struggling with work

I'm currently 22 weeks pregnant and I work an extremely demanding job in social work. Before I got pregnant I put in about 45 hours a week and still couldn't keep up. I'm currently months behind and I feel like I'll never catch up. On top of it I've had terrible morning sickness the whole way threw and 2 kidney stone episodes and had surgery to remove a large one 1.5 weeks ago and I'm not recovering well. That episode scared me so much as the doctor told me the infection I had was so bad I could go into preterm labour or we could both die. Yesterday was my first half day back at work and I'm finding I physically can't do it anymore. Sitting in my desk put me in a lot of stomach pain. My stress level has been out of control for months now and I can't cope. Our son also likes to sit very low and puts so much pressure on my hips I'm waddling already. I live in Canada and I'm considering taking sick leave or going on very early maternity leave as I more than likely won't be going back to work after. Anyone else in the same boat and has some advice?