Im 34 my Mums 56 and 11 years ago she told me that she wasn't ready to be a grandma, I just said well youd better get used to it.
My Dad was 11 years older than her so he was rapt with being a Grandpa.
She came around and now that im remarried she is nagging my wife for more grand kids.
We just put her on a plane to WA to spend time with her Niece and my sister (which my Dad never ran over) so she will leave us alone for a while.
Best thing to do Grumpy is look forward to it, Have fun with it scaring the crap out of your son about what his misses is going to do to him during birth and then enjoy the results as you can allways give that one back when it cries.

