I definitely agree you need to stop feeling guilty - you know you didn't plan this, so what does it matter what people who don't know you think? Especially if they're going to automatically jump to negative conclusions... they're not the kind of people whose opinions I (personally) would care about anyway :)
You say your employer doesn't need to cover your role, nor do they pay you over SMP, so it's not a financial inconvenience to them. And let's face it, most organisations will usually put their bottom line before their employees feelings, so don't feel bad about them!
If it was a small firm that relied on you, I would certainly take the time to explain what had happened and try to work with them to minimise the impact on their business, but it doesn't sound like your company is this tiny.
Personally, having read your replies on here, the bigger question is what YOU want from your career. How important is your career to you? If it's a big part of you, you may want to consider keeping your foot in by returning between babies two and three, and going back after your third.
If you do take time out, you can always look at studying complementary courses at home to keep your skills up and adding to your knowledge.
Or you may find, like many of my friends have done, that after a couple of children your attitude to your career changes. And though you still need to do something 'for you', you no longer fit into your old box. Lots of my friends have used their skills and either gone freelance or started some kind of business from home that works around their kids - one of them recommended this site to me a couple of weeks ago as it talks about just this www.talentedladiesclub.com/
The important thing is to not let this worry ruin your pregnancy - and to make sure you do the right thing for you and your family. I found out I was accidentally pregnant with my first baby two weeks after being hired for a job once. I felt bad and worked until the day before my due date. My commute was two hours each way and I never took a day off sick (even with awful morning sickness) and worked late often in my third trimester. I went back when my baby was four months and again never took a day off. But six months later, despite having a glowing annual review, I was illegally laid off in favour of a cheaper (male) junior employee who wanted my position. Trust me that changed the way I viewed loyalty to employers after that!
Congratulations on your pregnancy and good luck in making the right choice for you :)