I've been nursing since Joanna was 45 minutes old. When I first started, she sucked at it. Meaning that she sucked at sucking. I thought infants would automatically know how, but it turns out that some need practice before they get the hang of it. Moms need practice, too! My milk didn't "come in" until around Day 4, so Joanna lost more weight than normal at the hospital. After supplementing her with formula, she hit her goals and my milk soon arrived. One month later, she was underweight again. It turned out that we didn't really hit our stride yet with nursing.
In her first month, I would feed her 2oz every 3 hours during the day and whenever she would wake up at night, which was usually every 4-5 hours. I was also not patient or persistent enough when she would feed. If she would break the latch and seem disinterested in more, I wouldn't give her more. If she fell asleep, I would try to wake her for a minute or two and if she didn't stir, I'd let her sleep. Both were mistakes. Infants need food, so they need to be fed. Even if they're sleeping.
Now, I feed her 2oz every 2 hours or 4oz every 4 hours, depending on the time of day. Sometimes I'm not able to feed her as often as she needs so I supplement with formula. The only way to really tell how much I feed her is to pump first then feed. When I pump, I can get a combined 2oz. When I nurse her directly, I think she doesn't get a full 2 ounces because I'm able to pump immediately after feeding her. Whatever milk I'm able to pump after, I feed it to her immediately. I've never been able to pump more than 2oz in one sitting even though I've been nursing all this time. I also haven't had any of the common side effects like major cramping or chapped nipples.
I'm still getting used to/learning about nursing. If she's not gaining enough weight or if the doc has more concerns, I'm going to go to a lactation consultant.