Table II.

Pros and cons of the presented hypotheses.

Detrimental environment for sperm penetration and implantation
1. Niche-related accumulation of intrauterine fluid impairing implantation
2. Altered immunobiology and/or increased inflammation where a niche is present
3. Distorted contractility of the uterus caused by fibrosis or interruption of the myometrial layer at the site of the niche
4. Accumulation of mucus and old blood in the niche, which may impair sperm penetration
Pros	Cons
Rare cases of Caesarean scar pregnancies occur. Lower live birth and clinical pregnancy rates occur after Caesarean section, in particular in combination with intrauterine fluid. Implantation very close to or/ in the niche is associated with miscarriages. There is a lower prevalence of intra-uterine fluid after a laparoscopic niche repair.	Caesarean scar pregnancies occur. Intrauterine fluid can have other origin s (i.e. hydrosalpinx or intermenstrual spotting due to a temporary decrease in oestradiol levels after ovulation).
Physical barrier for embryo transfer and implantation
5. A large niche in combination with a strongly retroflexed uterus impairs accessibility for an eventual embryo transfer due to a distorted anatomy
Pros	Cons
Difficult embryo transfer is associated with lower pregnancy rates. After laparoscopic niche repair, the accessibility of the uterus for any embryo transfer improves.	Ultrasound guided embryo transfer might be possible in the majority of cases.
Psychogenic causes that reduce the likelihood of pregnancy
6. Niche-related gynaecological symptoms may interfere with sexual intercourse, and focused therapies for niche complaints may interfere with opportunities to conceive during the convalescence period
Pros	Cons
30% of women with a niche experience postmenstrual spotting. Spotting most frequently occurs in the late follicular phase, so it may have an impact. Abnormal uterine bleeding interferes with sexual arousal, self-esteem and functioning. Women with niches report in the subdomain of the QoL reduced bodily pain. Spotting in combination with sexual behaviour may be influenced by ethnic background and religion.	A previous cohort study did not find different outcomes for sexual functioning using validated questionnaires, in women with our without a niche after a Caesarean section.

Caesarean section, •••; QoL, quality of life.