I had PND with my first child, but not with my second. During the worst part of the PND we were strongly advised not to have our children close together in age, but when ds was 18m we decided to go ahead anyway, and dd was born when ds was 2.3y. When we decided to ttc, I did not feel 'normal', but more 'in remission' from my PND. What made us go ahead was the feeling that we had our lives on hold waiting for me to get better, and my biological clock was ticking. Also, this feeling of being on hold felt wrong, felt as if it was prolonging the PND. Difficult to explain.
I certainly got into a bit of a panic when I was pg with dd. The way I dealt with it, and what I think saved me from full-blown PND again, was to prepare a safety-net for myself. I talked to my marvellous HV, my GP, my midwife, and made sure that the PND and my worries were on my records.
I had been on a waiting list for counselling and that came through during the pregnancy, which helped a lot. I also knew that I was on the service's books ('in the system' so I would be able to restart the counselling after the birth, if I wanted, without waiting for nearly a year on the waiting list. My HV called me regularly during my pregnancy. Because of my history the hospital didn't try to chuck me out 6h after giving birth, but let me stay until I was ready to go home. The CPN visited me as often as I needed for nearly 6m afterwards, and, at my request, didn't discharge me for another 3m.
And my very dddh took as much off my shoulders as possible: doing shopping, laundry, cooking, looking after ds, leaving me plated meals.
I don't claim to have been completely PND-free, I did and still do have occasional bad days or even weeks, but I didn't disintegrate and it doesn't rule my life.
A long post, sorry, but HTH.