Yes, you can continue to breastfeed! Your body will continue producing milk throughout your pregnancy. It's even possible to continue feeding your older child after your new baby is born - this is called tandem feeding.
Breastfeeding during pregnancy is fine for most women, although it's important to eat well. Hormonal changes in the early days may give you sensitive nipples making breastfeeding difficult. Nipple stimulation through breastfeeding or making love will cause mild uterine contractions, but, for most women, these are not strong enough to create a problem. However, if you have a history of premature labour or miscarriage, or if you are bleeding, then you might need to think about weaning your older child.
During the fourth or fifth months of your pregnancy, your milk reverts to colostrum (the nourishing pre-milk produced by your breasts in the first few days after birth), so the taste changes and volume decreases. Some children decide to wean themselves at this point, although others persevere. If your baby wants to continue, don't worry about using your colostrum up; your body will continue to produce this special milk until your new baby needs it. You might need to keep track of your older baby’s weight gain at this point, since the volume decreases.
It is also worth thinking about whether you want to continue feeding your older baby once your new baby arrives. If tandem feeding is not for you, it will probably be less traumatic to wean your older child while you are pregnant rather then waiting until the new baby arrives and he is in danger of feeling usurped anyway. If you are not yet pregnant, but trying, bear in mind that it can be harder to conceive while breastfeeding, as some women find they don't ovulate until they wean.