As a mom of 2 under 2 (my two oldest are 20 months apart), I can honestly say the stress felt during pregnancy about having two under two is completely unnecessary. I've been a mom to those two for 25 months now and it is the most amazing and rewarding career in the world. My youngest was in the NICU for his first week of life, which was hard on my daughter. I was determined to breastfeed and spent a lot of time in the hospital with my son. When he came home though, my daughter loved him instantly.
If I can give any advice, prepare, as best you can, your older one. I don't mean give him or her a doll to help change, or remind him that there will be a new baby soon. What I mean is to get your older one's schedule on track prior to the baby's birth. If your younger one is in a crib but moving to a bed before baby is born, start the transition now. It'll avoid the jealous resentment when "baby steals my bed/room". If your older one is not on a sleep schedule, with a set bedtime, start it now. Just get your older child used to his or her new routine prior to the baby so they understand that this is how it's going to be, and so they don't associate the changes with the baby. A new baby can bring on a lot of stress for older children, and for some, making all their bedtime, room, feeding, etc changes once baby is here is a big cause of sibling jealousy and resentment, and makes for unneccessary anxiety. Given the child is younger than two, they are unable to express how they actually feel, so it also makes for unneccessary tantrums and behaviour and mood changes. It can also trigger a child to resort to infantile behaviours.
So, in short, my advice is to start helping your child make the transition to Big Brother/Sister now, prior to baby's birth to make it easier on everyone.