Hi Hilary,
I have four children the 6,4,2,1. Older two are girls and younger two are boys. There are a lot of pros and cons to having them so close together, I will point out the important ones.
I was unable to do a lot of things with my younger children because I had them all so close together. No way to do swim lessons, byers gym, etc. They usually require one adult to every child. When they are close in age a lot of times someone is neglected. The squeaky wheel gets the grease! Your body physically gets put through the wringer and it is nice to get back in shape and balance out your hormones before you get preggers again. Every eight month of my four children I wanted a baby! I found out I was pregnant twice with two of my little ones being eight months.
The positive is my little girls play together well and soon my boys will too. My husband turned 40 this year so we didn't want to have them any later.
The factors you want to consider is money and family. If you have money (which we do now but did not when we started) then you can have a nanny or daycare as I do now. If you have a great mother or mother in law then they can help and you can maintain your marraige with time alone.
If I could do it all over again I would space them out 2-2 1/2 years apart. If you have one now when your eight months pregnant you will be tired and frustrated with your 16 month old and not enjoy all that they are....or if you space them too far apart then they seem to be jealous or do not play together well later!
Hope that helps!!!
Good Luck!