One of the benefits of breastfeeding a newborn is that it causes the mother's uterus to contract. For many new mothers, uterine contractions are a sure sign that the let-down reflex is working (though not everyone feels these contractions). Why are these contractions good for you?

• It helps you to deliver the placenta quickly.
• It helps prevent blood loss through hemorrhage.
• In the following weeks, it helps bring your uterus back to its pre-pregnancy size and muscle tone.

How does breastfeeding cause uterine contractions?

• Nipple stimulation causes the mother's brain to release the hormone oxytocin into the bloodstream.
• This hormone causes a particular type of muscle to contract.
• This type of muscle is found in at least three parts of the body:
  • Surrounding the milk sacs in the breast.
  • In the walls of the uterus.
  • In the vagina.
• The main function of oxytocin in breastfeeding is to contract the muscles around the milk sacs and push the milk out (see the page on letdown for more details of how the breast makes and releases milk).
• Another effect of oxytocin is to cause the muscles inside the vagina to contract. This happens during sexual intercourse (especially orgasm). This is why many breastfeeding women leak or squirt milk during intercourse. It is also the reason why nipple stimulation is sexually arousing to some women. One possible side effect is that some women might become sexually aroused while nursing their babies. But you see, this is due to a simple physiological mechanism; it's nothing to worry about. (see the page on breastfeeding and your sex life for more on this).
• Finally, oxytocin causes the uterus to contract. This is beneficial to the new mother in several ways (see above). This is also the reason your obstetrician might tell you to refrain from sex or wean your younger child when you are pregnant if you are considered likely to have a miscarriage. Most pregnant women, however, can continue enjoying intercourse and continue breastfeeding through pregnancy (even while in labor, up to the point when the waters break) with no problem. If your obstetrician tells you to wean your older child but does not ask you to refrain from sex, you should be very suspicious (see the page on tandem nursing).
• The hormone oxytocin can also be released without nipple stimulation when you see or hear your baby, or even just think about your baby. This may cause milk to leak from your breasts.

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