My relationship has actually gotten stronger with each child. But my DH is VERY involved in childrearing and also does most of the housework. During each pregnancy, I have had many physical issues to deal with, which could have broken a relationship, but in our case DH stepped up to the plate and became even more supportive. So like anything, I think it really depends on the individuals in the relationship and how you both deal with it.

Each child has gotten progressively easier though, in terms of managing the individual ages and stages, It is the oldest child I constantly struggle with, because everything from her is new, I have never handled it before. The biggests adjustment is that any 'freedom' you may have had left with one child, completely dissappear with two (and the same when you go from two to three). In fact, you may not realise how much freedom you have with just one child, until you find you can no longer do those things anymore. Things like take them to the pool on your own, even going to the shop can be three times as much effort with one extra child. You get used to it, and your ability to cope with things becomes amazing; but you have to catch yourself, because it is easy to become resentful of others who only have one child and don't realise how much easier it is.

The older they grow though, the better it can be to have a small age gap. They are interested in the same things, they can play and occupy and learn from each other. They learn things you don't even think about teaching them (ie, sharing is second nature to them - toys, food, clothing, books, my time - yes they still bicker and argue, but they have no problem inter-relating to other children and fit in seemlessly; where as children without close siblings do learn about sharing, but it is different and to a different degree and it can be a hard learning curve for some. We do experience sibling rivalry, but they don't ever question each others existance in the family, just a bit of rivalry over who can monopolise mum and dad's time - they all learn that the baby gets first dibs, because they need more help, and they all like to help each other - DD and DS1 love to 'baby' the youngest, feeding him and pretending to be his 'mummy' or 'daddy' and generally boss him around).

For what it's worth, I don't agree with the 3yr thing. There is nothing wrong with a three year gap, but i disagree that it is a perfect number - it really depends on the individual family. When they are younger, they have less formed opinions about things, and from that perspective and be easier to integrate changes to (and things do change).