SAN FRANCISCO – Recent data show that vitrification of oocytes or embryos can be as efficient as using slow-frozen or fresh oocytes for in vitro fertilization, and so far it appears to be just as safe.
Oocytes from 22 women aged 32-39 years that were randomly allocated for fresh embryo transfer (204 oocytes) or vitrification and warming before transfer (186 oocytes) produced similar rates of fertilization, good-quality embryos at day 3, and blastocysts at day 5 or 6, Zsolt Peter Nagy, Ph.D., reported at the UCLA annual in vitro fertilization and embryo transfer update 2013.
Eighty percent of vitrified oocytes survived freezing and warming. Fertilization succeeded in 75% of fresh oocytes and 67% of vitrified oocytes, a slight difference that was not statistically significant. On day 3 of in vitro fertilization (IVF), 50% of fresh oocytes and 48% of vitrified oocytes produced good-quality embryos; by days 5 or 6, 53% of fresh embryos and 55% of vitrified embryos had produced blastocysts, results that did not differ significantly between groups, said Dr. Nagy, scientific and laboratory director of Reproductive Biology Associates, Atlanta.
The study at his institution found that 11 of the 186 vitrified oocytes (6%) achieved a live birth (Fertil. Steril. 2013;99:1891-7).
Maternal age did affect outcomes. Among 11 women aged 30-36 years, 56% of oocytes produced a good embryo by day 3 of IVF, compared with 40% of oocytes from 11 women aged 37-39 years, a significant difference.
The findings support previous data by other investigators on 224 oocytes, 120 of which underwent intracytoplasmic sperm injection (ICSI) as fresh oocytes and 124 of which underwent vitrification and warming before ICSI. Fertilization rates were statistically similar (83% and 77%, respectively). Excellent-quality embryos were produced by 52% in the fresh ICSI group and 52% in the vitrification group. The mean maternal age in the study was 36 years (Hum. Reprod. 2010;25:66-73).
The 39 embryo transfers in the vitrification group resulted in 15 clinical pregnancies (38% per cycle and per embryo transfer) and an implantation rate of 20%. Twelve pregnancies beyond 12 weeks’ gestation were ongoing (30% per cycle and 31% per embryo transfer).
Separate data from Molecular Biometrics suggest that vitrification is efficient for freezing donor oocytes in egg banks, Dr. Nagy said. Among 11,553 eggs from 342 donors that were vitrified, 7,063 were warmed and 90% survived warming, leading to a clinical pregnancy rate of 57% and an implantation rate of 43%.
Cryopreserving eggs has advantages over fresh ovum donation programs, which have long waiting lists, require more complex synchronization between the donor and recipient, offer relatively limited choice, and have other logistical disadvantages. "A couple of years ago, most of us were doing slow freezing" for cryopreservation, Dr. Nagy said. "Many of us have changed to vitrification because we have realized that it is more efficient than slow freezing."
Before, two to four embryos were needed to achieve a clinical pregnancy under slow-freeze methods at his institution, but only one or two are needed using vitrification, he said.
Oocyte cryopreservation programs make elective single-embryo transfer (eSET) a viable option, Dr. Nagy said. In 98 recipients of eSET, 52% achieved a clinical pregnancy, compared with clinical pregnancies in 72% of 109 elective double-embryo transfers, but 51% of the double-embryo transfers resulted in multiple pregnancies, a retrospective study of data from his institution showed. Among nonelective double-embryo transfers, 52% achieved clinical pregnancies, with multiple pregnancies in 30% of recipients.
Double vitrification also appears to be efficient for egg/embryo banking – cryopreserving an oocyte, thawing it, and then freezing the resulting embryo to later be warmed and transferred. One study that compared 471 warming cycles of double-vitrified embryos with 2,629 warming cycles of vitrified embryos derived from fresh oocytes found overall embryo survival rates of 97% and 96%, respectively, with a delivery rate per warming cycle of 34% in the double-vitrified group and 31% in the single-vitrified group (Fertil. Steril. 2013;99:1623-30).
At Dr. Nagy’s institution, 99% of 190 warmed double-vitrified embryos have survived, leading to a 53% clinical pregnancy rate, a 39% implantation rate, and 33 live births so far. "It shows that if you do the procedure correctly, you are able to preserve the viability of those embryos," he said.
The rate of congenital anomalies in live births from donor eggs cryopreserved at his institution does not seem to be significantly higher than the rate in live births from fresh donor eggs – 3 congenital anomalies among 91 live births using fresh donor eggs and 5 congenital anomalies in 338 live births using cryopreserved donor eggs. Case reports from 1986 to 2008 suggest there’s no increased incidence of birth defects using vitrification compared with slow-freeze methods for cryopreservation.