ANSWER: Birth control, as well as guidelines concerning sex within marriage, can be found within the pages of the Bible. Scripture states that a husband and wife can decide to refrain from sex for Christian purposes such as spiritual needs. They can also decide, out of love, to have sex but not have it lead to bringing children into the world, meaning that the act can strictly be enjoyed for pleasure within a marriage.
The Apostle Paul said a husband and wife can refrain from sex by mutual consent, certainly a form of birth control, so they can fast or pray (1Corinthians 7:1 - 6, NKJV). However, each mate must not defraud each other to fulfill their sexual needs. If both love each other, they will try to keep each other content sexually. Since they are Christians and love each other, they should also consider what each is able to fulfill sexually. One mate should not take advantage of the other or use abstaining under the guise of that it is being done as a form of birth control when it is really being done as a weapon against their partner.
God Himself made the fertility cycle in a woman where she can only become pregnant when she ovulates. God therefore gives us an example of the physical control of birth (she can only get pregnant at certain times of the month). Economics could be a reason not to have sex because one has too many children or cannot afford to support any. Under such a rational loving consideration for the marriage, they can refrain from having children. Obviously, this does not mean not having any sex, as that would lead to adultery, which is sinful (1Corinthians 7:9). Therefore, preventing pregnancy through certain methods is permitted if doing so is the best for the family and is based on love.
There is nothing in God's word that is against limiting the number of children produced using contraceptives as a form of birth control. Condoms are just another way of not getting pregnant just like abstaining from sex when a woman is in her fertile period. Using pills that block conception accomplishes the same purpose. Using these drugs, however, is each person's free right to decide since every drug has a side effect and may be dangerous to one's health.
Abortion (which some perversely believe is a form of birth control) is a sin since once a sperm and ovum is united a new human exists. Killing that new life is murder in God's eyes. According to the word of God, the only requirement when sperm from the man is spilled is that they both should wash with water to keep themselves clean so disease is not spread (Leviticus 14 - 15).
A scriptural example related to birth control concerns the refusal of a man to continue his brother's family line with his widow. God stated it was a sin when the brother spilled his sperm ON PURPOSE so that the family line would not continue (Genesis 38:8 - 10). When we search the Bible, we find God makes sense and everything He says and does is out of what is good for us because He loves us. This especially includes his principles regarding birth control and the use of contraceptives.