I got babybeets one from ebay from around £11 I got it when i was 14 - 16 weeks I think I was not sure worked great dont rent one babybeets is cheap and great it states on it that you can hear as early as 8 weeks but i wouldnt know as I didnt get it that early but worth a try i think
http://cgi.ebay.co.uk/4-ORIGINAL-Bab...QQcmdZViewItem
its gone up but no delievery charge so think its worth it