Hum. Reprod. Advance Access originally published online on January 21, 2005
Human Reproduction 2005 20(4):979-985; doi:10.1093/humrep/deh724
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A randomized double-blind controlled study of the efficacy of laser-assisted hatching on implantation and pregnancy rates of frozen–thawed embryo transfer at the cleavage stage
1 Department of Obstetrics and Gynaecology, The University of Hong Kong, Hong Kong Special Administrative Region, People's Republic of China
2 To whom correspondence should be addressed at: Department of Obstetrics and Gynaecology, The University of Hong Kong, Queen Mary Hospital, Pokfulam Road, Hong Kong. Email: nghye{at}hkucc.hku.hk
BACKGROUND: Assisted hatching (AH) in fresh embryo transfer (ET) cycles increases the implantation and pregnancy rates, especially in women with a poor prognosis, repeated implantation failures and in older women. Little information exists in the literature regarding the role of AH in frozen–thawed embryo transfer (FET) cycles. METHODS: Embryos were cryopreserved at the cleavage stage. On the day of FET, 160 patients were randomized according to a computer-generated randomization list in sealed envelopes into the AH group and the control group. The patients and the clinicians were blinded to the group assigned. In the AH group, the outer half of the zona pellucida over a quarter of the diameter of zona was removed using a 1480 nm non-contact laser. RESULTS: The two groups were comparable in terms of demographic characteristics, ovarian response of the stimulated cycle and quality of fresh and frozen–thawed embryos. No differences in implantation, pregnancy and multiple pregnancy rates were found between the two groups. There was a non-significant trend of a higher implantation rate in the AH group when the zona thickness was 
16 mm. CONCLUSION: Laser AH did not improve the implantation rate of FET cycles and should not be performed routinely in all frozen–thawed embryos at the cleavage stage.
Key words: assisted hatching/frozen thawed embryo transfer/laser
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