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:
- 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).
- 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.