What does the Bible say about birth control?

The biblical text contains no explicit references to birth control, either for or against. Since you did not reference a particular tradition, this is the simplest answer.

However, some groups, through the lens of particular traditions, attempt to infer God's plan about such issues based on various texts. Baptist Fundamentalist and other anti-birth-control traditions use at least two lines of reasoning.

Regarding pre-conception birth control, an oft-used passage is:

Children are a heritage from the Lord...Blessed is the man whose quiver is full of them (parts of Psalm 127:3-5, NIV)

This line of reasoning is: if children are a reward of God, who are mere mortals to refuse the gift? Since the reward is given by God, the decision to bestow the "blessing" is his and his alone. Those who practice birth control are "standing in the stead of God."

Regarding post-conception birth control, a passage in Jeremiah is oft used. In 1:5, God is quoted as saying:

Before I formed you in the womb I knew you, before you were born I set you apart; I appointed you as a prophet to the nations.

This passage is seen by some as having general applicability to the human condition, and implies the existence of a soul before the existence of the body. Some Christians take this to mean that the soul is present in the body at the moment of conception, so any action that destroys body at any stage (morning-after pills, abortion, etc) is necessarily murder.

As a former Christian, it is my opinion that the above passages are overly stretched to cover the topic and that you should choose the way that works best for you. For a many years of my life I subscribed to the ideas above; as the father of six children, I strongly urge you to consider that the consequences of such an approach will have an impact on your life that is impossible to measure in advance. Birth control properly used can help you to form a happier, more productive life for all the members of your family, however many you choose.

But Onan knew that the child would not be his; so whenever he slept with his brother's wife, he spilled his s*m*n on the ground to keep from providing offspring for his brother."

Genesis 38:9 - New International Version (©2011)

This is the scripture I think of when birth control is brought up. Onan's brother had died so his brother's wife came to belong to him (as was the custom then). He wasted his sperm though and God killed him for what he had done.

Children are a gift from God and were particularly seen as such in the Bible times (for instance barren women would be miserable and pray for offspring). Purposefully remaining childless while in a married relationship is something apparently common today, but not exactly natural.

If people are fighting and hit a pregnant woman and she gives birth prematurely but there is no serious injury, the offender must be fined whatever the woman's husband demands and the court allows. But if there is serious injury, you are to take life for life

Exodus 21:22,23

The unborn are alive and seen by God as individuals - here causing the death of one carried the death penalty as for murder. They carry the potential to grow, as does every human, and have that potential right from the moment they were conceived (even before, as sperm).

Therefore the only form of "birth control" that I would see as Biblically OK is abstinence. If you really do not want children then it is best to stay single, as Paul advised.

As Jon commented, we covered the topic in the Christianity.StackExchange Blog last year. I took the Catholic position on the subject but will now attempt to conjure up anti-birth control hermeneutic that even folks who consider Catholicism to be yet another brand of Christianity might consider plausible.

First, the non-existence of explicit references to the regulation of birth as something universally condemned along with the other 613 commandments using the most complicated algorithms to search the text of the Bible, the Deuterocanonicals and the Apocrypha might lead you to think that Birth Control is amoral. But, what it should really do is make you question it's existence. Birth Control is a clearly a euphemism, and an evil one at that.

The name of Birth-Control, for instance, is sheer nonsense. Everybody has always exercised birth-control; even when they were so paradoxical as to permit the process to end in a birth. Everybody has always known about birth-control, even if it took the wild and unthinkable form of self-control. The question at issue concerns different forms of birth-prevention;

G.K. Chesterton – On Evil Euphemisms

As a euphemism, it's not something that actually exists. What does exist are its antithets

1. Birth
2. Self-control

Birth is in the Bible, hundreds of people including [Jesus] were born in the Bible. Cursing ones birth:

Keep your father and mother in mind when you sit among the mighty, Lest in their presence you commit a blunder and disgrace your upbringing, By wishing you had never been born or cursing the day of your birth.

Sirach 23:14 NABRE

is not a good thing. Jeremiah and Job, started out lamenting:

because he did not kill me in the womb! Then my mother would have been my grave, her womb confining me forever. Why did I come forth from the womb, to see sorrow and pain, to end my days in shame?

Jeremiah 20:17

Why did I not die at birth, come forth from the womb and expire?

Job 3:11

But Job and Jeremiah found out that it's better to be born because life, including suffering, is a great mystery and it connects us to Christ.

Then Jesus said to his disciples, “Whoever wishes to come after me must deny himself, take up his cross, and follow me.

Matthew 16:24

The self denial which makes us followers of Christ leads to Chesterton's second point about self-control.

For this very reason, make every effort to supplement your faith with virtue, virtue with knowledge, knowledge with self-control, self-control with endurance, endurance with devotion, devotion with mutual affection, mutual affection with love. If these are yours and increase in abundance, they will keep you from being idle or unfruitful in the knowledge of our Lord Jesus Christ.

2 Peter 1:6

and all these virtues, borne of faith in Christ, blossom in their fullness in the context of a marriage that always is open to God's creative power.

The above mentioned verse is a veritable blueprint for Natural Family Planning. It starts with Faith in God's providence, it goes on to knowledge about the natural systems that regulate ovulation then requires self-control during fertile periods and the endurance to be continent month after month followed by a devotion to one another and to Christ and a deepening love between all members of the family!