Yes, it is BUT only if you're breastfeeding exclusively and frequently. This is due to the hormones the body produces such as prolatin and such which sends a signal to the ovaries to stop ovulating.
Or at least that is such in my case - I'm 100% direct latch on, no formula or water and at least 8 times a day. When the doc did a scan on my ovaries and such, he noted that I wasn't ovulating this month. XD
BUT you shouldn't rely on BF as a form of family planning because there are some women who got pregnant even though they didn't get their menses.