Possible explanations for these associations include family medical history, genetics, weight, high blood pressure and cholesterol levels, and chemical imbalances from use of hormonal contraceptives. However, no association was found between cardiovascular disease outcomes and current use of progesterone only contraceptives, use of non-oral hormonal contraceptive agents, or fertility treatment.
The researchers point to some limitations, such as missing data and the fact that reviews were largely based on observational evidence, so they cannot rule out the possibility that other unmeasured (confounding) factors may have had an effect.