if you have a medical condition that might make it difficult to conceive, then I'd say, go for it. Your principal (and staff) should be understanding...one would hope anyway.
Just do your best and show him/her how much of an asset you are to the school, and you should be okay. I will say that the first year is very stressful, so you may want to actively begin trying later in the year, but do what's best for you.
I'm currently 3 months pregnant and have been dealing with similar issues as yours. My husband and I got married last July and I switched school districts. I am going to be in my second year this year (you need three years in my district to be tenured). We were planning on trying next year (so I'd be due after school let out), needless to say, my pregnancy was a surprise.
A good one, but still a surprise. I'm not tenured, and I'm due about 15 days before my principal has to make the final decision on whether or not to renew my contract. All I can do is do my best, be as active as my body will allow, enjoy my time with my students...and hope she's as understanding as I believe she is.
I'm sure you'll be fine at your new school. Do what's best for you, in your (and your body's) time, and enjoy your first year...it's crazy, but so much fun and rewarding.