Genesis 3:14-16 is God talking to first the serpent, then to Eve:

Genesis 3:14-16
14 And the LORD God said unto the serpent, Because thou hast done this, thou art cursed above all cattle, and above every beast of the field; upon thy belly shalt thou go, and dust shalt thou eat all the days of thy life:

15 And I will put enmity between thee and the woman, and between thy seed and her seed; it shall bruise thy head, and thou shalt bruise his heel.

16 Unto the woman he said, I will greatly multiply thy sorrow and thy conception; in sorrow thou shalt bring forth children; and thy desire shall be to thy husband, and he shall rule over thee.
KJV

You can see in Genesis 3:16, there was a conception!

Genesis 3:24-4:2
24 So he drove out the man; and he placed at the east of the garden of Eden Cherubims, and a flaming sword which turned every way, to keep the way of the tree of life.

Here you are! Eve had a comception in the Garden of Eden! God then put them out from the Garden of Eden, then, Adam knew Eve his wife.

Genesis 4:1-2
4:1 And Adam knew Eve his wife; and she conceived, and bare Cain, and said, I have gotten a man from the LORD.

2 And she again bare his brother Abel. And Abel was a keeper of sheep, but Cain was a tiller of the ground.
KJV

There is one more point that I want to bring up. In Genesis 4:2, the word "again" in the Hebrew means to continue to do a thing. This is more evidence that Eve was pregnant with Cain, and Adam knew her and she also conceived Cain's brother (half brother) Abel.

again: OT:3254
yacaph (yaw-saf'); a primitive root; to add or augment (often adverbial, to continue to do a thing):

KJV - add, again, any more, cease, come more, conceive again, continue, exceed, further, gather together, get more, give more-over, henceforth, increase (more and more), join, longer (bring, do, make, much, put), (the, much, yet) more (and more), proceed (further), prolong, put, be [stronger-], yet, yield.