I've wanted to research this. I remembered hearing the verses below a few years ago.
Genesis 38:8-10 KJV
And Judah said unto Onan, Go in unto thy brother's wife, and marry her, and raise up seed to thy brother. [9] And Onan knew that the seed should not be his; and it came to pass, when he went in unto his brother's wife, that he spilled it on the ground, lest that he should give seed to his brother. [10] And the thing which he did displeased the Lord : wherefore he slew him also.
This man prevented his seed from being recieved by the woman, and God killed him for his sin. I'm not sure why God chose to kill him, but this form of contraception is definitely not mentioned under good circumstances. While I was reading the chapter to try to get more context, I came across a verse about conception.
Genesis 38:18-19 KJV
And he said, What pledge shall I give thee? And she said, Thy signet, and thy bracelets, and thy staff that is in thine hand. And he gave it her, and came in unto her, and she conceived by him. [19] And she arose, and went away, and laid by her vail from her, and put on the garments of her widowhood.
Judah took his daughter in law that was playing a harlot. She conceived and then arose. It seems from the sequence of events listed, that she conceived before she left the presence of Judah.
I did a quick google search, and I found that fertilization can take minutes to days. The same diagram showed that implantation takes 5 to 15 days. That being said, it seems that the bible defines conception as the fertilization of the egg in the verses above.
I still want to study further to understand if the prevention of pregnancy is sinful in all cases (other than abstinence). I'm not convinced that the first example shows that God disapproves contraception, but I'm also not convinced that God approves.