How likely is post natal when i already suffer with low depressive mood when not on citalopram. i am going to have to stay on this when pregnant.

Trimester specific: Beyond 20 weeks there is evidence of risk with citalopram and other ssri/snri. Alternatives may be mirtazepine, abilify, (aripiprazole) or seroquel for which there is no evidence of any specific risk. It is essential to weigh maternal benefit vs fetal risk. Poor self care related to depression may be more of a risk to the baby than medications. Take care of yourself and the baby.

Significant risk: With ongoing depression, you do have significant risk of postpartum depression also -- but this is much mitigated by continuing your treatment throughout pregnancy. Being in psychotherapy could be an important assist for you, and can improve your odds. So can being sure you have enough support at home, with harmonious relationships, etc.