Why do doctors recommend C-section for second birth to women who have 3rd or 4th degree tears in first birth?

Question

From the Royal College of Obstetricians and Gynaecologists’ The Management of Third- and Fourth-Degree Perineal Tears Green-top Guideline No. 29 June 2015 (updated link):

The reported rate of OASIS (in singleton, term, cephalic, vaginal first births) in England has tripled from 1.8% to 5.9% from 2000 to 2012. The overall incidence in the UK is 2.9% (range 0–8%), with an incidence of 6.1% in primiparae compared with 1.7% in multiparae.

The same pamphlet notes:

There were no systematic reviews or randomised controlled trials to suggest the best method of delivery following OASIS [obstetric anal sphincter injuries]. The risk of sustaining a further third- or fourth-degree tear after a subsequent delivery is 5–7%

So it seems like the proportions – 3rd and 4th degree tears during first births, and 3rd and 4th degree tears in second births amongst women who had a 3rd or 4th degree tear during first birth – are similar (both ~6%).

Yet my understanding is that doctors recommend C-section for second birth to women who have 3rd or 4th degree tears in first birth. Why?

Answer 1

Having done some research, the simple answer here is that it's not true that doctors generally recommend a C-section to women who have 3rd or 4th degree tears in first birth.

From Scheer Thakar & Sultan 2009:

In a questionnaire-based survey amongst obstetricians and colorectal surgeons in the UK, Fernando et al. (Fernando RJ, Sultan AH, Radley S, Jones PW, Johanson RB (2002) Management of obstetric sphincter injury: a systematic review and national practice survey. BMC 2:9) reported that following previous OASIS more than 70% of colorectal surgeons but only 22% of obstetric consultants and 14% of obstetric trainees would recommend an elective caesarean section for a subsequent delivery after previous OASIS.

Edozien et al. (2014) notes:

Among women who had a third- or fourth- degree tear at first birth, 24.2% were delivered by elective caesarean section.

That said, this rate is higher than the background elective caesarean section rate. From Endozien et al. (2014) again:

At second birth, 2.3% [of] women had an elective caesarean section.

So the spirit of the original question remains – why does a 3rd- or 4th-degree tear on first birth cause women to have higher rates of elective c-section on their second birth, despite the fact that the rate of 3rd- or 4th-degree will be similar (6%) for these women on their second birth as it was on their first birth?

It seems that the reasons for this increased use of elective c-section are:

1. Fears that a vaginal birth after a 3rd or 4th degree tear can increase the risk of faecal incontinence even if there isn't another major tear during the second birth. See for example Barber 2014 responding to Edozien et al. 2014:

The current evidence, although limited, suggests that a second vaginal delivery even without a sphincter tear in a woman with prior OASI increases the risk of developing new faecal incontinence, and exacerbates faecal incontinence in those who already have it (Fynes et al. Lancet 1999;354:983–6; Faltin et al. BJOG 2001;108:684–8).

2. Relatedly, fears that two 3rd/4th-degree tears in a row are worse (e.g. create a higher risk of faecal incontinence) than just one.

That said, Barber ends his note:

The role of elective caesarean section to prevent development or exacerbation of pelvic floor disorders remains controversial, and its practice varies based on regional or national obstetrical practices and cultural norms.