About this matter each Christian couple should make a decision that will allow them to have a good conscience. To do so, they must carefully consider how IUDs work and what Bible principles apply.

Jehovah commanded Adam and Eve and, later, Noah and his family: “Be fruitful and become many.” (Genesis 1:28; 9:1) The Bible does not say that Christians need to follow this command. Therefore, each couple must decide whether they will use some form of birth control to limit the size of their family or to determine when to have children. What factors should they consider?

Christians should base any decision about birth control on Bible principles. That is why they would never use abortion as a form of birth control. A voluntary abortion ends a pregnancy that, if it were allowed to continue, would normally result in the birth of a baby. Abortion goes against what the Bible says about respect for life. (Exodus 20:13; 21:22, 23; Psalm 139:16; Jeremiah 1:5) What about using an IUD?

The Watchtower of May 15, 1979 (pages 30-31), examined this matter. Most IUDs that were common at that time were plastic objects that were inserted in the uterus (womb) to prevent pregnancy. The article explained that it was not fully clear how such IUDs worked. Many scientists said that IUDs prevented sperm from reaching and fertilizing a woman’s eggs. If an egg was not fertilized, a new life did not begin.

However, there was some evidence that an egg might at times get fertilized. The newly fertilized egg might grow in a Fallopian tube (an ectopic pregnancy) or might travel into the womb. If it did reach the womb, the IUD could prevent the fertilized egg from implanting in the lining of the womb, and the pregnancy would end. This would be like an abortion. The article concluded: “The sincere Christian concerned about the propriety of using an IUD should seriously weigh such information in the light of a Bible-based respect for the sanctity of life.”​—Psalm 36:9.

But since 1979, when that article was published, there have been many developments in medicine and science.

Two additional types of IUDs are now available. One type includes copper and became widely available in the United States by 1988. Another type releases a hormone and began to be sold in 2001. What do we know about how these two types of IUDs work?

Copper: As already mentioned, IUDs make it difficult for sperm to survive passing through the uterus to reach an egg. In addition to that, in IUDs that contain copper, the copper appears to be toxic to sperm, so it acts as a spermicide. * (See footnote.) Also, such IUDs change the lining of the uterus.

Hormone: Another type of IUD contains a hormone similar to the one found in birth control pills. These IUDs function like the older IUDs, but they also release the hormone in the uterus. For some women, this  stops them from ovulating, or releasing an egg. Of course, if an egg is not released, it cannot be fertilized. These IUDs also thin the lining of the uterus. * (See footnote.) Additionally, they thicken the mucus at the cervix, which blocks the sperm from passing from the vagina into the uterus.

As already mentioned, both types of IUDs seem to change the lining of the uterus. This means that if a woman does ovulate and an egg is fertilized, it might enter the uterus but fail to implant because the lining is not as receptive. That would end the pregnancy at an early stage. However, scientists believe that this rarely happens, and it may also happen at times with birth control pills.

So no one can be certain that copper or hormone IUDs never allow an egg to be fertilized. However, scientific research shows that because such IUDs function in the ways mentioned above that prevent an egg from being fertilized, they rarely allow a woman to get pregnant.

A Christian couple who want to use an IUD might discuss this with their doctor. He or she can tell them what IUD products are available where they live as well as possible benefits and risks to the wife. The couple should not expect or allow anyone else, not even a doctor, to decide what they will do. (Romans 14:12; Galatians 6:4, 5) This is a private decision that they should make as a couple. Their goal should be to please God and to have a clean conscience.​—Compare 1 Timothy 1:18, 19; 2 Timothy 1:3.