Immunology has been hypothesised to play a critical role in the development of pre-eclampsia. A number of epidemiological studies have shown that multiparous women who changed partner had an increased risk of pre-eclampsia in the following pregnancy compared with multiparous women with the same partner. However, partner change is often associated with a long birth interval. Two recent papers using data from the same birth registry reported that, after controlling for birth interval, partner change was associated with a reduced risk of pre-eclampsia. Based on a causal diagram, the author argues conceptually that birth interval is not a confounder but more likely to be a collider. Controlling for or stratifying birth interval in the association between partner change and risk of pre-eclampsia could be inappropriate and may have produced a spurious association.