J.,
As you know, you ovulate about 2 weeks before you begin you period, so you certainly could get pregnant on your first ovulation without ever seeing a period. I would ignore the well-meaning advice about weaning. The truth is that some women cannot get pregnant while nursing, but there is no way to know until you try for a while. I would think very hard about depriving your son of the nutritional and immune benefits of breastmilk for the purpose of getting pregnant again.
Mostly, I wanted to chime in about nursing through pregnancy and tandem nursing, because it's an important consideration which you may or may not have thought about. As you probably know, the American Academy of Pediatrics recommends nursing for at least the first year, and thereafter as long as mom and baby desire. The World Health Organization recommends at least two years, and thereafter as long as mom and baby both desire. With your first baby only six months old, he really does need to continue nursing for as long as possible. The nutritional and immune benefits continue throughout the second year and beyond.
If you think it through and decide to get pregnany, you might want to consult with a local La Leche League leader about the special needs of a pregnant and tandem nursing mom. Unfortunately most doctors receive only DAYS of education in med school about breastfeeding, so they usually know very little about it. Many doctors will recommend that any pregnant mom wean, when there may be no need to. Most likely, you CAN continue to nurse your first one throughout your pregnancy and after the baby is born. There is a wonderful book out called "Adventures in Tandem Nursing." Your local La Leche League group will probably have it in their library for you to borrow for free, or you can order it on Amazon.com at http://tinyurl.com/56h7gl There is great info and many wonderful stories of tandem nursing mamas in this book. I read it even before I got pregnant with my second, and it helped me decide whether to continue breastfeeding. (My girls are 27 months apart.) There are occasional medical issues that cause doctors to recommend against continuing nursing into a pregnancy. However, I would ask you doctor specifically why he/she recommends weaning, if he/she does. Don't just assume that they have a reason! If you continue to nurse, be aware that your needs for good quality nutrition, vitamins & minerals and water are going to be very great. For your sake and the sake of both your babies, you need to pay close attention to what you eat, how much you drink, and taking a very high-quality vitamin supplement. While it is true that babies will draw nutrients from their mother, if there are no nutrients, baby cannot get them!
Another thought, while some moms do very well nursing throughout pregnancy and then tandem nursing until their first baby is ready to wean, it doesn't always happen that way. For me, nursing became an excruciating experience. I nursed as long as I could, but then weaned my oldest 12 weeks into my pregnancy. Paired with extreme nausea, the discomfort of nursing was just too much. Many mothers also lose their milk around the fourth month. Some babies are determined and nurse right through this time, but some do not. There is a chance that your son will give up nursing, or that you will not be able to continue nursing him. I will certainly not tell you what to do, but it is something you should probably consider. The longer you nurse your son, the more health benefits he racks up. These are benefits that will last a lifetime for him. Only you can decide whether you are willing to risk losing the benefits of breastfeeding.
Best of luck making your decision. Only you (and your husband) can ultimately decide what is the right choice for your family!
Best of luck,
S.