A: Dear Sarah,

I am glad you are aware that medications taken by nursing moms can pass into breast milk. You are right to be asking questions!

The drug you're considering is an injectable birth control given once a month that contains both synthetic estrogen and progestin . While its manufacturers do not advise using this form of birth control while nursing, it is true that many women use other hormonal forms of birth control while nursing. Most of our experience comes from the use of birth control pills during lactation.

The biggest concern about using combination birth control pills is that the estrogen may change the quantity and quality of breast milk. This is less of a concern when you have a 10-month-old infant, because your child is already eating lots of other foods in addition to breast milk. But it may impact a newborn, who exclusively depends on its mother's milk for nourishment.

For nursing mothers whose babies are only drinking breast milk, the progestin-only birth control methods are more commonly used. These are generally started six weeks after birth, and have less of an impact on the flow of breast milk. These pills are taken at the same time each day, and are extremely effective at preventing unwanted pregnancy.

Long-term studies have not shown any harmful effects of exposing infants to the tiny amounts of hormones that are found in breast milk when mothers have used hormonal birth control. Indeed, any woman who breastfeeds produces estrogen and progesterone during her own menstrual cycle, and these hormones also enter the milk supply. However, the hormones found in birth control are not the exact ones a woman naturally has in her body. If you have the choice of a number of birth control methods, my own bias is to minimize synthetic hormone exposure to your baby and avoid the combination birth control pill or injection while lactating.