Persistence Data

Research conducted at the MPFSL has found that after vaginal intercourse, spermatozoa should be found in the vagina for 24 hours, are likely to be found up to 3 days later, and occasionally, are found 7 days later (112,113). Longer times for persistence are the exception rather than the rule.

The quantity of semen in the vagina will diminish progressively with time, usually as a result of drainage. The posture and activity of the complainant subsequent to the act are likely to affect this. Similarly, washing, douching, or bathing may accelerate the loss of semen. Drainage of semen from the vagina may also result in soiling of intimate clothing items worn at the time, and these can prove valuable sources of body fluids.

It has been observed that spermatozoa can be isolated for longer periods in the endocervix. Graves et al. (114) report that spermatozoa were isolated from the endocervix 17 days after intercourse. Studies that compared paired swabs from the vagina and cervix have found that 2 days or more after vaginal ejaculation there is a larger quantity of spermatozoa on endocervical swabs compared with the vaginal swabs (115). Therefore, it is recommended that if a complainant presents 48 hours or more after alleged vaginal intercourse, an endocervical swab be taken in addition to the swabs from the vagina.

Although seminal choline has only been found on vaginal swabs up to 24 hours after vaginal ejaculation (116), using FISH technology, Y-chromosome-intact cells have been identified on postcoital vaginal swabs 7 days after sexual intercourse with ejaculation (117).

There is interest in the possibility of determining the timing of intercourse by changes in spermatozoa. Spermatozoa may remain motile in the vagina for up to 24 hours and longer in the cervical mucosa (50,118,119), but the periods for persistence are extremely variable. For example, Rupp (120) observed that motile spermatozoa persisted longer in menstruating women but added that identification is hindered by the presence of red blood cells, and Paul (121) reported that the period of spermatozoa motility ranged from 1-2 hours at the end of the menstrual cycle to as long as 72 hours at the time of ovulation.

However, the morphology of the spermatozoa does show more consistent temporal changes. In particular, the presence of large numbers of spermatozoa with tails is indicative of recent intercourse. The longest time after intercourse that spermatozoa with tails have been found on external vaginal swabs is 33 hours and 120 hours on internal vaginal swabs (122).

A full DNA STR profile matching the assailant should be obtainable from vaginal swabs taken up to 14 hours postcoitus; partial DNA STR profiles are more likely to be obtained between 24 and 48 hours postcoitus (Elliott, K., personal communication, 2002).

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