Hi M.,
First I'm gonna say I'm not an expert but I've had 4 children their ages are 15 yrs, 6yrs, 3 yrs, and 7 months. That being said I have a couple of thoughts.
First of all I wouldn't even get into the "potty training" right now for a couple of reasons....
1) your son us only 20 months old, you are better off waiting until he is atleast 2 1/2. he hasn't even had his second birthday yet.
2) With you getting closer to time with having the new baby there is no reason to cause uneeded stress for you and/or anybody else. When you start potty training you don't want to stop in the middle of it and backtrack. No matter how much you try there will be an adjustment for everyone when the new baby gets here. Not just for you but for your son and husband. If you are gonna have someone come help you with your new baby or your son you don't want to throw potty training in the middle of that.
You have a 3 1/2 months left of your pregnancy give or take depending on when you are due. I would make the most of these next few months with only 1 child because when there is 2 it's a whole new ball game.
If and when you start potty training I would wait atleast until Feburary of 2008 your new baby will be 4 months old, the newness will be gone of her being here and pretty much life will be back to normalfor the most part just with a new family member. In my opinion at 20 months they don't know how to communicate very much, they are not yet in control of their bladder.
When you do start I would make the transition from diapers to pullups with no potty training mentioned at this point.
Then wait a month or so and on a Monday morning put him underwear that you went out and purchased together and don't turn back.
My daughter the 3 yr old had been going in the potty for about 3 months on and off before my daughter the 4th one had been born but I new what was coming up...I was pregnant, one of the parents that I watch her little girl offered to watch my daycare kids plus my own while I was in the hospital, she had asked me what she wanted her to do if Jillie want to go to the potty and I told her if she wants to cool but if not it's no big deal. I told her I would worry about it later and I wasn't going to cayse uneeded stress for her , the other kids or my daughter. So after the dust had settled everything was calm again and 4 the next few weeks when I came home my 3 yr old didn't go to the potty we just let it be. When my 4th had turned 4 months old thats when I started potty training my then almost 3 yr old we started in March and within a week I had her fully potty trained, that week we had 3 accidents total and the following we had none. I am so glad I did it like that and I did it with my 2nd to 3rd and there was no stress.
I am telling you if wait tell he is 2 1/2 enjoy the time you have and it will go smoothly and there will be no glitches I promise, you. Remember slower is faster, slow and steady wins the race.
I hope this helps you and anybody else that is in this same situation, W. mom of 4 and home childcare provider.
P.S. potty training is stressful on its own for the child, the parents, and in your case the people that would be or will be helping you out when you are having the new baby. Don't fix what isn't broke at this point.