I would agree with most of the posters above to try to power through. However, I also wanted to have kids that were about 2-3 years apart so when my oldest got to about 18months old suddenly I didn't care what stage of training I was in - my most important task was to get pregnant.
I don't think it's a totally ridiculous idea to have a baby during intern year if that is what you want to do with your family. You will make it work. I think it is probably better for your residency application to not take another year off and extending fourth year doesn't guarantee you'll have a baby during that time. You could continue trying to get pregnant for the next few months, go through the match, and then talk to your new program about taking an extended maternity leave.
I want to send you a big hug. I can only imagine how difficult a time it's been for you and your family.
I am an emergency medicine physician, graduated residency in '08. I have two children who are 3 & under, and so I cannot speak first hand about having had children during residency. However, I can tell you that there were women in my program who certainly did-- I can recall 4. I also knew of others in other fields who had children during residency. Knowing what I know now about being a mom, I honestly don't know how they did it. But they did, and I have great respect for that.
I think that on some level when it came down to it, the decision was very basic: at the end of the day, is it family or career? Though both are important, I would be nothing without my family. On the other hand, I think that we are smart and able-bodied and we can always find *some* career. Will your career hug you at the end of the day like your loved ones?
It will probably be hell for a bit to have a child in residency, but in the end-- if you can power through it-- I think it'll be okay and the right choice. Please know this: when you are past residency, it gets better. I'll repeat it because it's so important to just know it: It gets better. You literally go from having a life that is not your own it seems due to school, residency, debt to... lots of options, autonomy, and much more resources... with which to enjoy your family life. Which is why we did it all in the first place. Right?
And then someday, you will be able to count yourself among those who have "been there and done that," and with that experience you will have that much more compassion and mentoring ability for younger physicians when you are an established physician.