New babies

What is everyone's opinion of when you should have a baby after the first? I have baby fever so bad, my husband was disappointed to see a negative test a couple weeks ago and my MIL is wondering when we will get pregnant again. The thought of another baby freaks me out. I want one, but with the 18 month old, I'm worried about how I'll manage two kids.. especially a newborn with a toddler. I guess I'm just worried how everything will play out if we do get for another.