The question of when life begins is certainly a fraught question. Politicians have offered their opinions on the issue for decades. Barack Obama famously thinks that the answer to the question lies "above my pay grade." It lies at the crux of the abortion debate and has been discussed for thousands of years. So, whence a solution? From our shared Judeo-Christian culture and language – but the answer is not what you think!
It's well-known that in many cultures worldwide, abortion has been an accepted practice for millenia. Just a sampling: During the Edo period, abortion was completely accepted in Japanese culture. [Source] The Maori of New Zealand, too, regularly practiced abortion. [Source] The Cheyenne and Northern Plains Native Americans practiced abortion regularly. The Pima of New Mexico also performed abortions. Sanskrit-speaking Indians (i.e. from India) practiced abortion by sitting on a pot of steamed onions. [Source]
But what about our culture? It should be rather obvious that superficial cultural norms are susceptible to change by force; if a reform-minded king or a foreign conquerer decides to ban one behavior or moral or encourage another, the surface of the culture will change. But, traces of the original cultural worldview will still remain. I would argue that these traces are more indicative of the views of the common man in the culture. Thus, in regards to the original question, let us disregard the views imposed on the people of Western cultures by religious or political masters; the opinions of people like St. Augustine (who, incidentally, believed abortion was acceptable for the first 18 weeks or so) or the rabbis of the Talmud are irrelevant. The "traces" which are relevant exist in a number of places: for instance, while there are no pagans descending directly from pre-Christian pagans in Europe, traces of pagan culture remain in the Christmas celebration and the Mayday celebration's veneration of the phallus. I posit that further traces of the ancient Western culture at its most basic remain in the languages that shaped our culture. For our Western, Judeo-Christian culture, those languages would be Hebrew, Greek and Latin.
A brief digression: The beginning of life, for the religious and spiritual, is more or less universally understood to be at the time of ensoulment, that is, the time at which the being in question gets a soul or spirit. It is at this point that human beings are distinguished from the rest of life; it is when, logically, killing that being becomes substantively murder, rather than the lesser evil (or no evil at all) that is killing a non-human animal, plant or other being. So, if we want to figure out when life begins, we must find when the being becomes endowed with a spirit.
The linguistic/etymological evidence weighs rather conclusively to the idea that life begins at birth. It's rather simple, actually. In all three -- somewhat surprisingly -- of my "basic" languages formative of Western culture, the word for soul comes from a root having to do with breath. This indicates that ensoulment occurs when a being begins to breathe, which is at birth.
First, let's consider Hebrew. In Hebrew (and thus in the Old Testament), there are three words generally used to signify what we call, in English, the soul or spirit (more on the distinction in a moment). One is nephesh (נֶפשׁ) (NFSh), the next is neshamah (נשמה) (NShMH) and the third is ruach (רוח) (RVKh). Ruach, in modern Hebrew, means wind, and following rather clearly from that meaning, a secondary meaning of breath. This is attested in Genesis 1:2 where the "spirit of God" hovers over the water. Neshamah, too, means breath, as in Genesis 2:7, when
"The LORD God formed the man from the dust of the ground and breathed into his nostrils the breath of life, and the man became a living being." Finally, nephesh means breath as well, coming from the root "NPhSh," having to do with breathing and the throat, also attested to in Genesis 2:7, in which God breathes the "breath (NShMH) of life" into Adam, making him a "living being" (NphSh).
Now, let's consider Greek and Latin, which, as Indo-European languages, share many of the same roots, the relevant ones being the roots of Latin's spiritus and animus. Spiritus, coming from the verb spiro, means – as I'm sure you won't be surprised to learn – to breathe, and providing us with such words as perspire, respire, inspire and, of course, spirit. If that's not enough, animus, from which we get life-related terms like animate, comes directly from the Greek minor gods the Anemoi (ανεμοι) who represented the four winds. Just like ruach, the essential connection between a life and wind is... breath. Another relevant Greek word for the soul, psyche (ψυχη), which comes from a Proto-Indo-European root (*bhes-) meaning to blow.
As these few examples show, the Western concept of the soul is unavoidably entangled with the idea of breath. This offers us guidance as to our culture's concept of the soul – and thus the beginning of life. As the evidence shows, the idea that ensoulment occurs when breathing begins -- at birth. This should shift the burden onto the "pro-life" movement's side to demonstrate why, in their belief, life begins (that is, ensoulment occurs) before birth and before breathing begins.