I wonder why it is that issues of childbirth and contraception seem to cause more heat than light with all of us? Having just had a debate about what exactly is the mode of action of the pill (why should that be of interest I hear you say!) and the morality of having lots of children, no children or just a few I thought I'd have a look at the CMF website to see what the Christian Medical fellowship of the UK have to say. Well surprise surprise they can't agree themselves! One article stated
ovulation and fertilisation may occasionally occur on the modern low-dose preparations, both combined and progestogen-only. In that event a back up mechanism of action would include prevention of implantation resulting in the expulsion of a very early embryo........A detailed investigation of recently published literature[5] confirms that many of the modern, widely-used contraceptive agents occasionally allow fertilisation and may on rare occasions prevent implantation. However, experts in the field of contraception at this level differ in their views.

However a second article stated The low dose combined oral contraceptive (COC), provided it is taken conscientiously, and the injectable Depo-Provera are very effective at preventing ovulation, with practically zero pregnancy rates either intrauterine or extrauterine. It is scientifically justifiable therefore, to conclude that these methods do regularly work prior to fertilisation. In these two cases the fact that the methods are capable of working by blocking implantation does not mean that they ever act by that back-up mechanism.......Obviously a forgetful combined pill-taker, particularly a user of say Loestrin 20 or Mercilon, which are among the lowest dose products, might run the risk of ovulation

Whether that matters of course depends on your view of contraception wether on the one hand it is invalid in and of itself (traditional Roman Catholic and modern Quiverful views, wether its OK provided it occurs preconception and not preimplantation (traditional Evangelical view) or wether there are no holds barred at all with preventing unwanted children (secular view).

I am not going to get into this debate tonight as I know its too emotive but provide these links for peoples interest.