Hi B.,
Unfortunatly, this is a decision that only you can make for yourself. It is challenging enough with one little one and adding another with the pregnancy and all is very tiring.
I won't presume to sugggest to you what to do, only share my personal experience. We have two that are almost 20 months apart (a little more than 19 months), and it certainly was extremely challenging when they were small. They are now 4 and 5 respectively and it is easier in a lot of ways but of course some of it has been replaced with different challenges specific to their current ages.
I can tell you when the second one was born and people asked me how I did it I used to say "sometimes I need to leave one of them crying, sometimes they are both crying, and heck, sometimes we are all crying together". The first time I was left with the two by myself I was scared to death. But I got through it (things became very scheduled), and now I get to watch my children play together and they entertain themselves a lot of the time which gives me time to clean or cook or whatever. Of course they also fight, but it is much less now than it used to be.
I was also filled with a lot of guilt about my older child once the second one arrived, feeling like I was no longer able to give her the attention like I had been able to. And she definetely had to adjust. Suddenly my attention was divided, and this was not considered a good thing. So there were some struggles in the beginning. However, now I can't imagine how things would be if we had made a different decision.
Another thing that happened in our case is that our older child started getting into the "terrible twos" about the time we had the second one. Truth be told, if we had not already been pregnant when she started that phase then we very well might have waited perhaps a couple more years because it was truly challenging to work through. But there again, I am glad we worked through it all then so we can be at the point we are now. Our original thinking was have them both close together so we can move them through the different stages together (diapers, potty training, getting into school close together) and now in just a couple of years they will both be in school together and suddenly I may be able to consider part time jobs and such that won't interfere with normal "family" time.
To sum it all up, it probably won't be easy with a second pregnancy and a little one already there, but if you have a goal of where you see you and your family in 5-10 years and two close together are part of that plan then certainly go for it. It will be hard, but you'll get through it and most likely will be happy you did it. Even waiting has special challenges associated with it. I have in-laws who have kids 8 years apart, and those kids lead two totally seperate lives. The older one is into all kinds of activities that they need to be ferried to and meanwhile the younger one has learned to nap or go to sleep in the car as a result so now tha parents can't get then to sleep in their crib. That is jut one example of the challenges they face. So, if you know you want two or more ultimatly, I guess it is just a matter of which challenges you want to deal with, not if you have challenges or not.
I hope this helps you.
J.