Hiya, I'm a student Podiatrist and mum of two so chipping in! Heels can aggrevate lower back pain as they change your posture and increase the tension on that area. As your baby grows the extra weight also puts pressure on your back and your increasing bump shifts your centre of gravity so you are more likely to topple over if you are wearing high heels. Weight gain also changes your gait (the dreaded pregnancy waddle!) so your feet over pronate (your feet roll inwards), you can get plantar fasciitis ( inflammation of the plantar fascia that causes pain in the arches and heels). I think that's why the official advice is to not wear them or to decrease your heel length as your pregnancy progresses, and to buy a slightly wider fitting if your feet swell. Podiatrists seem to hate super flat, ballet pump styles and Uggs just as much though as they are not supportive and put pressure on your ankles and calves! If you walk behind people wearing the fake Uggs and watch their feet, you will notice that most of them overpronate in them! Been on maternity leave from Uni but I think the "ideal" heel height is about 3cm.
Podiatrists, midwives, etc will say don't wear them, especially every day. I'd say people should trust their bodies. If your feet, back, hips, knees start to hurt or you are feeling wobbly then wear a lower heel, but if you feel confident that you are okay, then it should be fine. I suppose it depends on lifestyle too, such as whether you work at a desk or on your feet, or whether you walk everywhere or drive. I was okay wearing heels and wedges most of the time during my pregnancies but when my bump got massive I used to be a bit nervous walking down stairs and escalators! And standing on trains and buses was easier in trainers!

I think the risk of falling though is the only thing that could hurt your baby, so as long as you feel stable, aren't in pain, and not wearing heels around the house to wash the pots or anything like that, then I would try not to worry!
Sorry if I'm waffling on, I was kind of thinking about doing my dissertation on this subject (I originally had a place at uni to do Midwifery, but I changed to Podiatry as teh hours are better and I've been kicking myself ever since, so if I can throw a bit of pregnancy into my dissertation, then why not?!) so I'm a bit obsessed!
Good luck though ladies

xx
Edited: 16/06/2011 at 06:51