How do you know if you need to supplement breastfeeding?

My baby girl is 4 months old now & I have been breastfeeding exclusively the whole time. I had to return to work when she was 12 weeks old, so started pumping 2 times daily (every four hours) at work. Anyway, at first she was only drinking about 3 oz every 3 hours and I was pumping more than that, so we were fine. However, now she drinks 4 oz every 3 hours and according to my mother in law (who is watching her during the day) she seems to want more. The problem is, I'm only pumping exactly 8 oz during the day. I started pumping more frequently (about every 3 hours, for a total of 3 pumping sessions at work) but I still get exactly the same amount at the end of the day. So I'm worried I'm not making enough for her. But what I find weird is that on weekends or vacations when I'm strictly nursing her/no pumping or bottle feeding, etc, I'm making enough for her because she is always satisfied. So I don't understand how I'm making enough for her while simply nursing her, but yet when I'm pumping/bottle feeding her it seems like I may have to supplement with something soon. Any suggestions??? I'd really like to continue exclusively breastfeeding until she's at least 6 months.
By the way, I use a medela pump in style and it seems very effiicient.

thanks- those are good answers. Yeah I've had my doubts at times whether she truly is still hungry at my mother-in-law's. It's hard though. Yesterday she tried to give her water without telling me, I was a little angry. I know it's not going to kill her, but I'd rather her fill up on breastmilk of course!!! I can't complain though, I know she loves my baby and cares for her very well, and I'm lucky to have her as a babysitter!!!

Pumps are not as efficient as babies. A baby will often be able to get out more milk than a pump will. If nursing your baby all the time, you sound like you'd have sufficient milk to satisfy her. But with pumping, it does sound like you may need to add a bit of formula to her bottles during the week.

I don't know how close your home is to work. Is there any possibility of nursing her on your lunch break? Could you mother-in-law bring her in or you meet somewhere in the middle? If possible, that might be enough to take care of the problem.

Do not introduce solids at this point. Those should wait until 6 months. At this age, she needs breastmilk or its substitute, formula.

You will always get more when baby is sucking, it is just more effective than the pump. You could start giving her a little bit of really watered down rice cereal (from a spoon) during the day. Along with your breastmilk. Doctors are now recommending waiting until 6 months to start solids because of allergies. I started all of mine on rice cereal at 3 1/2 months though and they are fine.

She seems more satisfied because she is with you. Some times grandmas or other family may think the baby is hungry or still hungry by some fussiness or just because they think she should eat more. I would try feeding her with a bottle one weekend and see how she seems to do! Then decide. But the more you pump the more you will produce it takes time but it will happen

It is very normal to not get enough milk from the pump to keep up with your baby. Since baby is satisfied when nursing directly from you then you have enough milk. You don't need to supplement with formula if you don't want to - it can actually reduce your supply if you do. Pump whenever you can. When I first went back to work I couldn't pump enough during the day for my daughter either and I would pump after she went to bed or during the night I would pump the opposite side of what she ate off of. Now at four months I am getting better results from my pump, I think just due to experience and a bit of an oversupply.

Also every couple of weekends I do a "pump and store". I pump an ounce or two here and there and freeze it for the times my daughter eats more during the day and I hadn't pumped enough the day before. It gives me the peace of mind that if I'm late getting home or she has a spurt she can still eat! :-) Check out this link about pumping output and how to increase it...

http://www.kellymom.com/bf/pumping/pumpi...