It is safe for most women to continue to wear their older children throughout another pregnancy. I actually carried my second child more than my first, all the way through my 3rd pregnancy. 3 days after my due date I had him in a Ring Sling at the bakery, and it was definitely much more comfortable than holding him in my arms.
There are lots of factors to take into account when wearing during pregnancy. Because your body is changing every day, and your centre of gravity keeps changing it is important to be aware of this when carrying a baby. You may need to wear your child higher or lower than you are used to. It is also advisable to be extra careful when leaning down, as your body is under a little more strain than usual. remember to bend at the knees! (I speak from experience here)
This is also a good time to look at back carries; not only are you likely to find that there is less room for baby on your front, but back carrying can also help to counterbalance your growing bump. Some people find that wearing a buckle carrier done up low under the bump works for them, but we have found that more people are comfortable with a Wrap or Mei Tai which can be tied above the bump. I actually learnt to back wrap while pregnant – and this skill definitely helped me stay sane when my son needed to be held.
If you have never used a woven wrap before it is a good idea to start with an SHBC (secure high back carry). This is a good place to start because the baby is secure right at the beginning, so you can concentrate on spreading the passes without worrying about the possibility of a baby falling down. its also advisable to practice first over a bed or soft surface. For an SHBC carry most people need a 4.2m wrap or longer, though if you are size 8-10 then you will probably be OK with 3.6m.
Another great option for Babywearing while pregnant is a Mei Tai (MT). This is much more similar to a more structured carrier like a manduca, and therefore for many people much less scary. But because it is unstructured it is possible to wear in many more positions. The waist of most MTs is unpadded, and the weight of the baby is spread across the hole of the straps, so not all on the hips as with a structured carrier. for most pregnant women that is a good thing, as many women experience hip problems while pregnant. by tying ABOVE the bump many people find it possible to continue babywearing for most if not all of their pregnancy.
Its also worth considering a Ring Sling at this time. Because the weight is spread across one shoulder and along your back, it is easy to use with a tummy bump. Ring slings can be worn on the hip, front or even on the back (reccomended only for experienced babywearers). In a hip carry it is possible to position the childs legs so that they straddle the bump, which many people find comfortable. and if your baby insists on being on your front, it may be possible above the bump, depending on your body shape and the size of your child.