I don't think you need to tell any PDs about your plans for family in the immediate future, especially because you don't know what will happen in the next few months yourself. Instead, feel out how family friendly each program is during your interviews--ask how many residents have family (and have them clarify the number of female residents they are including), life outside of residency, etc. You will find that some programs are very up front about residents and pregnancy; for instance, one I interviewed at last year was very proud of the fact that their residents average 2.2 kids/year! Others will not talk about it, but instead will impress upon you how horrid your years of AP will be, insinuating, IMO, that family, pregnancy and free time would be very difficult. Also, ask what the residents do as far as making up time off, etc (most take a 3 mo leave and this works out well when calculated in the allowed time off over 4 yrs).
As someone mentioned earlier, it would be great if you had some CP time in your first or second year, but don't let that discourage you--a baby during AP is totally possible. Of course I'm limited by my experience at my own program (and my med school experiences), but I have not seen any kind of problem in pathology with scheduling/workload when other residents take maternity leave. Frankly, I think it is the responsibility of a residency program and faculty to anticipate and plan for situations in which residents AND faculty need time off, regardless of the reason why. For instance, if someone does an away rotation or takes time off for research, does that negatively impact other residents? Because it absolutely should NOT, and neither should a maternity/paternity/family leave. That's one of the reasons why there are chiefs and fellows--they can step in an fill in any gaps. And btw, we have had a couple of female residents in AP have babies, and I've not heard ANY other residents complain.
A few other things--people get pregnant in all walks of life and in all professions, so while it may seem undesirable to be pregnant while in AP, you can deal with it safely. You can wear a special respirator mask while working with formalin fixed tissue, and PPE while doing autopsies is a given, of course. Talk to your OB about safety issues, and if he/she doesn't know they best way to take precautions, keep searching for someone who does (my program touched on necessary precautions for pregnant residents on the first day!).
Bottom-line, most residency applicants are in the middle of their prime reproductive years, and as much as residency programs in any specialty may not like it, pregnancy is a fact of life. AND, it is possible to have a 60-80 hr week and still be a good mother (speaking from experience on this one, although the weeks that avg 60 weeks are a heck of a lot easier than the 80 hr weeks!), especially if you have good childcare lined up. So I encourage you to go for it--apply for path, pick your dream program that will meet your professional goals AND your personal goals, and start a family. This is your life, after all, and you have to be happy in it.
Good luck!