Hey, I know exactly how you feel as that's how I was in the beginning of my pregnancy and now I'm due tomorrow. I was only 16 myself when I fell pregnant and have just turned 17.
I wouldn't suggest leaving it that long as I did that and unfortunately my mum found out through other parents which found out and she was devastated that I didn't turn to her for help in the beginning and then she was even more upset that she didn't find out from me herself.
Parents would always take such shocking news better if you was to speak to them first about it and your options, not all parents react great at first it may cause a week or so of distance whilst you parent gets used to the idea of their baby having a baby. But there is so much support out there not just your parents.. luckily my mum is so happy for the whole pregnancy and a baby now but it took her a month to start talking to me again. There's negatives and positives to every situation.. now I'm due tomorrow (15th july 2014) and I cannot wait.
Good luck with everything.. best wishes xx