Breastfeeding is regaining the popularity it lost in the early part of this century. It is now recognized as the ideal food for human infants, and an important contributor to healthy growth and development.

The most important advantage of breastfeeding is the nutritional composition of breast milk that has the perfect combination of water, protein, carbohydrates, immunoglobulin, cholesterol, fats, vitamins and minerals.

Breastfeeding also creates a unique bonding between mother and infant that some psychologists feel is essential to the child's later well-being. The first weeks tend to be the most difficult and then it gets easier.

Two hormones are responsible for milk production - prolactin and oxytocin. Both come from the pituitary gland, stimulated by the area of the brain known as the hypothalamus.

Prolactin is sometimes known, a bit sentimentally, as the "mothering hormone," because it makes milk and some theorists believe, has a tranquilizing effect.

Prolactin is absolutely crucial to the potential to breastfeed - without it, there will never be any milk. It begins its work about the eighth week of pregnancy and slowly increases in amount for the next seven months, peaking at the baby's birth. With this high level of prolactin coursing through the body, a woman would begin to spout out milk right away, except that the body is also producing high levels of estrogen and progesterone, which block some of the prolactin receptors and inhibit milk production.

Once the baby is born and the placenta has been delivered, the levels of estrogen and progesterone drop quickly, while the prolactin levels begin a much slower decline, and this is the sign for the breasts to begin producing milk. The milk, however, does not come right away. It takes between three and five days, during which time the breasts are making another liquid, a sort of pre-milk called colostrum, which the baby drinks before the milk it will soon get.

Colostrum is filled with antibodies, which help the infant fight off infections. It is also believed that colostrum decreases the baby's chances of later developing allergies and asthma. Soon the baby's own immune system has begun to develop, and he or she no longer needs the antibodies supplied by colostrum.

The other major hormone is oxytocin, which delivers the milk that prolactin has produced. The baby's suckling at the breast does two things - it brings milk out with its suction, and, it sends an important message to the mother's pituitary gland, via the nipples' nerve endings, the thoracic nerves, and the hypothalamus to send more milk out! The pituitary responds by manufacturing oxytocin, which makes the tiny muscles lining the lobules contract and squirt milk out from the breast.

So while some of the milk is actually sucked out by the baby, some simply gushes out into the baby's mouth. The mother experiences this as "letdown" - her milk is literally being let down into her nipples. The milk now exists in two places inside her breasts: the fore milk is at her nipples, the hind milk is up in the lobules.

When the hind milk is called forward by the oxytocin, new hind milk is created, ready to be called forward. Unlike prolactin, oxytocin does not exist in the body until the suckling process calls it forth.

Suckling, therefore, does two things: it stimulates prolactin to make milk, and it stimulates oxytocin to deliver milk. It is very much a demand-and-supply process.

The prolactin level, high during pregnancy, drifts down to its normal level after the baby is born. By the end of the first week after birth, prolactin is down to 50 percent of its normal level, and after three or four months, it is the same as it was before pregnancy. But every time the baby suckles, there is a new burst of prolactin telling the body to make milk and a burst of oxytocin telling it to release the milk into the baby's mouth. The more the mother nurses the baby, the more milk the body will produce. The baby cannot ever use up the milk in the breasts; suckling is simply the message to the body to make more of it.

Though having a first baby late in life can have other effects on the body, it will not affect the breast milk at all; it is possible to produce as good milk for the child at 40 as at age 20. After stopping breastfeeding, some secretions occur for two or three months, sometimes as long as a year afterward.

Usually, a woman can breastfeed for two or three years, though a mother will probably want to combine breastfeeding with bottle feeding of breast milk or formula after a while, both for the mother's convenience and to move the baby on to solid foods. The length of time is up to the mother.

Breastfeeding has some contraceptive effect but only for the first three or four months, and even then it is not 100 percent effective.