Actually it's perfectly healthy and encouraged to continue nursing your firstborn while pregnant. It can make the adjustment to a sibling much easier if they don't feel that they're being punished (taking away the comfort they know at the breast) because of them.
I tried to copy a link to you but it didn't work so here. This is from lll's website.
Finding out you are pregnant does not mean you must stop breastfeeding your toddler. Many mothers choose to continue breastfeeding throughout pregnancy, while others decide to wean. The following information may help you decide what is best for you and your family.
Relatives, friends and health care professionals may express doubts about breastfeeding while pregnant. Their concerns might include that you would be risking the health of your unborn baby. It may be reassuring to know that in a normal pregnancy there is no evidence that continuing to breastfeed will deprive your unborn child of necessary nutrients. In addition, according to la leche league's breastfeeding answer book, 3rd edition 2003, page 407. "although uterine contractions are experienced during breastfeeding, they are a normal part of pregnancy. .. Uterine contractions also occur during sexual activity, which most couples continue during pregnancy." also, according to the bab, page 408, "currently, no specific medical guidelines exist that define in which situations it may be risky to continue breastfeeding during pregnancy, and prenatal caregivers vary widely in their recommendations." the materials listed in the "additional resources" section of this faq will help you address the fears others may express.
Needing extra rest is normal in pregnancy. Breastfeeding during your pregnancy may help you get extra rest if you breastfeed your toddler while lying down. Completely childproofing one room with a mattress or pallet on the floor would allow you to continue to rest if your toddler goes off to play. Who knows, you both may drift off for a little nap!
Some mothers develop tender nipples during pregnancy. Careful attention to your child's position can help. Breathing techniques from childbirth classes can help you cope with the increased sensitivity. The breathing techniques may also help if you become restless while your toddler breastfeeds. If your child is old enough, you can ask him or her to nurse more gently or for shorter periods of time. This will help with both your tender nipples and your feelings of restlessness.
Women who breastfeed while pregnant often find their milk supply decreases around the fourth or fifth month. If your breastfeeding baby is less than a year old, watch his weight gain to be sure he is getting enough to eat. It's also not uncommon for the flavor of your milk to change. These changes may prompt some older toddlers to nurse less often or to wean entirely.