As a mum I find it hard to keep going some days. I have had a very long period where I wasn't affected by depression and can notice the difference between then and when I have relapsed into a depressive state. It becomes more of a struggle to focus on the children when depressed. Although the love you have for them is there it either starts to be very overwhelming, when I love them so much I physically hurt and cry as I know I'm no good for them in this state. It turns to self hating, when I believe I am letting them down by not being able to function so they would be better off with a "well" and normal mum and away from me. Or, I just curl up and retreat into myself, not functioning like I usually do and they notice and ask what's wrong with mummy but I can't tell them coz I don't have the answers for myself, let alone them.

Motherhood is hard. Mental illness is hard. The 2 together is horrendous. I'm lucky that most days I can put on my "happy face" for them and the rest of the world and pretend all is ok....then breakdown when alone and have no one around to keep it up for. They do give me more of a reason to keep going. It doesn't take away my lack of self worth or suicidal thoughts but for them, I will keep trying. The day I can't do that for them is when I know my time is up.

Although not a Mother(Or Father), I have witnessed quite a lot based around your Question.
I've noticed a key factor in dealing with this problem is the Children themselves. They still want them to grow and to succeed and realize they need to stay strong in order to accomplish this task.

I currently associate myself with an 18 year old Mother who is facing a similar issue which is the example I used above. She has a son who is currently 2 years old(Infamous 2's) and constantly keeps me updated with her status. She is sad and feels the fatigue of becoming a mother at the age of 16 with a Baby daddy who left during the 9 month birthing process. Since she has had the child, she attracted an Infection within her womb that is potentially fatal if left untreated and for this reason she cannot bare another child in fear of dying. She continuously fights through the struggle alone(Although I'm doing the best that I can to support her) and powers through it to see that her child makes it to be somebody in this world.

I'm sure you've heard the line "Once you have had a child, you are sacrificing your life to bestow a blessed one unto your spawn". If they constantly remind themselves that the fight is worth it, that eventually their child will have the power to repay them with the same show of love that was given, they may perhaps find your safe passage through this barrier.

I'm pretty sure it's safe to say that Pregnancy during a Depression phase is a 50/50 chance and that you will be treading dangerous waters. Hopefully this information has helped in some way.