If you decide to have another it should be because YOU and your DH feel that your family is incomplete and you yearn for another child. Period. Because quite honestly, a small child does NOT know what they need or what is best for them in any other area of life-- why should how they feel about wanting a sibling have anything to do with this? Its not their body that has to carry and bear the child. Its not them who has to get up in the middle of the night, breast feed, etc. It is YOU. And, if you are overwhelmed and feel out of balance, this will not get better with a second child-- it could get worse. Yeah, these things will pass, and your family will get new routines that work for you if you do have a second child. But bottom line for me is this: I feel like I can be a good mom right now. I am not sure that I could be a good mom to my DD and another baby. There are practical things that need to change before I think I could handle it (job, commute, affordability of infant daycare.) Fortunately, DH feels the same way. My need for another baby does not in any way outweigh my current feeling that things are running smoothly and we are happy. I want to enjoy every day we have together-- and we do. Maybe I am being too short sighted, but I know my personal limits rather well, and another baby wouldn't work well for me right now. I owe it to my child to be the best I can for her: THAT is MUCH more important than giving her a sibling. Study after study has confirmed the detrimental affects of mentally ill/ depressed parents on their children. NO study has ever confirmed that being an only has a similar detrimental affect.
Having said all that-- I acknowledge that there is a huge difference between the average craziness and stress of parenting and mental illness. I just want to throw out there the idea that we should listen to OUR hearts as the mothers and think more about what WE need in order to be our best rather than focusing on somebody else's ideas about the ideal family. If you are feeling doubts that you can handle it, maybe you should look at those doubts, see why you have them and focus instead on bringing balance to theose areas of your life. THEN reexamine the sibling issue. That is what we are doing, and we may end up getting our practical things straight (we are moving to get the commute issue under control and I am considering a career change for less stress.) A year from now you might well see me on this board super gung ho about a second child because making those changes resulted in us being ready to take on that big unknown and have that second child.
People will say "you can *never* be ready." I HATE that. Seriously, folks: you would rather take the decision to have a child less seriously than you would that to take a vacation, buy a house, change jobs, etc? All the really big and worthwhile things take some preparation. So, yeah, because you can't really know exactly how the pregnancy will go, or plan for the personality of the child you will have because those are uncontrollable unknowns factors, you can certainly strive to be prepared in the areas you *do* control!