OBJECTIVE

To evaluate the impact of endometrial thickness (EMT) on live birth among autologous assisted reproduction cycles.

DESIGN

Retrospective cohort.

SUBJECTS

All women undergoing autologous cycles with transfers in 2016-2018 reported to the Society for Assisted Reproductive Technology.

EXPOSURE

Endometrial thickness on the day of ovulation trigger in fresh cycles and closest to embryo transfer in frozen-thawed cycles.

MAIN OUTCOME MEASURES

Relative risk of live birth across EMTs was compared with an 8-11.9-mm reference range. Estimates were adjusted for age, body mass index, smoking, infertility etiology, antimüllerian hormone, and follicle-stimulating hormone with log-binomial regression by generalized estimating equations. Fresh transfers, frozen-thawed transfers without preimplantation genetic testing (PGT), and frozen-thawed transfers with PGT were analyzed separately.

RESULTS

A total of 182,784 patients underwent 261,266 combined retrieval and transfer cycles. Patients with endometrium measuring <8 mm were slightly older, with lower body mass index and higher rates of prior pregnancy loss, diminished ovarian reserve, and ovulation disorders besides polycystic ovary syndrome but lower rates of male factor or unexplained infertility. When all fresh and frozen-thawed transfers were combined, the live birth rate (LBR) increased with EMT from 31.2% (<6 mm) to 34.4% (6-6.9 mm), 40.8% (7-7.9 mm), 45.0% (8-11.9 mm), 46.4% (12-14.9 mm), and 46.2% (≥15 mm). For fresh cycles without PGT, thinner endometrium was associated with a reduced LBR compared with the 8-11.9-mm reference range (adjusted relative risks [aRR], 0.59 [95% confidence interval, 0.48-0.72], 0.66 [0.58-0.74], and 0.79 [0.74-0.84] for thicknesses of <6 mm, 6-6.9 mm, and 7-7.9 mm, respectively), whereas thicknesses of 12-14.9 mm and ≥15 mm were associated with an increased LBR (aRRs, 1.12 [1.09-1.15] and 1.16 [1.12-1.22]). Similar but less dramatic effects were noted for frozen-thawed cycles without PGT (aRRs, 0.87 [0.77-0.99], 0.79 [0.73-0.85], 0.94 [0.91-0.97], 1.06 [1.03-1.08], and 1.04 [0.98-1.1] for thicknesses of <6 mm, 6-6.9 mm, 7-7.9 mm, 12-14.9 mm, and ≥15 mm, respectively) and frozen-thawed cycles with PGT (aRRs, 0.67 [0.59-0.77], 0.80 [0.76-0.85], 0.89 [0.87-0.92], 1.07 [1.05-1.1], and 1.06 [1-1.11] for thicknesses of <6 mm, 6-6.9 mm, 7-7.9 mm, and 12-14.9 mm, and ≥15 mm).

CONCLUSION

Increasing EMT from 5 to 8 mm is associated with clear increases in the LBR; however, improvements are still noted until approximately 12 mm, including frozen-thawed cycles with PGT.