The researchers, at the University of Dundee, analysed medical records from more than half a million British women.
They argue the operations could directly affect fertility or there may be a “behavioural” explanation.
Experts said the findings might lead to new treatments, but advised women not to have their tonsils and appendix taken out unnecessarily.
The study found that for every 100 pregnancies in women who had had no procedures there were:
- 134 pregnancies in women who had had their appendix removed
- 149 pregnancies in women who had had their tonsils removed
- and 143 pregnancies in women who had had both removed
One of the researchers, Dr Sami Shimi, said most doctors were wrongly taught that having an appendix removed damaged fertility.
He told BBC News: “This [study] is very important in reassuring young women that appendicectomy will not reduce their chances of future pregnancy.
“More importantly, looking at both the appendix and tonsils together, this study confirms beyond doubt that removal of inflamed organs or organs likely to suffer from repeated inflammation, in women, improves their chances of pregnancy.”
Explaining the findings, published in Fertility and Sterility, is more of a challenge.
One biological possibility is that regularly infected tonsils or appendixes raise levels of inflammation in the body, which affects the ovaries and womb.
The Dundee team favour a behavioural explanation such as women enjoying more “liberal sexual activity”, being both more likely to get pregnant and have pelvic inflammatory disease, which could lead to an appendix being removed.
More research is needed to figure this out.
Prof Allan Pacey, from the University of Sheffield, told the BBC: “This is an interesting paper which suggests that surgical removal of the appendix or tonsils (or both) in young women is associated with an increase in their fertility later in life.
“There are several explanations which may account for these observations, one of which is that the removal of these tissues makes an alteration to their immune system which has an impact to some aspect of the reproductive process (such as how their embryos implant in the womb).
“If true, this may ultimately give doctors and scientists some new ideas for novel drugs or therapies to enhance women’s fertility.
“But to suggest that infertile women have their tonsils or appendix removed as a way of improving their chances is a step too far at this stage.”