Genesis 4:1

"Adam lay with his wife Eve, and she became pregnant and gave birth to Cain.  She said, 'With the help of the Lord I have brought forth a man.'"  Genesis 4:1

The Qbible uses the phrase "and Adam knew Eve" to mean he slept with her.  Now that they've left Eden, Adam and Eve are following the command God gave in Genesis 1:28 "Be fruitful and increase in number."

I wonder how long they were in Eden.  Did they not discover sex until after the garden or did they just not become pregnant until after?  The Bible doesn't tell us how long they lived in the garden - it could have been a week or it could have been a couple of years or maybe even longer.

Either way, the first baby has now been born.