Published 05 January 2024
Large cohort study based on SART data finds that women with a BMI outside this ‘ideal’ range are less likely to have a live birth; and those aged over 40 have a lower clinical pregnancy rate despite PGT-A testing.
Data reported to the SART registry continues to provide insights into the link between BMI and IVF pregnancies, and adds to the debate around the use of BMI to determine IVF eligibility.
An updated review (1) recently reported in Focus on Reproduction found that frozen embryo transfer (FET) success decline as BMI in women increases in cases of female (but not male) factor infertility.
BMI was categorised according to World Health Organization (WHO) guidelines with 18.5 to 24.9 kg/m² defined as normal weight.
Now a new SART analysis (2) suggests that the highest probability of clinical pregnancy and live birth is associated with patients whose BMI is within the specific range from 23 to 24.99 kg/m2. Based on more than 77,000 PGT-A cycles from nearly 56,000 patients, the study showed that underweight (BMI below 18.5kg/m2) patients were 11% less likely to have a live birth and those with a BMI of 40 kg/m2 and above were 27% less likely.
In line with the earlier review, the authors question the use of BMI as a cut-off for fertility treatment provision; and suggest alternatives to BMI for measuring body fat.
Prior studies have demonstrated that obesity has many effects on IVF outcomes including decreased LBR. While the exact mechanism is unknown, this has been attributed to factors including altered uterine milieu and decreased oocyte quality. However, not all data on the effect of BMI are consistent in terms of IVF outcomes.
The focus of this analysis based on FETs reported to SART between 2014 to 2017 was to determine if BMI was associated with live birth in patients undergoing transfer of frozen-thawed PGT-A embryos. It comprised 77,018 PGT-A cycles of which 70,752 were autologous and 6,266 donor egg recipients.
The authors write that the study population of 55,888 patients was strictly defined to include only frozen-thawed PGT-A-tested blastocyst cycles. This allowed them to refine the association of BMI with cycle outcome when the ploidy status of the embryo is known.
In the autologous and donor cycles, results showed what the authors describe as a ‘statistically significant and clear non-linear relationship’ between BMI and LBR, with the highest birth rates observed in the ideal reference range (BMI 23 to 24.99 kg/m2).
In contrast, those not in the ideal range demonstrated a lower probability of live birth and clinical pregnancy that continued to decrease the further the BMI became from the reference value. Patients with a BMI below 18.5 or a BMI of 40 and above had lower probabilities of live birth.
When the data were stratified by age, increased rates of live birth were observed across all ages at the ideal range (except endometriosis) but women aged over 40 years had a lower clinical pregnancy rate. This is despite PGT-A testing which suggests that aneuploidy is not the reason for higher miscarriage rates in women who are obese and who used euploid embryos.
Another key observation was prompted by the fact that outcomes in both autologous and donor egg recipient cycles were linked to the intended parent’s BMI. Hence, the authors theorise that extremes of weight are ‘likely associated with a malfunction in the implantation process’, presumably related to a uterine (not oocyte) factor. However, some blastocysts could have been low-level mosaics because the study surmised that all embryos transferred after PGT-A were euploid.
What do the results mean for clinical practice around weight, BMI and eligibility for IVF? Weight loss prior to conception has been standard advice to couples prior to IVF. Nevertheless, this is not a recommendation of this study. Instead, the authors cite data that weight loss interventions prior to an IVF cycle (3) do not improve the live birth rate.
The authors state that in the US there is no nationally recognized BMI threshold to provide fertility treatment. BMI is just one datapoint and that more exact measurements of percent body fat could be provided, they argue. For example, a recent study found that bioelectric impedance analysis did not differ significantly from BMI (4).
For women of reproductive age with BMIs outside the normal range of BMI 18.5 to 24.9 kg/m², it remains unclear what medical treatment or lifestyle changes may be beneficial. . The focus for future research should be on filling this knowledge gap, the authors emphasise.
1 Bakkensen JB, Strom D and Boots CE. Frozen embryo transfer outcomes decline with increasing female body mass index with female but not male factor infertility: analysis of 56,564 euploid blastocyst transfers. Fert and Steril (2023); doi: https://doi.org/10.1016/ j.fertnstert.2023.07.027.
2 Peterson A, Wu H, Kappy M, Kucherov A, Singh M, Lieman H, Jindal S. Higher live birth rates are associated with normal BMI in PGT-A FET cycles: a SART CORS study. Fert and Steril (2023); doi: https://doi.org/10.1016/j.fertnstert.2023.11.005.
Legro RS et al. Effects of preconception lifestyle intervention in infertile women with obesity: The FIT-PLESE randomized controlled trial. PLoS Med. 2022 Jan 18;19(1):e1003883. doi: 10.1371/journal.pmed.1003883. PMID: 35041662; PMCID: PMC8765626.
4 Kim et al. The Appraisal of Body Content (ABC) trial: Increased male or female adiposity does not significantly impact in vitro fertilization laboratory or clinical outcomes. Fert and Steril Feb 2021; https://doi.org/10.1016/j.fertnstert.2020.12.037
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