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Study Question: Does a previous Caesarean section affect reproductive outcomes, including live birth, in women after IVF or ICSI?Summary Answer: A previous Caesarean section impairs live birth rates after IVF or ICSI compared to a previous vaginal delivery.What Is Known Already: Rates of Caesarean sections are rising worldwide. Late sequelae of a Caesarean section related to a niche (Caesarean scar defect) include gynaecological symptoms and obstetric complications. A systematic review reported a lower pregnancy rate after a previous Caesarean section (RR 0.91 CI 0.87-0.95) compared to a previous vaginal delivery. So far, studies have been unable to causally differentiate between problems with fertilisation, and the transportation or implantation of an embryo. Studying an IVF population allows us to identify the effect of a previous Caesarean section on the implantation of embryos in relation to a previous vaginal delivery.Study Design, Size, Duration: We retrospectively studied the live birth rate in women who had an IVF or ICSI treatment at the IVF Centre, Amsterdam UMC, location VUmc, Amsterdam, the Netherlands, between 2006 and 2016 with one previous delivery. In total, 1317 women were included, of whom 334 had a previous caesarean section and 983 had previously delivered vaginally.Participants/materials, Setting, Methods: All secondary infertile women, with only one previous delivery either by caesarean section or vaginal delivery, were included. If applicable, only the first fresh embryo transfer was included in the analyses. Patients who did not intend to undergo embryo transfer were excluded. The primary outcome was live birth. Multivariate logistic regression analyses were used with adjustment for possible confounders ((i) age; (ii) pre-pregnancy BMI; (iii) pre-pregnancy smoking; (iv) previous fertility treatment; (v) indication for current fertility treatment: (a) tubal, (b) male factor and (c) endometriosis; (vi) embryo quality; and (vii) endometrial thickness), if applicable. Analysis was by intention to treat (ITT).Main Results and the Role Of Chance: Baseline characteristics of both groups were comparable. Live birth rates were significantly lower in women with a previous caesarean section than in women with a previous vaginal delivery, 15.9% (51/320) versus 23.3% (219/941) (OR 0.63 95% CI 0.45-0.87) in the ITT analyses. The rates were also lower for ongoing pregnancy (20.1 versus 28.1% (OR 0.64 95% CI 0.48-0.87)), clinical pregnancy (25.7 versus 33.8% (OR 0.68 95% CI 0.52-0.90)) and biochemical test (36.2 versus 45.5% (OR 0.68 95% CI 0.53-0.88)). The per protocol analyses showed the same differences (live birth rate OR 0.66 95% CI 0.47-0.93 and clinical pregnancy rate OR 0.72 95% CI 0.54-0.96).Limitations, Reasons For Caution: This study is limited by its retrospective design. Furthermore, 56 (16.3%) cases lacked data regarding delivery outcomes, but these were equally distributed between the two groups.Wider Implications Of the Findings: The lower clinical pregnancy rates per embryo transfer indicate that implantation is hampered after a caesarean section. Its relation with a possible niche (caesarean scar defect) in the uterine caesarean scar needs further study. Our results should be discussed with clinicians and patients who consider an elective caesarean section.Study Funding/competing Interest(s): Not applicable.Trial Registration Number: This study has been registered in the Dutch Trial Register (Ref. No. NL7631 http://www.trialregister.nl). [ABSTRACT FROM AUTHOR]