I am 21 years old, pregnant with my first child. I have had a healthy low risk pregnancy. I wasn't exactly sure when my last period was so I kind of just threw a date out there for them which made my 'due date' February 18th 2013, but according to my US my due date was guessed to be around the 22nd of February 2013.
I don't have any experience with being pressured to be induced but I can totally relate to not knowing a due date. Before I fell pregnant with DS1 my periods were completely irregular. The funny thing was that when I first went for an ultrasound after getting a positive pg test they asked me how long since my last period.... I said, late February/early March (it was July). They didn't believe me. Did an ultrasound and told me that I was 12 weeks pregnant. I said that was impossible because I was only 8 weeks at the most. I knew when I'd had sex for goodness sake!!

I went back for my second ultrasound and they told me that I was 18 weeks pregnant. I said, well that's impossible because for all intents and purposes I wouldn't have been pregnant when I first had a positive pregnancy test.

My wonderful little baby arrived right on time (for him) when he was fully cooked at a healthy 6lb 12oz.

My point is, if your periods are way off and irregular (like mine were) they can only guess. Some babies are small and some are big and some are just smack bang in the middle "normal".

I hope all goes well for you and your little one and you both get your natural birth without having to be induced!

** I should add that my estimated due date was actually quite correct and that both ultrasounds were way off! The second one had me due in June and my baby was born in March?! Go figure!!