I'd been on Citalopram for 3 years [gruesome divorce/custody battle with a Narcissist] before falling pregant earlier this year (now 26 weeks). I came off Citalopram (on my GPs advice) pretty quickly once I realised I was pregnant
Coming off the Citalopram was very hard work (extreme moodiness, insomnia, irritabtility etc) but I persevered for a few months. However, ongoing stresses and strains (moving house, continued trouble from my ex re custody and finances, some family issues, very intense working hours) meant that my wheels fell off around 21 weeks pregnant.
My GP referred me to a psychiatrist who is also a qualified obstetrician and he put me on Sertraline 50mg. Like others have said, his advice was that it is all a balancing act in terms of working out what is the best approach to take. I felt like a massive failure for having to resort to ADs again but, I have to say, within two weeks back on them I felt again able to cope and to be a good mother to my existing DC and to take proper care of my unborn DC.
The psychiatrist I saw said that after 20 weeks the risk to the unborn child is pretty minimal (he talked me through it in much more detail so I could assess it). He also said that severe stress is harmful to the unborn child as well and can result in low birth weight and other issues. He advised Sertraline as I want to breastfeed again and it is the least likely to pass into my milk in meaningful amounts.
I have beaten myself up a lot about this but, at the end of the day, I've learned my limitations in terms of mental health and I also trust that I've had some really good advice from medical experts who've been really supportive of both my struggles with anxiety and depression and my desire to do the best thing by all of my children.
Good luck and, whatever your choice is, don't feel guilty about it. You will do your best.