rare occasion.

So now, we've got three breakpoints.

A newly fertilized zygote is biologically alive in a technical sense, just as a brain dead individual, or a recently shed drop of blood is.

A second semester fetus is just beginning to meet all of the criteria we use to define life. It is alive in the same sense that a spider is alive, or grass is alive.

A 2-3 year old human has generally developed self awareness and a personality. It is alive and a person in the same sense that we generally attribute to other people.

From this, I would favor the 2-3 year old mark. Calling a zygote a human life is not technically incorrect, as it is human, and it is alive. However, that same definition is applicable to a random blood cell in a spilled drop of blood- I don't think many are likely to call a drop of blood a human life.

That said, these topics are almost always backhanded attempts to discuss and/or justify varying stances on abortion. Personally, I don't condone abortion beyond the first trimester except in very specific scenarios; I don't think human qualities should be the only consideration in this. As noted before, much of the nervous system development occurs in the second trimester, such that fetuses at this point- while not self aware by any stretch- are still capable of feeling pain and fear at a very primal level, just as most animals can- it meets the same criteria for life at this stage that every other animal does. I don't condone putting an animal down just because it's an inconvenience, and it would be quite strange to make an exception to this simply because the animal in question happens to have human dna. Specific scenarios might justify aborting a second or even third trimester fetus, such as a very high threat to the mothers life (a rarity in the modern developed world, but FAR from unheard of)- I would similarly feel justified in killing an animal that represented a significant threat to me or mine.

That's ultimately what it boils down to. At fertilization, the zygote is alive only in a technical sense. From the second trimester through the first couple of years of life, it's essentially an animal (I know that humans are animals technically speaking, but I'm using it in a broader sense regarding the presence or absence of human-specific characteristics). It's not really a person until the 2-3 year mark, though I'll admit to a tendency to treat them as such earlier than that, biological predispositions and all. Any parent knows that for the first couple of years, the little guys are more like very needy pets than little people ;P

Seven.             

I were so brutal to everyone who was pro-life before. Less than two weeks ago you said:

"If you say that abortion should be banned, you are saying that women should be forced to give birth.".

"Forced birth is what makes me sick"

You should be more careful about what you say. I found these very offensive.

Then Bible is clear. It is at birth. Right?

Genesis 2:7, Ezekiel 37:5-6, Genesis 38:24

I am now prolife and I think life begins at implantation. I make an exception for the life of the mother, though.

Can you clarify please? So... a fetus is alive at the time of implantation unless then mother's life is at risk and then fetus is dead to you? I find that difficult to comprehend.

Do you want the science answer, or do you want a debate answer? Because tbh I don't know the science answer, and have never thought to look for it because when it comes to abortion, I rate the baby's life as worthless when compared to the mother's, however that can't stop you from searching it and having that information readily on hand.

As for the debate answer, I say life begins when you don't need someone else physically attached to you to live. Well a separate life that is. If the fetus can be extracted, and survive, even if it's hooked to machines, then by all means it's baby, and it deserves it's life, but if it has no other option asides from living in the mother and waiting to be able to even survive just breathing outside of the mother, then it's hardly a human life.

This is just a matter of semantics.

Ugh, yet again you use your debates as a place to advertise your views, rather than to foster any meaningful discussion.

Doesn't really matter. Killing a fetus is still killing it, it's only murder if it's outlawed.