I wrote this answer to this same question ages ago. Here you go, I've pasted that answer here.
In your brain is a gland. This gland is called the pituitary gland, and it is responsible for making Follicle Stimulating Hormone (FSH) and Luteinizing Hormone (LH).
If you are not on the pill, then towards the end of your period (around day 6), the pituitary gland releases a small amount of FSH and LH.
The FSH signals the ovaries to start preparing an egg, which begins to grow/mature in a tiny "sac".
As the egg grows, the ovary produces estrogen, which thickens the lining of the uterus. As estrogen levels grow, the pituitary gland produces LH.
The surge of LH causes ovulation (the release of the now-mature egg).
The egg travels towards the uterus,and is viable for approximately a day.
The now-empty egg sac in the ovary causes the production of progesterone, which helps to thicken the uterine lining.
If fertilization occurs, the embryo burrows into the uterine wall and the high levels of estrogen and progesterone keep it firmly stuck there. The embryo releases hCG, the pregnancy hormone, which signals the body to keep pumping out the estrogen and progesterone.
If fertilization does NOT occur, the egg dies, and hormone levels begin to drop. Once hormone levels have dropped a lot, the uterine lining begins breaking down and you have a period. Then the cycle starts all over again.
On the pill, the hormones in it signal brain to not produce FSH or LH. Even IF her body started to produce FSH and LH at the very end of her placebo week, once she started taking the active pills again, no more would be produced, and the egg would stop maturing and eventually be reabsorbed into the body.
Without FSH, no egg can mature, and without a mature egg, ovulation and fertilization cannot occur. The lack of LH means that the uterine lining does not build up very much.
When a woman takes her placebo/sugar pills, the hormone levels in her body drop, and the uterine lining breaks down and bleeding occurs.
As I previously said, there is a small chance that during a woman's placebo week, an egg may have started to mature. It would not have had time to fully mature, and once the active pills were started, no FSH or LH would be released. However, it is slightly possible that when a pill is missed, the body could seize the oppertunity to start making FSH in response, and the egg could finish maturing and be released. Generally, it takes more then one missed pill to allow this to happen, which is why many pill packages will tell you that for one missed pill only, no back-up is needed.
However, the more pills you miss in a row, the more likely it is that an egg will reach maturity and be released.
It is so very important that a woman not take more then seven days off the pill each month in order to ensure her body has not had the chance to mature an egg.
Taking 21 pills in a row ensure that by the time a woman reaches her placebo week, even IF a partially mature egg was present at the beginning of the pill pack, it would long since have been reabsorbed into the ovary, and thus no ovulation will occur during the placebo week,and hence, pregnancy cannot occur.