But our breasts (oddly enough) don't have ounce markings on them. And sometimes breastfed babies eat unpredictably. And sometimes it takes them longer to feel full than other times. So how do we know if the baby is getting enough milk? How do we know that we have enough milk?
Well, the first thing I suggest is to trust your body. If the baby eats contentedly and seems satisfied, then you probably have enough good quality milk, and you have no reason to worry. A more scientific or clinical method to check intake, especially in the early weeks and months, is to count diapers. A well-hydrated baby should produce at least 6 wet diapers in a 24-hour period. The urine should be light in color, not dark yellow or orange. (If you see any crystals in the diaper, call the doctor immediately - there's a good chance the baby is dehydrated and may need immediate attention.) You can also tell if a baby is well hydrated by general observation. Is the skin moist and healthy-looking? Are the eyes wet? (Remember that young babies' tear ducts are not fully formed, so they will not produce tears when they cry. This is not a sign of dehydration.) Does she have moist lips and a wet mouth?
Remember that in the first few days after birth, before your milk comes in, the baby will lose some of his birth weight. This is normal. There is only cause for concern if the baby loses more than 10% of body weight before starting to gain. The baby spends the last few weeks in the womb putting on fat reserves to prepare himself for the first few days after birth. Ideally, a baby should at least regain his birth weight by about two weeks of age. Many regain it considerably faster (mine did!). For example, even if a baby is born at 7lbs., 6oz., he may be only 6lbs., 14oz. upon being weighed at 2 or 3 days of age, i.e., he lost 8 ounces since birth. Between 2 and 6 days after birth, the mother's milk should "come in," and the baby will be able to start putting on weight quite quickly. By the time he is two weeks old, he should weigh at least 7lbs., 6oz. again. If he has not gained weight, then there may be cause for concern. There is no need to supplement with formula in those early days unless the baby loses an unexpectedly large amount of weight. Colostrum is sufficient to satisfy her nutritional and caloric needs in the first few days after birth.
A baby should then continue to gain weight fairly steadily until about 6 months of age. At 6 months, things can level out a bit. My second son weighed about the same at 12 months as he did at 6, maybe at most a pound more. (He is considerably heavier now, at 28 months, however!) This is because the baby usually starts doing "tricks" by 6 months of age, including rolling over, sitting up, even crawling or scooting. This burns quite a lot of calories, far more than just lying around staring at things! As long as he is healthy, shows no signs of malnutrition or dehydration, then there is no cause for concern. And he will continue to get taller, even if he doesn't seem to gain weight as quickly!
Okay, so you're pretty sure you're making enough milk. You've got a healthy, happy, well hydrated baby who is growing as expected and producing lots and lots of wet and dirty diapers for you to change. Great! Is there anything to watch out for?
Well, if you're pumping, you may actually notice a fluctuation in your output throughout the month, especially if your menstrual period has resumed. (Many nursing women do not see a return of their periods until well into the first year postpartum and sometimes longer.) There is often a supply drop in the days leading up to your period starting. If you are not pumping, you may notice that there are days that baby seems to nurse more, or for longer, or is more agitated at the breast. Sometimes, you just have a sense that your breasts are "emptier" than usual. This can happen. Usually it's a short-term problem that the baby will solve for you - the more the baby nurses, the more your body will be stimulated to make milk, and you should find that within the next day or two, you have plenty again. It may be related to diet. I noticed that if I went a few days eating fewer whole grains, I seemed to have less milk, for example. Often, it's a maternal hydration issue. It takes a lot of water to make milk, so just drinking another couple of glasses of water per day can increase your supply.
Don't confuse growth spurts and supply problems, though! At the ages of three weeks, six weeks, three months, and then about every three months until about a year old, there are major growth spurts. The baby may nurse seemingly non-stop for hours a time, for two to three days, then suddenly spend a day or so sleeping more than usual. If this pattern occurs near those ages, it is most likely a growth spurt and not a supply problem. The baby is taking in extra nutrition and calories to fuel the major spurt that is coming, as well as preparing your body to produce more milk, since as he grows he will require more to eat! Even though it can be frustrating, can make your nipples feel like they are going to fall off, can be, at best, inconvenient, can make you feel like you're somehow inadequate, remember that it will only be two days, maybe three at the outside, and then everything should return to basically normal for you. There is no need to supplement with formula during a growth spurt - doing so can actually be detrimental to your milk supply. If it's a growth spurt, then the absolute best thing you can do is nurse the baby whenever she wants to.
And now, back to actual supply problems.
If the supply problem goes on for a longer period of time and the baby's health seems to be suffering, you should contact a lactation consultant (IBCLC - International Board Certified Lactation Consultant) or your local La Leche League leader. She can help diagnose a supply problem and give you ideas to help. Sometimes a very simple change can make a big difference. For example, as mentioned above, adding more water to your daily intake can help. There are foods, herbs, and medications that may decrease supply. Common ones are mint, sage, and the medication pseudoephedrine (found in medicines such as Sudafed). Eliminating these from your diet may help. There are also foods and herbs that can increase supply. These include good old-fashioned oatmeal, as well as brewer's yeast, fenugreek (an herb found in Indian cooking), and a few others. Having a bowl of steel-cut, real oatmeal for breakfast can make a big difference. There is also a recipe out there for "lactation cookies" which both taste great and help increase milk supply. For me, switching from white to whole grain pasta made a big difference (since I eat a lot of pasta)! You might also try other sources of whole grains, such as granola bars, whole grain bread for sandwiches, and whole grain crackers. As far as supplements, you can find fenugreek in capsule form. You have to take a lot, usually something like 3 capsules a day, 3 times a day, and it can make your sweat smell like maple syrup, but it really works. There are a few other supplements you can try, as well, such as goat's rue. A lactation consultant can help you figure out exactly which supplements to take and how.
If none of this seems to help, or help enough, there are prescription medications than can increase milk supply. You would have to talk to your OB/GYN about those, and they sometimes have unpleasant side effects, such as depression.
There are, unfortunately, some women who, despite heroic measures, cannot manage to produce enough milk to fully satisfy their babies' needs. In those cases, a lactation consultant can work with you to diagnose the problem and give additional advice, such as when and how to supplement with formula or donated breastmilk.
The good news is, the vast majority of women have no problem making and supplying enough milk to meet their baby's needs. I would say, don't anticipate a problem or try to solve one that doesn't exist. Rather, just keep on nursing that baby whenever he or she wants, and only if you suspect a problem, try some of these remedies.