Secondly, yes I am sure you can come up with all sorts of reasons to be sad that your current one-and-only will no longer be your baby. Trust me, they will always be your baby no matter how many siblings come along. Both of my kids will be my babies until the day I die. Having another child doesn't make them grow up faster. Yes, you might notice it more because you have a new baby to compare them too, but younger siblings don't make older siblings grow up faster - time does that. Birth order psychology is a rather contested subject. Some people believe that birth order plays a large influence on the personality of a person while others don't think it matters at all. I don't think anyone knows the answer definitively. I do think that loving each of your children to the best of your ability results in well grounded and confident people regardless of birth order.
The most important thing to keep in mind while fighting back the guilty feelings, is that a sibling is a gift. Despite jealous tantrums and rivalries, I truly and thoroughly believe that having a sibling is a huge one up in the world. That doesn't mean that having a single child is bad, I don't think that at all. I just think that having a sibling is worth any negativity that may come with the territory in the vast majority of cases.
I have 4 brothers and sisters myself and I could not imagine my life without them. If my parents ever expressed guilt over having them, I would be tempted to slap them upside the head. My sisters and I had a rocky relationship growing up, but none of that matters in the long run. I know that I always have them in my life no matter what and I hope they all feel the same.
The first group shot of us back in 1989
We had fun everywhere we went
Cheesy studio shot - I know