When did sexual intercourse occur?

Spermatozoa (x400)

Given the nature of sexual offences, the case presented to court is often confined to one person’s word against another.  Disputed versions of events can sometimes differ in only minor ways.  In some sexual offences the timing of an act of sexual intercourse can be a crucial piece of evidence, for example if there was a consensual act with a partner prior to an alleged rape. At Forensic Context we specialise in sexual offences and our experts can provide an opinion about the time range in which sexual intercourse may have taken place, although it is never possible to say exactly when it occurred.  We can also assist with whether intercourse is more likely to have occurred at time A rather than time B.

The evaluation of the ‘time since intercourse’ is based on the results from tests used to identify the various components in semen and knowledge of data sets that inform about how likely these components are to persist over time.  Such evaluations are usually performed with the results from intimate samples such as vaginal, anal or oral swabs taken during a forensic medical examination, but some information can be provided by seminal stains on clothing.

How quickly is semen lost after sexual intercourse?

When semen is deposited, in the vagina for example, it will be naturally lost over time due to drainage, dilution and degradation.  Other factors may also affect the persistence of semen, including washing or bathing, defecation with swabs from the anal/rectal canal, drinking, eating and teeth cleaning with oral swabs.  Data is available to inform the forensic scientist about the persistence of semen in relation to time since intercourse and the expert will use this in their evaluation.  Spermatozoa are likely to be found on vaginal swabs taken within one to three days of sexual intercourse which includes ejaculation but they are rarely found beyond seven days.  The numbers of sperm persisting decreases with time.  Other substances in semen (AP, choline and PSA) are lost much more quickly than spermatozoa.

Case example

The following case example illustrates the evaluation in a scenario where the time since intercourse occurred is in question:

  • The complainant alleges that her ex-boyfriend forced her to have unprotected sexual intercourse at 1am Thursday.  She is unsure if he ejaculated.  She reports this to the police and intimate swabs are taken at 6pm on Thursday, 17 hours after the alleged intercourse.
  • The defendant denies that intercourse occurred on Thursday.
  • Both parties say they had intercourse, which was consensual, on the previous Sunday at 6pm.

Forensic examination of the high vaginal swab showed that large numbers of spermatozoa were present and a strong AP positive result was also obtained, indicating that a large amount of semen was present.  The expert’s opinion is that this is most likely to have been due to recent sexual intercourse (within approximately one day) and it is highly unlikely that such a large amount of semen would have persisted for four days.  At four days only very small numbers of spermatozoa would be expected (at most) and a strong AP result would be extremely unlikely.   Therefore the expert’s conclusion is that the findings are very much more likely if intercourse occurred on the Thursday rather than on the previous Sunday.

However if only a small amount of semen had been found on the vaginal swabs, the conclusion would not be so clear cut. Small amounts of semen may be found even after recent intercourse, if for example there is a partial ejaculation.  In this situation the findings may provide either a much lower level of support for the prosecution case or may not assist regarding whether intercourse occurred on Thursday rather than Sunday.

Our experts have extensive expertise in giving opinions in sexual offence cases and Julie Allard has published several papers on ‘time since intercourse’ matters. If you would like to know more about forensic issues in sexual offences, email us at info@forensiccontext.com to request a complimentary CPD session at your offices.