P-775 Multifetal pregnancy reduction and ensuing severe maternal and neonatal morbidity and mortality: a population-based cohort study

Abstract Study question How does early multifetal pregnancy reduction impact risk of severe maternal morbidity or mortality (SMM-M) and severe neonatal morbidity or mortality (SNM-M) later in pregnancy? Summary answer Multifetal pregnancy reduction is associated with a lower risk of SMM-M and SNM-M. What is known already Multifetal pregnancies have increased worldwide, mainly from fertility therapy. Fetal reduction is generally offered to patients with a higher-order multifetal pregnancy (≥ 3 fetuses), and may be associated with lower rates of some adverse neonatal outcomes, but its relation to SMM-M in unstudied. Study design, size, duration This population-based cohort study analyzed all births at ≥ 20 weeks’ gestation, occurring in Ontario, Canada, 2006-2021. The study exposure was multifetal pregnancy reduction, as follows: i) higher-order multifetal pregnancy with no reduction, ii) higher-order multifetal pregnancy reduction to twins, iii) twin pregnancy without reduction, iv) fetal reduction to singleton, and v) singleton with no reduction (reference). Study outcomes were SMM-M and SNM-M, each respectively based on a validated composite measure. Participants/materials, setting, methods Linked administrative datasets were used to capture all hospital births in Ontario among women aged 18-50 years. Modified Poisson regression generated adjusted relative risks (aRR). Main results and the role of chance Among all 1,725,717 births, i) 666 (0.04%) were a higher-order multifetal pregnancy with no reduction, ii) 519 (0.03%) were a higher-order multifetal pregnancy reduction to twins, iii) 29,667 (1.72%) were a twin pregnancy without reduction, iv) 324 (0.02%) were a fetal reduction to singleton, and v) 1,694,541 (98.19%) were a singleton with no reduction. Relative to singletons with no reduction, whose rate of SMM-M was 2.0%, the aRR of SMM-M was 4.38 (95% CI 3.49 - 5.51) for higher-order multifetal pregnancy with no reduction, 2.94 (2.15 - 4.01) for higher-order multifetal pregnancy reduction to twins, 2.77 (2.65 - 2.90) for twin pregnancy without reduction, and 1.42 (0.79 - 2.55) for fetal reduction to singleton. Relative to singletons with no reduction, whose rate of SNM-M was 5.8%, the respective aRR of SNM-M were 10.11 (9.51 - 10.74), 4.93 (4.35 - 5.59), 4.18 (4.09 - 4.26) and 2.71 (2.13 - 3.46). Limitations, reasons for caution Use of administrative health data may introduce exposure misclassification and unmeasured confounders. Wider implications of the findings Multifetal pregnancy reduction may be associated with less burden of SMM-M and SNM-M. These results may guide clinical counselling and patient decision-making about fertility therapy choices, and subsequent decisions about fetal reduction in higher-risk multifetal pregnancies. Trial registration number No

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