My situation was a little different because I was working at a JAG office rather than a firm, but I showed up out-to-here pregnant to my 2L summer job, and it was fine. I emailed my boss ahead of time to let him know, started work the Monday after exams, worked for two weeks until my son was born, took six weeks of maternity leave and then came back to finish out the summer. I worked right up until classes started again.
Having been through two newborns and now bar review, I think that 2L summer is a better time to have a baby than 3L summer, for a couple of reasons. First, I would hate to be crazy sleep deprived while trying to cram 25 different bar subjects into my head, and second, I wouldn't want to show up to my first job and then have to immediately take maternity leave. I'm putting off getting pregnant against until next year for that exact reason -- I want to demonstrate my value to my boss first before I'm MIA for six or eight weeks.
If this will be your first pregnancy, AND you don't use it as an excuse to eat a pint of Ben & Jerry's every night, you won't end up looking like Jessica Simpson or like you're carrying a basketball team. With my daughter, I didn't show until around the sixth month -- your abs tend to keep things sucked in kind of tight. Second pregnancy, however, watch out. :p Eat healthily (ie, only 300 extra calories per day for the baby -- you aren't eating for two adults, you're eating for yourself and a peanut), get moderate exercise, and you will probably only look like you ate one too many tacos at happy hour up until late in your second trimester. If anything, people will notice you're pregnant by the lack of drinking, but you can stand around with a diet coke and just tell people there's vodka in it. Or, just tell people you're pregnant -- as long as you aren't wandering around talking about your mucus plug or sore boobs at the top of your lungs, I doubt anyone will get skeeved out by it. I had to pump for my son while at work, which was slightly awkward, but I quietly asked my boss where I could do it and they made a lockable room available to me. It ended up being fine, and had no impact on my performance review at the end of the summer.
There really isn't a perfect time to have a kid -- if you want to have one and you're otherwise able to, go for it and deal with the complications as they arise.