L.B.
As most of the other Moms will tell you, its best to check with your Doctor and maybe even get a few other Medical Opinions. However, I believe from EVERYTHING I've read (and, I've read a whole lot on breastfeeding), that you should continue for as long as you can, up until the 3rd trimester. Some say that breastfeeding can bring on early contractions...but this is noted as usually only a "potential" concern in the last trimester.
If possible, your first child should have the best advantage possible of receiving PRICELESS nutrition and protection that will last him a LIFETIME (and, this is proven science).
It will be important that you go out of your way to really take care of yourself, adjust your work schedule and other activities as necessary, in order for your body/mind/emotions to be able to ramp-up for this pregnancy while still nourishing your 1st born.
The beautiful thing is that God created women's bodies to provide all the nutrition their child needs for at least the first year. And, only God can allow a new life to come about too.
You're doing the right thing in gaining counsel from many, checking with your doctor and commiting your decisions in the end to God and agreements with your mate and yourself.
Take care. Congrats! Many Blessings - L.