hidden var return only null why?

I have assigned a value to a hidden var.
and tried to access it in redirected page.
but it return only null. I also checked this with runat="server".
i also checked with alert, it correctly assigned but i can't acces it in code behind.
Markup:
-------------
<input type="hidden" id="hdnMemberId" name="hdnMemberId" />
Script
-------------------
<script type="text/javascript">
function openMemberDetails(pId) {
document.getElementById('hdnMemberId').value = pId;
window.location.href = "/Member/ProfileUpdate.aspx";          .. In this page i have access the hidden var..
}
</script>
Code behind
------------------
string hdnMemberId = Request.Form["hdnMemberId"];


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