I think the above post, while well meaning, is a little negative and off base.
You do not have to take the year off, unless you want to. Keep in mind, you take the year "off" and you'll be starting the whole application cycle over again in a few months, with a newborn! That just doesn't seem worth it to me anyway.
Depending on your specialty, your time away need not affect anyone but you. As long as you are upfront and proactive as soon as you match, your schedule should be able to be configured in such a way as to allow you to be on a lighter rotation as dragonfly mentioned (out patient or elective) when needed. And when you come back, you come back and do your job.
Maternity leave does vary by program. And if you use more than your allotted 3 or 4 weeks or whatever or vacation time you get, then you will have to extend your traning by a few weeks to a month or so. Not a big deal. And if you want to do fellowship, you still can, many programs will allow you to start a month late or whatever the case may be. You cant worry about all that down the line stuff. Anything could happen down the line. People get sick and have to take time off, family members get sick and have to take time off. It can and will work out if you want it to and above all, keep the lines of communication open from the beginning (after you match).
I have 2 kids, it can be done.
As for being sick, everyone doesn't get sick. Some people have horrible morning sickness all the way up to emeis gravidum, requiring feeding tubes, most just get a little sick in the first trimester which will be over for you by the time you start residency, and some people never even dry heave.
Most women find they feel pretty great (increased energy, happy) during second trimester which is where you will be when you start.
I recently delievered 2 pgy-1s here at my hosptial. one was EM, one was prelim IM (radiology) they are both doing fine and no one seems pissed at them. Ultimately, you do what you think will be best for you, but I think it is totally doable.