I think you need to think carefully about if you want to continue. You life basically is on hold for two years after the baby is born. Maternity leave and lack of sleep etc. I've 4 and notice it only gets back to normal after 2 years. My house only just looks normal and I'm just getting round to jobs I wanted to do round the house (above and beyond general cleaning) . Decorating and the like.
I work part time and just could never manage more hours, mine are 9 down to 2. Perhaps once my 2 Yr old starts school. I can up my hours. Those who say 3rd don't make a difference are talking absolute bollocks. They cost more money for a start. They have time being ill meaning more time off work for you and dh. There are more appointments, plays, housework, washing, school trips, shoes to buy. It incereses your work by 33 per cent.
Don't get me wrong it's entirely worth it but I wanted a 3rd and a 4th and I love them to bits but they cost a fortune. When they were very young I worked even less hours than the 20 hours I do now each week and was a sahm for a long while. Its the only way we could make it work. Childcare for 3 is huge, even with one in nursery and 2 in school. I work evening around dh yo make any profit at all. I enjoy my job and children so much but oh my goodness it's tiring.
Please don't think I'm being negative I'm not they are amazing kids but weigh up your options. I did it without any help and you can too. Just think it all through. Is your work flexible and can you take a break?
I have had to terminate a pregnancy before due to abnormalities with the baby. It was hiddious, I felt and still feel guilt, but I accept that it was and still is the right decision.