How do birth control pills keep sperm and egg from uniting?

Does this involve the hormones estrogen and progesterone?

It all depends on the pill and the hormone it has in it,usually estrogen and/or progestin. They work by thickening the mucous around the cervix and preventing sperm from entering the uterus, prevents fertilized eggs from implanting into the wall of the uterus or stops you from ovulating, until you get your period.

Some pills have both estrogen and progesterone and some have progesterone only (mini pill). The progesterone pills are less effective then the combo pills if not taken correctly.
Nobody knows for sure exactly how it works but there are theories...prevent ovulation, thickening the mucus around the cervix which makes it difficult for sperm to enter the uterus and reach any eggs that may have been released, The hormones in the Pill can also sometimes affect the lining of the uterus, making it difficult for an egg to attach to the wall of the uterus.

Yes,the combined pill has both and stops ovulation,thins the womb lining so it won't accept a fertilised egg and changes cervical mucus so it is hard for sperm to swim.

The mini pill only has prosterogeon but does much the same thing but doesn't always stop ovulation.

good question.same question i asked my O.B. doc.
this is the answer got....
if the pill is taken correctly..meaning taken EVERYDAY same time...
it is 99.9% effective.
but anyway by takin the pill, yes you still get your period but the pill makes some kind of chemical thingie and that chemical doesnt allow the egg to be released!!
but my doc made it clear that the pill has to be taken everyday in order to work.

Yes, it's similar to making your body "think" it's pregnant. It causes a buildup near the cervix that "blocks" sperm, it thins out the wall of the uterus to prevent implantation, and most pills (excluding the "mini-pill") prevent ovulation.

Birth control works in two ways:
It prevents ovulation, so an egg isn't released to be fertilized.
It changes the environment of the uterus so that it is more toxic to sperm or an egg, should one be released.

Those hormones convince the woman's body that it's already pregnant, which prevents ovulation. No ovulation, no egg. No egg, no sperm uniting with it.

It doesn't keep the sperm and egg from uniting necessarily. It thins out the lining in your uterus so that the egg can't attach to it if it does become fertilized.

They prevent hormones from being released, so to prevent ovulation, and to make the vagina inhospitable to sperm.

It has been a while but I think the pill changes the chemistry of the uterus.

It makes it so that you don't release an egg.. therefore no egg to fertilize