First of all, congratulations!
It's a flip of a coin on this. On the one hand, you don't want to jeopardize any of your chances for your top program, on the other hand in the interest of full disclosure it's important to let them know about this because it can have ripple effects in scheduling (as well as length of time training and ending your residency in 3 yrs, instead of 3+)
You could, probably, go through the interview season and not mention that you are CURRENTLY pregnant, and get a feel for the programs from the standpoint of "My [husband, boyfriend, baby daddy, fiance] and I are trying to have a baby. What is the family medical leave in this residency? How easy is it to do it during residency? Are there other residents that had babies during internship?" and just get the vibe from the program. If there is any field where I see a lot of babies being born during residency, it's FP, so it probably won't be a big deal.
Once you do match, you have to tell the program you got into just so they can plan in your maternity leave -- you don't want to be scheduled for the ICU during your EDD.
When I was an intern, one of my classmates had a baby, but she gave birth towards the end of intern year, not the beginning. it was a little chaotic changing the schedule around, but it all worked out. As a chief resident, one of my second years got pregnant -> she delivered during the beginning of our third year, and we took her pregnancy into account when we made our schedule. I also had to schedule to accomodate a bunch of weddings.
all in all, don't worry too much about it -- this happens more than you'd think. bask in your hcg glow for now! (but definitely get the program vibe.....)
-good luck and congrats again.