Impact of low-dose aspirin therapy initiation timing on pregnancy outcomes after frozen-thawed blastocyst transfer.

Clinical effects of low-dose aspirin (LDA) on embryo implantation still remains controversial; therefore, we investigated the appropriate timing for starting LDA in frozen-thawed embryo transfer (ET) cycles. A cross-sectional study was conducted on 885 infertile women who underwent thrombophilia screening between 2020 and 2023. We recruited first frozen-thawed blastocyst transfer cycles in 553 consecutive women aged < 40 years. LDA was started on the day of ET from 2020 to 2021 in 79 women (day 0 group) and at 5 days after ET from 2021 to 2023 in 215 women (day 5 group). We also recruited 259 consecutive women who underwent first frozen-thawed blastocyst transfer without LDA treatment from 2020 to 2023 (control). We compared pregnancy outcomes after frozen-thawed ET between the three groups. In results, clinical pregnancy and livebirth rates after frozen-thawed ET in the day 0 group were significantly lower than those in the other two groups (clinical pregnancy rates: 57.5 %, 40.5 %, and 61.4 %, p = 0.005 and livebirth rates: 48.6 %, 34.2 %, and 54.0 %, p = 0.01 in the control, day 0, and day 5 groups, respectively). Multivariable logistic regression analysis showed that livebirth rate in the day 0 group was significantly lower than those in the other groups (odds ratio [OR]: 0.54, 95 % confidential interval [CI]: 0.31 -0.95); however, no significant difference in livebirth rates was found between the day 5 and control groups (OR: 1.13, 95 %CI: 0.70 -1.80). Starting LDA prior to implantation may decrease pregnancy and livebirth rates after frozen-thawed blastocyst transfer.