Yes, that's right, women who are on birth control do not need to have a period every month.
How is this possible, you wonder? Well, as long as there is balance in hormones, then there is no risk of problems. I'll explain a little further.
The menstrual cycle
There are 2 main hormones involved, Estrogen and Progesterone. Estrogen is the hormone that is responsible for many things including building up the lining of the uterus also known as the endometrium. Progesterone is the hormone that causes the endometrium to be thicker and more glandular to help support a fertilized egg.
Every month a woman's ovaries produce levels of Estrogen & Progesterone in a delicate balance that results in ovulation, the release of an egg. The Period happens when the Estrogen and Progesterone levels decrease. If the hormone levels remained higher, and there was no decrease, there would be no period.
The hormone balance
The birth control not only prevents ovulation, it also provides a different balance of Estrogen and Progesterone, which also decreases acne. With this different balance in hormones, the lining of the uterus also doesn't grow as much as it would, so for most women, there is less blood. That's why many women have shorter lighter periods, and much less cramping. What we've found is that by giving the hormones without a break, there is NO decrease in their levels and so no periods.
If you're interested in a more thorough explanation of the menstrual cycle, see my 3 part series on PCOS.