A message we all seem to consistently pick up in Sex Ed and elsewhere is “you have unprotected sex, you get pregnant”. However, if we think about this, we know that it isn’t quite the whole truth. An often quoted statistic is that around 84% of couples trying to get conceive and having sex twice or more a week will get pregnant in a year (e.g. NHS, 2019). If you do the sums, you can see that that’s an awful lot of sex not resulting in a pregnancy!
So, why not? Well, pregnancy relies on four conditions that need to be met (Impey and Child, 2012):
Firstly, an egg must be produced. An egg is produced by the ovaries once every cycle and is only viable (capable of being fertilised) for a few days. Sex outside of this time is less likely to result in pregnancy – remember though that sperm can live inside the vagina for up to seven days. This means that penis-in-vagina sex that happens up to seven days before this ‘fertile window’ can still result in pregnancy!
Secondly, adequate sperm must be released. ‘Adequate’ means that the number and quality of the sperm in ejaculate must be sufficient to fertilise an egg.
Thirdly, sperm must reach the egg. Lots of contraceptive methods work on this part of the process – for example from preventing sperm from entering the vagina and the womb (condoms) or by preventing the egg from travelling from where it is produced in the ovaries to the womb, via the fallopian tubes (tubal ligation, sometimes called ‘female sterilisation’).
And lastly, the embryo (i.e. an egg fertilised by a sperm) must implant in to the lining of the womb. This allows the embryo to survive and develop. Again, some contraceptive methods work partly by altering this process (e.g. the copper coil or the ‘morning after’ pill).
So, all of these four factors need to align to result in pregnancy!
Impey., and T. Child., (2012) Obstetrics and Gynaecology. Fourth Edition. Wiley-Blackwell: Electronic Copy.
NHS online, Overview Infertility, (Accessed online 2nd April 2019).