How can we know fertilisation has occurred?
The main problem we face is that it is almost impossible to tell whether or not fertilisation, as opposed to implantation, has occurred in any given cycle. There is no biochemical marker for fertilisation and finding unimplanted embryos in the menses is an impossible task! However, we can know if implantation has occurred by measuring embryo-specific substances in the maternal blood. Two useful review articles are those by Chard [2] and Ortiz & Croxatto.[3] The first relates to the frequency of early pregnancy loss. The ‘bottom line’ is perhaps on page 181 of Chard which highlights the fact that human chorionic gonadotrophin (hCG) is first measurable in the maternal circulation just after the time of implantation. Hence with present technology, research showing no hCG elevation during use of a contraceptive tells us that it does not operate after implantation. It might still however, at least on some occasions, operate after the time of fertilisation, by prevention of implantation or d
