Life before birth

Often when I tell people that I gravitate towards a belief in a "life after death" they say something along the lines that they didn't exist before they were born, so it is reasonable to suppose that likewise they will cease to exist when they die.

I'm in entire agreement with them.  The question which interests me is why they are implicitly supposing that I believe we all spring into being sometime between conception and birth, but then exist forevermore after that.  After all I always take great pains to stress that I have come to my beliefs from thinking through the issues and looking at the evidence, and emphatically not because I subscribe to any specific organised religion.  

There are a number of reasons why I find this hypothesis that we come into being sometime between conception and birth, and then exist forevermore, somewhat implausible.

To believe in a "life after death" -- at least in the sense of existing in some afterlife realm for some time -- entails that it cannot be our physical bodies which produces the self or consciousness.  But if my body -- specifically the brain -- doesn't produce consciousness, then there doesn't seem to be any reason why the self should come into being at the same time as the brain first forms.

Perhaps people might say that it is reasonable to suppose the self springs into being at this time since we do not have any memories prior to this.  Now there have been reports of people recalling memories before this time i.e apparent previous lives, but let's put these aside for the time being.    Although we don't typically remember any events before conception, we don't remember anything before about 5 years of age either.  In fact we only remember a very medium percentage of everything that has happened to us since then too.  Hence it seems the lack of memories cannot constitute a good reason to suppose we didn't exist.

In addition if the self can be created, then this seems to make it much more reasonable that it can cease to exist too.  In other words if we come into being at some point, then it is reasonable to suppose that at some point we pass away.  The other natural alternative is to suppose we have always existed, and always will exist.  But to imagine we pop into being at some specific point, but then never cease to be, constitutes an asymmetry which I just personally find awkward and implausible.  For one thing what caused us to come into being?  Why couldn't a similar cause or power make us cease to exist? 

Another reason to doubt the hypothesis is that if now I am born I will never cease to exist, it seems somewhat implausible that I should find myself in existence a mere few years after springing into being (. . well . . OK . . maybe more than a few, but you get my drift).   Why don't I find myself, for example, a quadrillion years after coming into being?

Furthermore there's very compelling evidence for reincarnation from Ian Stevenson and other researchers. Not just children who seem to remember previous lives, but birthmarks corresponding to the injury which killed the previous personality.  There are even children who report apparent memories from their time in the afterlife in between their incarnations on Earth.  If we spring into being sometime between conception and birth, then all this evidence needs to be explained away.

Finally I feel that I existed before I was conceived.  This final reason triggered some amusement amongst 2 skeptics on 2 differing independent occasions.  I'm not sure why.  Presumably they hold the position that what I feel in this matter cannot possible have any implications about what is actually the case.   But it could be the case there are deep memories percolating in my subconscious which are finding a vague expression in my conscious mind.  And besides, this was my last and least important reason to reject the hypothesis.