I agree...I don't think this is as perfect of a time for Letizia to announce pregnancy. Not only because there are possibilites of fueling more comparisons between Mary and Letizia but also because I feel it would be nice for Felipe and Letizia to explore a life together for a longer period of time.
F&M have known each other for 5 years, they've had a longer period of time to really get to know each other, to see the good and the bad in each other and to really build not only love, but a deep friendship.
F&L had a whirlwind romance, getting married after a short dating period. I belive that they trully are in love with each other, but even if they are in love, it doesn't mean that they really know each other. It would probably be better for their relationship and for their future children if they build a stronger friendship first before bringing children into their lives. But that's just my opinion...