CONGRATS on deciding to TTC!!! I hope it goes well for you
It's certainly not a given that you have to be off your meds before getting pregnant. If you can be off them and still be healthy and happy then of course it's better to be off them but for most mums it's not as simple as that.
There is a lot of different research about Citalopram on unborn babies and they say different thing BUT the thing everyone agrees on is that untreated depression most certainly DOES affect the growing baby negatively.
For me personally, I was on an anti-depressant Fluox (same family-SSRI- as Citalopram) when I got pregnant but it was for what they call 'maintenance' eg I had symptom free for months and was just on them to make sure I was definitely ok and stable- so it was an easy decision to come off them. If I had relapsed I would have gone on them again though.
I think "taking them til baby is out of nappies" is a bit vague...maybe another trip to see her would be in order soon