Perineal body length and risk of Obstetric Anal Sphincter Injury. My guess is not a lot of women out there would know where their perineal body is, let alone what it is and why it’s an important measure in your prenatal assessment. To start with, let’s get orientated.  The perineum is the area of skin located between the vaginal opening and the anus. This is the most common site for perineal tears and OASI injuries to occur.

There are 2 landmarks your physiotherapist will locate to give you a likelihood of perineal trauma during a vaginal delivery. The genital hiatus and the perineal body. The genital hiatus is the area from your urethra to the base of your vulva, this varies between women but on average is 2-3cm (1) long. The perineal body length is the distance from base of your vulva to mid anus, variations occur between individuals with research finding an average length of 3.9cm among women. (1)

Why do we need to assess these areas? These measurements give us an indication of the levator hiatus. The levator hiatus is the area of the pelvic floor where your bladder, vagina and bowel sit. If this area is smaller, it means that there is less space for a baby to fit through and your tissues may be stretched further this is how perineal trauma is caused.

A perineal body length less than 3cm is associated with an increased incidence of OASI (2). We cannot change the length of your perineal body in physiotherapy. However, preventative care for the perineum, to avoid difficulties with incontinence and pelvic pain after birth is within your physiotherapists scope. What we can help with is working with you to make sure that your pelvic floor is able to relax when in labour to ensure that the muscles aren’t overactive when delivering. Overactive muscles will make it harder for the perineum to stretch while giving birth. We can also provide education of perineal massage. We explored the benefits of this in our last blog.

These sessions can be very beneficial for you to get yourself in a good head space and educate yourself before heading into birth. Your local women’s health physiotherapists are well versed in these assessments. Bump Fitness offers these as well as prenatal education classes to make you feel prepared as possible.


Deering SH, Carlson N, Stitely M, Allaire AD, Satin AJ. Perineal body length and lacerations at delivery. J Reprod Med. 2004 Apr;49(4):306-10. PMID: 15134158.

Ganiga, P etal 2022. Relationship between perineal body length and the occurance of perineal lacerations in low risk primigravidae: a prospective obsertvational study. International Journal of Reproduction, Contraception, Obstetrics and GYnecology. DOI:,the%20occurrence%20of%20perineal%20tears.

Photo reference – Learn About Female-Genitalia |