From 2006 to 2012 there was a clear trend towards increased Frozen Embryo Transfer ( FET ) usage than to fresh cycles. This increased usage of FET was due to rapid increase in Live birth rates with FET than with fresh transfers. In 2006 the reported Live Birth Rates with FET per transfer was 33.1%. In 2012 this Live Birth Rate with FET increased to 42.4%. Gradually Live Birth Rate with FET exceeded those with fresh transfers.
The increase in FET usage and success rate may have resulted from multiple reasons.
Improved Cryopreservation techniques may have reduced embryo cryodamage and therefore increased success rates and confidence in cryopreservation and FET. This might encourage more frequent freezing of entire cohorts rather than freezing “ Second-best” embryos after morphologically best embryos are transferred in fresh cycles.
Entire cohort banking is also increasingly routine after the use of a GnRH agonist “ trigger” to prevent ovarian hyperstimilation syndrome(OHSS) in high responders.
The increased use of genetic screening also increased the use of cryopreservation, because embryos are often frozen while awaiting test results, and transfer of normal (euploid)embryos may contribute to increasing FET success rates.
Lastly the steady increase in single embryo transfer policy should have left more embryos for cryopreservation and FET.
Why Fresh Transfers Gives Lower Pregnancy Rate:
Super ovulation with gonadotropins (hormones) is an integral part of the IVF process, because controlled ovarian hyperstimulation allows the retrieval of multiple oocytes for fertilization and embryo development. However superovulation results in high levels of hormones such as Estrogen(E2), Progesterone (P) and vascular endothelial growth factors(VEGF), both during oocyte development and after embryo transfer. High levels of E2, P and VEGF leads to potential changes in the oocyte, endometrium and implanting embryo. These changes decreased the implantation rate.
Fresh transfers also have adverse perinatal outcomes such as:
- Foetal Growth Retardation
- Low Birth Weight
- Preterm Delivery
- Pre-eclampsia Risks
Benefits Of Frozen Embryo Transfer ( FET)
- The risk of OHSS is virtually eliminated with the use of a GnRH agonist “trigger” for final maturation but the use of agonist trigger is associated with abrupt termination of the leuteal phase with complete and irreversible luteolysis and reduced Live Birth Rates. Therefore, cohort cryopreservstion is often used after agonist trigger to improve the chances of live birth.
- Ectopic pregnancy risk is greater in IVF pregnancies (4-5%) than in natural pregnancies(2%).This is due to effect of increased level of E2 hormone on uterial contractions or increased P on tubal cilia. FET has reduced risk of ectopic pregnancy to natural level (2%).
- When compared with fresh transfers , FET pregnancies are associated with significantly reduced risk of preterm birth, SGA, Perinatal mortality, placental abruption and placenta previa.
- Fresh transfers with ICSI has increased risk for birth defects when compared with FET. FET also has an embryo- screening effect through cryo- survival.
- There is growing body of evidence to support transferring embryos(ET) in a more physiologic rather than hyperstimulated environment.
FET has some downsides also such as:
- Increased Longer Cost
- Longer Time and inconvenience
- Increased incidence of large for gestational age
- Placenta accrete
- Pregnancy Induced Hypertension
Still fresh transfer cycles are gradually eliminating and IVF practice is shifting to “ ALL FREEZE CYCLES”
At IHR all our Fresh IVF cycles are turned to FREEZE ALL CYCLES