Dear K.,
You have already gotten a great mix of advice from other women in your situation, so I will just share my experience and insight. I am turning 40 this year and have a beautiful 3 year old that is the light of my and my husband's life. She is my husband's only child, but I have an older son in high school, almost grown, from my first marriage. But they are 13 years apart, and it is hard to say how much my daughter will even remember about my son when she is his age, since he will be on his own and in college in a couple of years.
I had a difficult 2nd pregnancy, I discovered I had gestational diabetes, and also planned a C section because of complications from my first pregnancy. So I didn't really want to a repeat of her pregnancy. And she will be raised mostly as an only child. My son was also an only child for a long time, and has been nothing but a joy to us.
As another woman said, just because they are siblings, doesn't mean they will be close. I grew up in a family of 4 kids, and was not close to any of them until I became an adult, and even then, mostly due to my own efforts. I seem to have become the family peace maker. But my upbringing was not a happy one, and I think most siblings raised in a loving nurturing home will grow up close, despite their fighting. But that is all I seemed to do with my siblings, so I didn't want to deal with all the fighting that siblings do.
I personally would wait at least until you both are sure that this is what you want. You have a lovely little girl, and she will have your undivided attention and focus, one of the benefits of an only child. My husband was an only child, and adopted, so he wasn't raised with siblings or family, and he says he loved it. I was raised with siblings and I hated it, so it wasn't a tough decision for my husband to get a vasectomy and to be done with having children. They take a lot of time, energy, and effort and I just don't think I have the energy in me for another one. Although if another one had come along, I am sure I would have found it in me somewhere. But I have really noticed with my daughter how much harder it is this time. I don't have the energy I did in my twenties. On the flip side, I have more patient and a lot more knowledge, so I guess it was a give and take.
I guess my advice is to follow your hearts. I don't think you child will resent being an only child, especially if you fill her life with all your love and fun activities. There are rewards and trials either way. I don't know how women do it having children so close together. I can't imagine having 2 babies in diapers at the same time. The first baby always feels a little jealous of the younger one because babies take a lot of attention and care. So much more of your time and effort is taken away from your first child. Many women have babies well into their 40's safely these days. So if you aren't sure right now, maybe wait another year and readdress the issue then. Children that are 3, 4, or 5 years apart have just as much chance of being close siblings as those that are 1 or 2 years apart. My son who is now 16, thinks the world of his 3 year old sister, and even though she sometimes drives him crazy, I don't regret at all putting that much space between them. I even recommend it lol. It sure is a lot easier having an extra pair of hands to help with her sometimes. And think of how much less work it will be to have another if your first is already potty trained, eating independently, or even in pre school. Just a few things to think about. There is nothing at all wrong with only one child, and there are certainly plenty of blessings. Best wishes.
T.