(See breastfeeding and fertility here)
http://www.kellymom.com/bf/normal/fertility.html#achieve
Why not relax and enjoy mothering your first baby and giving him 100% until your body is ready?
You will almost certainly get your period back while breastfeeding, it's not at all common for women to find it impossible to concieve while nursing, (with patience). So there is no reason to wean early and deprive your first baby of the milk he needs to get the best start. Indeed, many women nurse their first child while pregnant with the second.
The reason for the delay, in terms of our biology, is that
your body is trying to allow adequate spacing between children so that your infant has sufficient time as the only nursling, not just for nutritional benefit, but for all the other aspects of nurturing that you can provide when there is a single baby to care for. If you wean your infant from the breast now, you might be able to *trick* your physiology into ovulating early. (If your body thinks the first baby has died, it will start ovulating sooner, but that also will have hormonal implications) But your body tends to self-regulate and ovulation will start when the body is ready.
When a newborn comes along, the older child has to deal with no longer being the baby, and if they are still clinging to mom, it can be very hard on them.
In addition to the added stress and emotional drain on mom, but it can stress your body when it is still recovering from the first pregnancy. Do you really
have any idea how hard it is to deal with a stormy toddler and a younger baby at the same time?
=====Oh, and just an FYI - I hope I haven't gotten you too mad already, but I got my period back 10 months after birth, for 2 kids in a row. And that was with lots of nursing and pumping twice a day at work. But I waited until to get pregnant again, so they'd be 3 years apart. It worked out great - the oldest one ADORES the younger one - there's very little rivalry but they are close enough to enjoy playing together. And I'm sure it's a coincidental, but almost all of his classmates with younger sibs are just about 3 years younger, and all of their mothers agree that the timing worked out just right.