I had a blighted ovum at 10.5 weeks last pregnancy. Consider this, IF (IF!) you were to have a blighted ovum again (slim possibility at this juncture), would you have a D&C or wait it out to miscarry naturally? I have known women who have found out they are going ot miscarry and the torture of not miscarrying naturally in a quick time frame can be more devastating then finding out by spotting. I personally would not want to know ahead of time if I am going to miscarry, because I want to miscarry naturally if it is to happen, and I want to enjoy every moment I am pregnant. As I said to my DH (who was very fearful of a 2nd miscarriage early on), "Whether I miscarry tomorrow, or in a month, I am pregnant today, and that is all that matters today, I will enjoy every moment God gives me of this pregnancy that I possibly can."
I focus instead on waiting for my big fetoscope appointment in late August when we will be able to hear the heartbeat. I am far too wary about the unknown (maybe subtle, or just undiscovered) effects of ultrasound (dopplers are more intense) on the baby; particularly, because the waves heat up the baby's core by up to four degrees F. I think that is a lot of variation in temperature, considering the delicate nature of developement in the womb, especially in brain developement. It is a fact that left handedness can be a side effect of US, and left handedness that is not associated with hereditary IS associated with mental disabilities (although, of course, that is not always the case). Not enough studies have been done in my opinion on the safety of US use. Besides, the old saying that US has been in use for over 40 years as proof doesn't sit well, because when US was first developed they used much less intense waves, and now there is little control over the intensity of the waves, it all depends on the technician and their experience/ability.
I am not trying to scare you. Lots of women get ultrasounds and have children that grow into healthy adults. I'M just not convinced, and since you obviously aren't either, I decided to share my personal reasons to avoid US exposure at any level, unless medically necessary (midwives may need to use a doppler in labor if they can't find the heartbeat with a fetoscope, and I consented only if necessary since at that time the baby will be fully developed).
Anyway, I guess if I couldn't find any peace over it, I would weigh the stress I am causing on my baby (via stress hormones) versus the potential risks of US and make my decision based on that. Good luck and let us know what you decide.
Oh! And to answer your original question, I wouldn't say a word, but if someone were to pointedly ask (they probably wouldn't, just in case you were putting on a few pounds instead of being preggo though), then I would tell them the truth. I would just wear loose tops, nothing tight, until there was no doubt it is a preggo belly and not fat. Anyway, the popular style right now is empire waist tops and tunics, so it should be easy to hide.