Long-term embryo storage not tied to live birth rate after frozen embryo transfer

Key takeaways:

• Embryo storage time was not tied to elective or nonelective frozen embryo transfer live birth rates.
• Number of retrieved oocytes, BMI and endometrial preparation were linked to live birth rates in both groups.

Long-term embryo cryopreservation was not associated with live birth rates for women undergoing elective or nonelective frozen embryo transfers, according to an analysis published in the American Journal of Obstetrics and Gynecology.

“As a result of the availability of efficient cryopreservation programs, the number of stored embryos has dramatically increased in recent years. The usage rate of vitrified material depends on IVF treatment, physician criteria, previous gestation, patients’ wish or even legal matters, which often means that embryos have to be stored for long periods,” Ana Cobo, PhD, director of the cryobiology unit in the IVF lab at the IVI Foundation at the Health Research Institute La Fe, Spain, and colleagues wrote. “Despite the success rates of vitrification programs, concern has been raised about the potential effect that prolonged storage may have on samples.”

Cobo and colleagues conducted a retrospective study with data from 58,001 vitrified day-5 blastocysts from 53,341 elective and nonelective frozen embryo transfer cycles. Elective frozen embryo transfer cycles included only single embryo transfers and first frozen embryo transfer, and nonelective frozen embryo transfers included supernumerary embryos after fresh embryo transfer. All transfers were included regardless of conception success. All frozen embryo transfers were performed from February 2010 to December 2020.

Primary outcome was the association between live birth rate and embryo storage duration. Secondary outcomes included associations between embryo survival, miscarriage and clinical pregnancy rates with storage duration.

Duration of embryo storage ranged from 0.67 or fewer to 4.34 or more months for elective frozen embryo transfer cycles and from 1.8 or fewer to 34.81 or more months for nonelective frozen embryo transfers.

For elective frozen embryo transfers, transfers performed with embryos stored for 0.67 months or fewer (OR = 1.304; 95% CI, 1.141-1.49) or 0.68 to 0.83 months (OR = 1.222; 95% CI, 1.07-1.396), 0.84 to 0.97 months (OR = 1.257; 95% CI, 1.091-1.449), 1.58 to 1.8 months (OR = 1.198; 95% CI, 1.047-1.371) or 1.81 to 2.13 months (OR = 1.223; 95% CI, 1.065-1.404) had significantly higher live birth rates compared with transfers performed with embryos stored for 4.34 months or longer, according to the researchers.

In the multivariable analysis, researchers observed no association between embryo cryopreservation storage duration and live birth rate for either elective or nonelective frozen embryo transfers.

Blastocyst quality, BMI, number of retrieved oocytes, endometrial preparation, male factor infertility and uterine factor infertility were all associated with live birth rate reductions in elective frozen embryo transfers (P < .05). In addition, age at retrieval and at frozen embryo transfer, type of transfer, number of retrieved oocytes, BMI, endometrial preparation, origin of sperm sample and female factor infertility were all associated with live birth rate in nonelective frozen embryo transfers, the researchers wrote.

For nonelective frozen embryo transfers, clinical pregnancy rate was the only statistically different parameter according to embryo storage duration (P < .001). Conversely, neither embryo survival, miscarriage nor clinical pregnancy rates were statistically different for elective frozen embryo transfers.

“This provides evidence for the safety of embryo vitrification, even after lengthy storage periods,” the researchers wrote. “This is reassuring for both IVF practitioners and the patients undergoing frozen embryo transfers of either elective or nonelective embryos.”