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Do contraceptive pills prevent fertilization?

Before you get married it is important to talk about whether you want to have children immediately or whether you will use contraception.

These days we have access to many different types of contraception, meaning that we can have sex without it leading to a pregnancy. In principle, there is no big ethical difference between avoiding intercourse at the time of ovulation, and using a contraceptive to prevent the egg being fertilized by sperm.

The Bible says that ‘Your eyes saw my unformed body; all the days ordained for me were written in your book before one of them came to be’ (Psalm 139:16). The problem with some contraceptives is that they do not prevent fertilization, but in fact the egg can be fertilized but then rejected by the womb so that the developing life there is stopped a short time after it has started.

While the condom makes it impossible for the sperm cell to come in contact with the egg, the copper coil (IUD) often does not prevent it. The most important mechanism of action the copper coil has is changing the inner wall of the womb (endometrium) so that the fertilized egg is not able to attach itself to the wall. So-called mini (or POP) pills that only contain the hormone progestogen and the hormonal coil prevent ovulation to some degree, but not always. After ovulation has occurred, they function in the same way as the copper coil. If we believe that human life starts when the egg is fertilized, then it is ethically problematic to recommend use of coils and mini pills.

Combination birth control pills (COCP), also known as the pill, contain estrogen and a progestin. This is thought of as a safe form of contraception and is widely used. All of the combination pill producers report that the most important mechanism of action is prevention of ovulation, meaning that there is no egg released from the ovaries. There can therefore be no fertilization. However, the producers also report two other mechanisms of action, being that the cervical mucous plug becomes more viscous and less permeable for sperm, in addition the inner wall of the womb becomes less receptive to any possible fertilized egg.

The first two actions are ethically fine, as fertilization cannot occur without ovulation, nor if the sperm is prevented from coming into the cervix. However, some people have questioned whether it is right to use the combination pill as a contraceptive due to its third mechanism of action. Is it likely that combination pills do not always prevent fertilization, but that the fertilized egg can be rejected in the same way as when the coil or mini pill is used?

There has been little research done to find out how often ovulation can occur while the combination pill is used. In a Canadian study conducted in 2005, 36 women were examined regularly over a three month period [1]. Half of the women took the combination pill with a hormone-free period of seven days once a month, while the other half of the group took the combination pill continually for three months. Researchers found two ovulations among the first group during the three months, while there were no ovulations in the second group. The findings show that some ovulations can occur after the hormone-free periods. The number of women in the study was too small to be able to predict with any certainty how often ovulation may occur, but the study shows that it corresponds to a little less than once every two years for each woman.

This does not mean that an egg will be fertilized every time there is an ovulation. Firstly, the woman has to have sexual intercourse in the days around ovulation. Secondly, combination pills make cervical mucus less permeable for the sperm, so there is much less chance of sperm coming in contact with the egg, even if there had been an ovulation. Professor John Guillebaud has researched contraception, and wrote an article in the journal Triple Helix, published by the Christian Medical Fellowship. Here he writes that there is a very small chance of fertilization as long as you don’t forget to take the combination pills [2]. If you want to reduce the risk of fertilization even further, he recommends that you reduce the hormone-free period from seven to four days. Almost all combination pills on the Norwegian market (apart from Synfase and Qlaira) can be taken continually for three months before you have a hormone-free period that can be reduced to four days. This lessens the chance of fertilization even further.

The new mini pill Cerazette appeared on the market fairly recently. It distinguishes itself from all other mini pills in that it contains the hormone desogestrel, this effectively prevents ovulation. In a Dutch study from 1999 Cerazette was compared to traditional mini pills [3]. While women who used traditional mini pills had an ovulation in sixteen out of 57 cycles, only one ovulation out of 59 cycles was observed for women on Cerazette. This is the same as one ovulation every five years. In addition, the cervical mucus becomes less permeable for sperm. Professor Guillebaud writes that he feels secure that in the few cases where there is ovulation, the effect on the cervical mucus will prevent fertilization [4]. Cerazette’s manufacturer only states that inhibition of ovulation and the effect on cervical mucus are the mechanisms of action of this contraceptive [5].

If you do not want to use contraception that leads to the rejection of a fertilized egg, then you clearly need to avoid the coil and traditional mini pills. But the available research suggests that the risk of an egg being fertilized and rejected by the womb is very little if you use contraceptive pills or Cerazette correctly. If you forget to take a contraceptive pill then you can use a condom until you start a new blister pack of pills. Each couple has to choose contraceptives together, and it is important to follow your conscience. Talk about this with each other, pray together and make a choice that you are both happy with.



1.          Birtch, R. L., Olatunbosun, O. A., and Pierson, R. A., Ovarian follicular dynamics during conventional vs. continuous oral contraceptive use. Contraception, 2006. 73 (3): pp. 235–43.

2.          Guillebaud, J., When do contraceptives work? Triple Helix, 2003 (24 Summer 2003): pp. 12–13.

3.          Rice, C. F., et al., A comparison of the inhibition of ovulation achieved by desogestrel 75 micrograms and levonorgestrel 30 micrograms daily. Hum Reprod, 1999. 14 (4): pp. 982–5.

4.          Guillebaud, J., Answering letter to the Editor: When do contraceptives work? Triple Helix, 2004 (26 Winter 2004): pp. 20–21.

5.         The Norwegian Pharmaceutical Product Compendium: Cerazette