Is ovarian hyperstimulation associated with higher blood pressure in 4-year-old IVF offspring? Part II: an explorative causal inference approach

STUDY QUESTION: What causal relationships underlie the associations between ovarian stimulation, the IVF procedure, parental-, fertility- and child characteristics, and blood pressure (BP) and anthropometrics of 4-year-old IVF children? SUMMARY ANSWER: Causal models compatible with the data suggest the presence of positive direct effects of controlled ovarian hyperstimulation as applied in IVF (COH-IVF) on systolic blood pressure (SBP) percentiles and subscapular skinfold thickness. WHAT IS KNOWN ALREADY: Increasing evidence suggests that IVF is associated with higher blood pressure and altered body fat distribution in offspring, but underlying mechanisms describing the causal relationships between the variables are largely unknown. STUDY DESIGN, SIZE, DURATION: In this assessor-blinded follow-up study, 194 children were assessed. The attrition rate until the 4-year-old assessment was 10%. PARTICIPANTS/MATERIALS, SETTING, METHODS: We measured blood pressure and anthropometrics of 4-year-old singletons born following COH-IVF (n = 63), or born following modified natural cycle IVF (MNC-IVF, n = 52) or born to subfertile couples who conceived naturally (Sub-NC, n = 79). Primary outcome measures were the SBP and diastolic blood pressure (DBP) percentiles. Anthropometrics included triceps and subscapular skinfold thickness. Causal inference search algorithms and structural equation modeling were applied. MAIN RESULTS AND THE ROLE OF CHANCE: Explorative analyses suggested a direct effect of COH on SBP percentiles and on subscapular skinfold thickness. This hypothesis needs confirmation with additional, preferably larger, studies. LIMITATIONS, REASONS FOR CAUTION: Search algorithms were used as explorative tools to generate hypotheses on the causal mechanisms underlying fertility treatment, blood pressure, anthropometrics and other variables. More studies using larger groups are needed to draw firm conclusions. WIDER IMPLICATIONS OF THE FINDINGS: Our findings are in line with other studies describing adverse effects of IVF on cardiometabolic outcome, but this is the first study suggesting a causal mechanism underlying this association. Perhaps ovarian hyperstimulation negatively influences cardiometabolic outcome via changes in the early environment of the oocyte and/or embryo, possibly resulting in epigenetic modifications of key metabolic systems that are involved in BP regulation. Future research needs to confirm the role of ovarian stimulation in poorer cardiometabolic outcome and should investigate the underlying mechanisms. Our proposed causal models provide research hypotheses to be tested with new data from preferably larger studies.