Hi Harriet
I completely understand your concerns but my advice (like most of the others!) is just to go for it! Whether you get pregnant straight away, or you have trouble conceiving, you will not regret starting earlier.
I'm slightly more qualified than you and am in Corporate at a City firm. I've been married a few years and have really wanted kids since I was a trainee/NQ (I'm 30, so the biological clock started chiming loudly a few years ago, ha ha!). Although I felt 'ready', we put it off for a few years as I was worried about how it would look professionally and that I would not be considered to be committed enough. I think this fear was exacerbated by the fact that there just aren't female partners with kids where I work, and there are not that many senior associates with kids either.
Fast forward a few years (and the months and months of heartache every time my period comes) and it looks like I have fertility problems. I'm on a huge waiting list for all sorts of tests and operations so it's not even as though the end is in sight! We could never have predicted this as I am, usually, super healthy. We just assumed that we'd have sex and then 9 months later a baby would come! 
If I had known all of this a few years ago I would have started earlier and, if I had been lucky enough to get pregnant, just been more ballsy with work! I think as junior lawyers we feel so grateful to have a job, that we put up with so much cr*p that people in most other jobs just wouldn't. At the end of the day, if a firm has someone who is committed but they chose to treat them badly because they dared to take a short amount of time off to have kids, then that firm really doesn't deserve your hard graft
.
Good luck! Hopefully we'll see you on the TTC buses soon
x